HIV is similar in structure to other retroviruses. It is roughly spherical [ 22 ]  with a diameter of about 120  nm , around 100,000 times smaller in volume than a  red blood cell . [ 23 ]  It is composed of two copies of positive- sense   single-stranded   RNA  that codes for the virus' nine  genes  enclosed by a conical  capsid  composed of 2,000 copies of the viral protein  p24 . [ 24 ]  The single-stranded RNA is tightly bound to nucleocapsid proteins, p7, and enzymes needed for the development of the virion such as  reverse transcriptase ,  proteases ,  ribonuclease  and  integrase . A matrix composed of the viral protein p17 surrounds the capsid ensuring the integrity of the virion particle. [ 24 ]


This is, in turn, surrounded by the  viral envelope , that is composed of the  lipid bilayer  taken from the membrane of a human host cell when the newly formed virus particle buds from the cell. The viral envelope contains proteins from the host cell and relatively few copies of the HIV envelope protein, [ 24 ]  which consists of a cap made of three molecules known as  glycoprotein (gp) 120 , and a stem consisting of three  gp41  molecules that anchor the structure into the viral envelope. [ 25 ] [ 26 ]  The envelope protein, encoded by the HIV  env  gene, allows the virus to attach to target cells and fuse the viral envelope with the target  cell's membrane  releasing the viral contents into the cell and initiating the infectious cycle. [ 25 ]


As the sole viral protein on the surface of the virus, the envelope protein is a major target for  HIV vaccine  efforts. [ 27 ]  Over half of the mass of the trimeric envelope spike is N-linked  glycans . The density is high as the glycans shield the underlying viral protein from neutralisation by antibodies. This is one of the most densely glycosylated molecules known and the density is sufficiently high to prevent the normal maturation process of glycans during biogenesis in the endoplasmic and Golgi apparatus. [ 28 ] [ 29 ]  The majority of the glycans are therefore stalled as immature 'high-mannose' glycans not normally present on human glycoproteins that are secreted or present on a cell surface. [ 30 ]  The unusual processing and high density means that almost all broadly neutralising antibodies that have so far been identified (from a subset of patients that have been infected for many months to years) bind to, or are adapted to cope with, these envelope glycans. [ 31 ]


The molecular structure of the viral spike has now been determined by  X-ray crystallography [ 32 ]  and  cryogenic electron microscopy . [ 33 ]  These advances in structural biology were made possible due to the development of stable  recombinant  forms of the viral spike by the introduction of an intersubunit  disulphide bond  and an  isoleucine  to  proline   mutation  ( radical replacement  of an amino acid) in gp41. [ 34 ]  The so-called SOSIP  trimers  not only reproduce the antigenic properties of the native viral spike, but also display the same degree of immature glycans as presented on the native virus. [ 35 ]  Recombinant trimeric viral spikes are promising vaccine candidates as they display less non-neutralising  epitopes  than recombinant monomeric gp120, which act to suppress the immune response to target epitopes. [ 36 ]


The RNA genome consists of at least seven structural landmarks ( LTR ,  TAR ,  RRE , PE, SLIP, CRS, and INS), and nine genes ( gag ,  pol , and  env ,  tat ,  rev ,  nef ,  vif ,  vpr ,  vpu , and sometimes a tenth  tev , which is a fusion of  tat ,  env  and  rev ), encoding 19 proteins. Three of these genes,  gag ,  pol , and  env , contain information needed to make the structural proteins for new virus particles. [ 24 ]  For example,  env  codes for a protein called gp160 that is cut in two by a cellular protease to form gp120 and gp41. The six remaining genes,  tat ,  rev ,  nef ,  vif ,  vpr , and  vpu  (or  vpx  in the case of HIV-2), are regulatory genes for proteins that control the ability of HIV to infect cells, produce new copies of virus (replicate), or cause disease. [ 24 ]


The two  tat  proteins (p16 and p14) are  transcriptional transactivators  for the LTR  promoter  acting by binding the TAR RNA element. The TAR may also be processed into  microRNAs  that regulate the  apoptosis  genes  ERCC1  and  IER3 . [ 37 ] [ 38 ]  The  rev  protein (p19) is involved in shuttling RNAs from the nucleus and the cytoplasm by binding to the  RRE  RNA element. The  vif  protein (p23) prevents the action of  APOBEC3G  (a cellular protein that  deaminates   cytidine  to  uridine  in the single-stranded viral DNA and/or interferes with reverse transcription [ 39 ] ).  The  vpr  protein (p14) arrests  cell division  at  G2/M . The  nef  protein (p27) down-regulates  CD4  (the major viral receptor), as well as the  MHC class I  and  class II  molecules. [ 40 ] [ 41 ] [ 42 ]


Nef  also interacts with  SH3 domains . The  vpu  protein (p16) influences the release of new virus particles from infected cells. [ 24 ]  The ends of each strand of HIV RNA contain an RNA sequence called a  long terminal repeat  (LTR). Regions in the LTR act as switches to control production of new viruses and can be triggered by proteins from either HIV or the host cell. The  Psi element  is involved in viral genome packaging and recognized by  gag  and  rev  proteins. The SLIP element ( TTTTTT ) is involved in the  frameshift  in the  gag - pol   reading frame  required to make functional  pol . [ 24 ]


The term  viral tropism  refers to the cell types a virus infects. HIV can infect a variety of immune cells such as  CD4 +  T cells ,  macrophages , and  microglial cells . HIV-1 entry to macrophages and CD4 +  T cells is mediated through interaction of the virion envelope glycoproteins (gp120) with the CD4 molecule on the target cells' membrane and also with  chemokine   co-receptors . [ 25 ] [ 43 ]


Macrophage-tropic (M-tropic) strains of HIV-1, or non- syncytia -inducing strains (NSI; now called R5 viruses [ 44 ] ) use the  β -chemokine receptor,  CCR5 , for entry and are thus able to replicate in both macrophages and CD4 +  T cells. [ 45 ]  This CCR5 co-receptor is used by almost all primary HIV-1 isolates regardless of viral genetic subtype. Indeed, macrophages play a key role in several critical aspects of HIV infection. They appear to be the first cells infected by HIV and perhaps the source of HIV production when CD4 +  cells become depleted in the patient. Macrophages and microglial cells are the cells infected by HIV in the  central nervous system . In the  tonsils  and  adenoids  of HIV-infected patients, macrophages fuse into multinucleated  giant cells  that produce huge amounts of virus.


T-tropic strains of HIV-1, or  syncytia -inducing strains (SI; now called X4 viruses [ 44 ] ) replicate in primary CD4 +  T cells as well as in macrophages and use the  α -chemokine receptor,  CXCR4 , for entry. [ 45 ] [ 46 ] [ 47 ]


Dual-tropic HIV-1 strains are thought to be transitional strains of HIV-1 and thus are able to use both CCR5 and CXCR4 as co-receptors for viral entry.


The  α -chemokine  SDF-1 , a  ligand  for CXCR4, suppresses replication of T-tropic HIV-1 isolates. It does this by  down-regulating  the expression of CXCR4 on the surface of HIV target cells. M-tropic HIV-1 isolates that use only the CCR5 receptor are termed R5; those that use only CXCR4 are termed X4, and those that use both, X4R5. However, the use of co-receptors alone does not explain viral tropism, as not all R5 viruses are able to use CCR5 on macrophages for a productive infection [ 45 ]  and HIV can also infect a subtype of  myeloid dendritic cells , [ 48 ]  which probably constitute a  reservoir  that maintains infection when CD4 +  T cell numbers have declined to extremely low levels.


Some people are resistant to certain strains of HIV. [ 49 ]  For example, people with the  CCR5-Δ32  mutation are resistant to infection by the R5 virus, as the mutation leaves HIV unable to bind to this co-receptor, reducing its ability to infect target cells.


Sexual intercourse  is the major mode of HIV transmission. Both X4 and R5 HIV are present in the  seminal fluid , which enables the virus to be transmitted from a male to his  sexual partner . The virions can then infect numerous cellular targets and disseminate into the whole organism. However, a selection process [ further explanation needed ]  leads to a predominant transmission of the R5 virus through this pathway. [ 50 ] [ 51 ] [ 52 ]  In patients infected with subtype B HIV-1, there is often a co-receptor switch in late-stage disease and T-tropic variants that can infect a variety of T cells through CXCR4. [ 53 ]  These variants then replicate more aggressively with heightened virulence that causes rapid T cell depletion, immune system collapse, and opportunistic infections that mark the advent of AIDS. [ 54 ]  HIV-positive patients acquire an enormously broad spectrum of opportunistic infections, which was particularly problematic prior to the onset of  HAART  therapies; however, the same infections are reported among HIV-infected patients examined post-mortem following the onset of antiretroviral therapies. [ 3 ]  Thus, during the course of infection, viral adaptation to the use of CXCR4 instead of CCR5 may be a key step in the progression to AIDS. A number of studies with subtype B-infected individuals have determined that between 40 and 50 percent of AIDS patients can harbour viruses of the SI and, it is presumed, the X4 phenotypes. [ 55 ] [ 56 ]


HIV-2 is much less pathogenic than HIV-1 and is restricted in its worldwide distribution to  West Africa . The adoption of "accessory genes" by HIV-2 and its more  promiscuous  pattern of co-receptor usage (including CD4-independence) may assist the virus in its adaptation to avoid innate restriction factors present in host cells. Adaptation to use normal cellular machinery to enable transmission and productive infection has also aided the establishment of HIV-2 replication in humans. A survival strategy for any infectious agent is not to kill its host, but ultimately become a  commensal  organism. Having achieved a low pathogenicity, over time, variants that are more successful at transmission will be selected. [ 57 ]


The HIV virion enters  macrophages  and CD4 +   T cells  by the  adsorption  of  glycoproteins  on its surface to receptors on the target cell followed by fusion of the  viral envelope  with the target cell membrane and the release of the HIV capsid into the cell. [ 58 ] [ 59 ]


Entry to the cell begins through interaction of the trimeric envelope complex ( gp160  spike) on the HIV viral envelope and both  CD4  and a chemokine co-receptor (generally either  CCR5  or  CXCR4 , but others are known to interact) on the target cell surface. [ 58 ] [ 59 ]  Gp120 binds to  integrin  α 4 β 7  activating  LFA-1 , the central integrin involved in the establishment of  virological synapses , which facilitate efficient cell-to-cell spreading of HIV-1. [ 60 ]  The gp160 spike contains binding domains for both CD4 and chemokine receptors. [ 58 ] [ 59 ]


The first step in fusion involves the high-affinity attachment of the CD4 binding domains of  gp120  to CD4. Once gp120 is bound with the CD4 protein, the envelope complex undergoes a structural change, exposing the chemokine receptor binding domains of gp120 and allowing them to interact with the target chemokine receptor. [ 58 ] [ 59 ]  This allows for a more stable two-pronged attachment, which allows the  N-terminal  fusion peptide gp41 to penetrate the cell membrane. [ 58 ] [ 59 ]   Repeat sequences  in gp41, HR1, and HR2 then interact, causing the collapse of the extracellular portion of gp41 into a hairpin shape. This loop structure brings the virus and cell membranes close together, allowing fusion of the membranes and subsequent entry of the viral capsid. [ 58 ] [ 59 ]


After HIV has bound to the target cell, the HIV RNA and various enzymes, including reverse transcriptase, integrase, ribonuclease, and protease, are injected into the cell. [ 58 ] [ failed verification ]  During the  microtubule -based transport to the nucleus, the viral single-strand RNA genome is transcribed into double-strand DNA, which is then integrated into a host chromosome.


HIV can infect  dendritic cells  (DCs) by this CD4-CCR5 route, but another route using  mannose-specific C-type lectin receptors  such as  DC-SIGN  can also be used. [ 61 ]  DCs are one of the first cells encountered by the virus during sexual transmission. They are currently thought to play an important role by transmitting HIV to T cells when the virus is captured in the  mucosa  by DCs. [ 61 ]  The presence of  FEZ-1 , which occurs naturally in  neurons , is believed to prevent the infection of cells by HIV. [ 62 ]


HIV-1 entry, as well as entry of many other retroviruses, has long been believed to occur exclusively at the plasma membrane. More recently, however, productive infection by  pH -independent,  clathrin-mediated endocytosis  of HIV-1 has also been reported and was recently suggested to constitute the only route of productive entry. [ 63 ] [ 64 ] [ 65 ] [ 66 ] [ 67 ]


Shortly after the viral capsid enters the cell, an  enzyme  called  reverse transcriptase  liberates the positive-sense single-stranded  RNA  genome from the attached viral proteins and copies it into a  complementary DNA  (cDNA) molecule. [ 68 ]  The process of reverse transcription is extremely error-prone, and the resulting mutations may cause  drug resistance  or allow the virus to evade the body's immune system. The reverse transcriptase also has ribonuclease activity that degrades the viral RNA during the synthesis of cDNA, as well as DNA-dependent DNA polymerase activity that creates a  sense  DNA from the  antisense  cDNA. [ 69 ]  Together, the cDNA and its complement form a double-stranded viral DNA that is then transported into the  cell nucleus . The integration of the viral DNA into the host cell's  genome  is carried out by another viral enzyme called  integrase . [ 68 ]


The integrated viral DNA may then lie dormant, in the latent stage of HIV infection. [ 68 ]  To actively produce the virus, certain cellular  transcription factors  need to be present, the most important of which is  NF- κ B  (nuclear factor kappa B), which is upregulated when T cells become activated. [ 70 ]  This means that those cells most likely to be targeted, entered and subsequently killed by HIV are those actively fighting infection.


During viral replication, the integrated DNA  provirus  is  transcribed  into RNA. The full-length genomic RNAs (gRNA) can be packaged into new viral particles in a  pseudodiploid  form. The selectivity in the packaging is explained by the structural properties of the dimeric conformer of the gRNA. The gRNA dimer is characterized  by a tandem three-way junction within the gRNA monomer, in which the SD and AUG  hairpins , responsible for splicing and translation respectively, are sequestered and the DIS (dimerization initiation signal) hairpin is exposed. The formation of the gRNA dimer is mediated by a 'kissing'  interaction between the DIS hairpin loops of the gRNA monomers. At the same time, certain guanosine residues in the gRNA are made available for binding of the nucleocapsid (NC) protein leading to the subsequent virion assembly. [ 71 ]  The labile gRNA dimer has been also reported to achieve a more stable conformation following the NC binding, in which both the DIS and the U5:AUG regions of the gRNA participate in extensive base pairing. [ 72 ]


RNA can also be  processed  to produce mature  messenger RNAs  (mRNAs). In most cases, this processing involves  RNA splicing  to produce mRNAs that are shorter than the full-length genome. Which part of the RNA is removed during RNA splicing determines which of the HIV protein-coding sequences is translated. [ 73 ]


Mature HIV mRNAs are exported from the nucleus into the  cytoplasm , where they are  translated  to produce HIV proteins, including  Rev . As the newly produced Rev protein is produced it moves to the nucleus, where it binds to full-length, unspliced copies of virus RNAs and allows them to leave the nucleus. [ 74 ]  Some of these full-length RNAs function as mRNAs that are translated to produce the structural proteins Gag and Env. Gag proteins bind to copies of the virus RNA genome to package them into new virus particles. [ 75 ]
HIV-1 and HIV-2 appear to package their RNA differently. [ 76 ] [ 77 ]  HIV-1 will bind to any appropriate RNA. [ 78 ]  HIV-2 will preferentially bind to the mRNA that was used to create the Gag protein itself. [ 79 ]


Two RNA genomes are encapsidated in each HIV-1 particle (see  Structure and genome of HIV ). Upon infection and replication catalyzed by reverse transcriptase, recombination between the two genomes can occur. [ 80 ] [ 81 ]  Recombination occurs as the single-strand, positive-sense RNA genomes are reverse transcribed to form DNA. During reverse transcription, the nascent DNA can switch multiple times between the two copies of the viral RNA. This form of recombination is known as copy-choice. Recombination events may occur throughout the genome. Anywhere from two to 20 recombination events per genome may occur at each replication cycle, and these events can rapidly shuffle the genetic information that is transmitted from parental to progeny genomes. [ 81 ]


Viral recombination produces genetic variation that likely contributes to the  evolution  of resistance to  anti-retroviral therapy . [ 82 ]  Recombination may also contribute, in principle, to overcoming the immune defenses of the host. Yet, for the adaptive advantages of genetic variation to be realized, the two viral genomes packaged in individual infecting virus particles need to have arisen from separate progenitor parental viruses of differing genetic constitution. It is unknown how often such mixed packaging occurs under natural conditions. [ 83 ]


Bonhoeffer  et al. [ 84 ]  suggested that template switching by reverse transcriptase acts as a repair process to deal with breaks in the single-stranded RNA genome. In addition, Hu and Temin [ 80 ]  suggested that recombination is an adaptation for repair of damage in the RNA genomes. Strand switching (copy-choice recombination) by reverse transcriptase could generate an undamaged copy of genomic DNA from two damaged single-stranded RNA genome copies. This view of the adaptive benefit of recombination in HIV could explain why each HIV particle contains two complete genomes, rather than one. Furthermore, the view that recombination is a repair process implies that the benefit of repair can occur at each replication cycle, and that this benefit can be realized whether or not the two genomes differ genetically. On the view that recombination in HIV is a repair process, the generation of recombinational variation would be a consequence, but not the cause of, the evolution of template switching. [ 84 ]


HIV-1 infection causes  chronic inflammation  and production of  reactive oxygen species . [ 85 ]  Thus, the HIV genome may be vulnerable to  oxidative damage , including breaks in the single-stranded RNA. For HIV, as well as for viruses in general, successful infection depends on overcoming host defense strategies that often include production of genome-damaging reactive oxygen species. Thus, Michod  et al. [ 86 ]  suggested that recombination by viruses is an adaptation for repair of genome damage, and that recombinational variation is a byproduct that may provide a separate benefit.


The final step of the viral cycle, assembly of new HIV-1 virions, begins at the  plasma membrane  of the host cell. The Env polyprotein (gp160) goes through the  endoplasmic reticulum  and is transported to the  Golgi apparatus  where it is  cleaved  by  furin  resulting in the two HIV envelope glycoproteins,  gp41  and  gp120 . [ 87 ]  These are transported to the plasma membrane of the host cell where gp41 anchors gp120 to the membrane of the infected cell. The Gag (p55) and Gag-Pol (p160) polyproteins also associate with the inner surface of the plasma membrane along with the HIV genomic RNA as the forming virion begins to bud from the host cell. The budded virion is still immature as the  gag  polyproteins still need to be cleaved into the actual matrix, capsid and nucleocapsid proteins. This cleavage is mediated by the packaged viral protease and can be inhibited by antiretroviral drugs of the  protease inhibitor  class. The various structural components then assemble to produce a mature HIV virion. [ 88 ]  Only mature virions are then able to infect another cell.


The classical process of infection of a cell by a virion can be called "cell-free spread" to distinguish it from a more recently recognized process called "cell-to-cell spread". [ 89 ]  In cell-free spread (see figure), virus particles bud from an infected T cell, enter the blood or  extracellular fluid  and then infect another T cell following a chance encounter. [ 89 ]  HIV can also disseminate by direct transmission from one cell to another by a process of cell-to-cell spread, for which two pathways have been described. Firstly, an infected T cell can transmit virus directly to a target T cell via a  virological synapse . [ 60 ] [ 90 ]  Secondly, an  antigen-presenting cell  (APC), such as a macrophage or dendritic cell, can transmit HIV to T cells by a process that either involves productive infection (in the case of macrophages) or capture and transfer of virions  in trans  (in the case of dendritic cells). [ 91 ]  Whichever pathway is used, infection by cell-to-cell transfer is reported to be much more efficient than cell-free virus spread. [ 92 ]  A number of factors contribute to this increased efficiency, including polarised virus budding towards the site of cell-to-cell contact, close apposition of cells, which minimizes fluid-phase  diffusion  of virions, and clustering of HIV entry receptors on the target cell towards the contact zone. [ 90 ]  Cell-to-cell spread is thought to be particularly important in  lymphoid tissues , where CD4 +  T cells are densely packed and likely to interact frequently. [ 89 ]   Intravital imaging  studies have supported the concept of the HIV virological synapse  in vivo . [ 93 ]  The many dissemination mechanisms available to HIV contribute to the virus' ongoing replication in spite of anti-retroviral therapies. [ 89 ] [ 94 ]


HIV differs from many viruses in that it has very high  genetic variability . This diversity is a result of its fast  replication cycle , with the generation of about 10 10  virions every day, coupled with a high  mutation rate  of approximately 3 x 10 −5  per  nucleotide base  per cycle of replication and  recombinogenic  properties of reverse transcriptase. [ 95 ] [ 96 ] [ 97 ]


This complex scenario leads to the generation of many variants of HIV in a single infected patient in the course of one day. [ 95 ]  This variability is compounded when a single cell is simultaneously infected by two or more different strains of HIV. When  simultaneous infection  occurs, the genome of progeny virions may be composed of RNA strands from two different strains. This hybrid virion then infects a new cell where it undergoes replication. As this happens, the reverse transcriptase, by jumping back and forth between the two different RNA templates, will generate a newly synthesized retroviral  DNA sequence  that is a recombinant between the two parental genomes. [ 95 ]  This recombination is most obvious when it occurs between subtypes. [ 95 ]


The closely related  simian immunodeficiency virus  (SIV) has evolved into many strains, classified by the natural host species. SIV strains of the  African green monkey  (SIVagm) and  sooty mangabey  (SIVsmm) are thought to have a long evolutionary history with their hosts. These hosts have adapted to the presence of the virus, [ 98 ]  which is present at high levels in the host's blood, but evokes only a mild immune response, [ 99 ]  does not cause the development of simian AIDS, [ 100 ]  and does not undergo the extensive mutation and recombination typical of HIV infection in humans. [ 101 ]


In contrast, when these strains infect species that have not adapted to SIV ("heterologous" or similar hosts such as  rhesus  or  cynomologus macaques ), the animals develop AIDS and the virus generates  genetic diversity  similar to what is seen in human HIV infection. [ 102 ]   Chimpanzee  SIV (SIVcpz), the closest genetic relative of HIV-1, is associated with increased mortality and AIDS-like symptoms in its natural host. [ 103 ]  SIVcpz appears to have been transmitted relatively recently to chimpanzee and human populations, so their hosts have not yet adapted to the virus. [ 98 ]  This virus has also lost a function of the  nef  gene that is present in most SIVs. For non-pathogenic SIV variants,  nef  suppresses T cell activation through the  CD3  marker.  Nef 's  function in non-pathogenic forms of SIV is to  downregulate  expression of  inflammatory cytokines ,  MHC-1 , and signals that affect T cell trafficking. In HIV-1 and SIVcpz,  nef  does not inhibit T-cell activation and it has lost this function. Without this function, T cell depletion is more likely, leading to immunodeficiency. [ 103 ] [ 104 ]


Three groups of HIV-1 have been identified on the basis of differences in the envelope ( env ) region: M, N, and O. [ 105 ]  Group M is the most prevalent and is subdivided into eight subtypes (or  clades ), based on the whole genome, which are geographically distinct. [ 106 ]  The most prevalent are subtypes B (found mainly in North America and Europe), A and D (found mainly in Africa), and C (found mainly in Africa and Asia); these subtypes form branches in the  phylogenetic tree  representing the lineage of the M group of HIV-1.  Co-infection  with distinct subtypes gives rise to circulating recombinant forms (CRFs). In 2000, the last year in which an analysis of global subtype prevalence was made, 47.2% of infections worldwide were of subtype C, 26.7% were of subtype A/CRF02_AG, 12.3% were of subtype B, 5.3% were of subtype D, 3.2% were of CRF_AE, and the remaining 5.3% were composed of other subtypes and CRFs. [ 107 ]  Most HIV-1 research is focused on subtype B; few laboratories focus on the other subtypes. [ 108 ]  The existence of a fourth group, "P", has been hypothesised based on a virus isolated in 2009. [ 109 ] [ 110 ]  The strain is apparently derived from  gorilla  SIV (SIVgor), first isolated from  western lowland gorillas  in 2006. [ 109 ]


HIV-2's closest relative is SIVsm, a strain of SIV found in sooty mangabees. Since HIV-1 is derived from SIVcpz, and HIV-2 from SIVsm, the genetic sequence of HIV-2 is only partially homologous to HIV-1 and more closely resembles that of SIVsm. [ 111 ] [ 112 ]


Many HIV-positive people are unaware that they are infected with the virus. [ 113 ]  For example, in 2001 less than 1% of the sexually active urban population in Africa had been tested, and this proportion is even lower in rural populations. [ 113 ]  Furthermore, in 2001 only 0.5% of  pregnant women  attending urban health facilities were counselled, tested or received their test results. [ 113 ]  Again, this proportion is even lower in rural health facilities. [ 113 ]  Since donors may therefore be unaware of their infection,  donor blood  and blood products used in medicine and  medical research  are routinely screened for HIV. [ 114 ]


HIV-1 testing is initially done using an  enzyme-linked immunosorbent assay  (ELISA) to detect antibodies to HIV-1. Specimens with a non-reactive result from the initial ELISA are considered HIV-negative, unless new exposure to an infected partner or partner of unknown HIV status has occurred. Specimens with a reactive ELISA result are retested in duplicate. [ 115 ]  If the result of either duplicate test is reactive, the specimen is reported as repeatedly reactive and undergoes confirmatory testing with a more specific supplemental test (e.g., a  polymerase chain reaction  (PCR),  western blot  or, less commonly, an  immunofluorescence assay  (IFA)). Only specimens that are repeatedly reactive by ELISA and positive by IFA or PCR or reactive by western blot are considered HIV-positive and indicative of HIV infection. Specimens that are repeatedly ELISA-reactive occasionally provide an indeterminate western blot result, which may be either an incomplete antibody response to HIV in an infected person or nonspecific reactions in an uninfected person. [ 116 ]


HIV deaths in 2014 excluding the U.S.: [ 117 ]


Although IFA can be used to confirm infection in these ambiguous cases, this assay is not widely used. In general, a second specimen should be collected more than a month later and retested for persons with indeterminate western blot results. Although much less commonly available,  nucleic acid testing  (e.g., viral RNA or proviral DNA amplification method) can also help diagnosis in certain situations. [ 115 ]  In addition, a few tested specimens might provide inconclusive results because of a low quantity specimen. In these situations, a second specimen is collected and tested for HIV infection.


Modern HIV testing is extremely accurate, when the  window period  is taken into consideration. A single screening test is correct more than 99% of the time. [ 118 ]  The chance of a false-positive result in a standard two-step testing protocol is estimated to be about 1 in 250,000 in a low risk population. [ 119 ]  Testing post-exposure is recommended immediately and then at six weeks, three months, and six months. [ 120 ]


The latest recommendations of the US  Centers for Disease Control and Prevention  (CDC) show that HIV testing must start with an  immunoassay  combination test for HIV-1 and HIV-2  antibodies  and p24  antigen . A negative result rules out HIV exposure, while a positive one must be followed by an HIV-1/2 antibody differentiation immunoassay to detect which antibodies are present. This gives rise to four possible scenarios:


HIV/AIDS research includes all  medical research  that attempts to prevent, treat, or cure  HIV/AIDS , as well as fundamental research about the nature of HIV as an infectious agent and AIDS as the disease caused by HIV.


Many governments and research institutions participate in HIV/AIDS research. This research includes behavioral  health interventions , such as research into  sex education , and  drug development , such as research into  microbicides for sexually transmitted diseases ,  HIV vaccines , and  anti-retroviral drugs . [ 122 ]  Other medical research areas include the topics of  pre-exposure prophylaxis ,  post-exposure prophylaxis ,  circumcision , and  accelerated aging effects .


The management of HIV/AIDS typically involves the use of multiple  antiretroviral drugs . In many parts of the world, HIV has become a chronic condition, with progression to  AIDS  increasingly rare.


HIV latency  and the resulting  viral reservoir  in CD4 +  T cells, dendritic cells, and macrophages is the main barrier to eradication of the virus. [ 19 ] [ 123 ]


While HIV is highly virulent, transmission through sexual contact does not occur when an HIV-positive individual maintains a consistently undetectable  viral load  (<50 copies/ml) due to antiretroviral treatment. This concept was first proposed by the Swiss Federal Commission for AIDS/HIV in 2008 in what is known as the  Swiss Statement . [ 124 ] [ 125 ]  Although initially controversial, subsequent studies have confirmed that the risk of transmitting HIV through sex is effectively zero when the HIV-positive person has a consistently undetectable viral load, a concept now widely known as U=U, or "Undetectable = Untransmittable." [ 126 ] [ 127 ]


Studies that established the U=U principle include Opposites Attract, [ 128 ]  PARTNER 1, [ 129 ]  PARTNER 2 [ 5 ] [ 130 ]  (which focused on male-male couples), [ 131 ]  and  HPTN052 [ 132 ]  (which focused on heterosexual couples). [ 131 ]  These studies involved couples where one partner was HIV-positive and one was HIV-negative, and included regular HIV testing. Across these four studies, a total of 4,097 couples participated from four continents, reporting 151,880 acts of condomless sex with zero phylogenetically-linked HIV transmissions when the positive partner had an undetectable viral load. [ 133 ]  Following these findings, the U=U consensus statement advocating the use of the term 'zero risk' was endorsed by numerous individuals and organizations, including the  CDC , the  British HIV Association , and  The Lancet  medical journal. [ 134 ]


Additionally, reactivation of  herpes simplex virus-2  (HSV-2) in individuals with  genital herpes  is associated with an increase in  CCR-5  enriched CD4 +  T cells and inflammatory dendritic cells in the dermis of ulcerated genital skin, persisting even after ulcer healing. HIV's tropism for CCR-5 positive cells contributes to the two- to threefold increased risk of HIV acquisition in persons with genital herpes. Notably, daily antiviral medication, such as  acyclovir , does not reduce the subclinical post-reactivation inflammation and therefore does not decrease the risk of HIV acquisition. [ 135 ] [ 136 ]


The first news story on "an exotic new disease" appeared May 18, 1981, in the gay newspaper  New York Native . [ 137 ]


AIDS was first clinically observed in 1981 in the United States. [ 138 ]  The initial cases were a cluster of injection drug users and gay men with no known cause of impaired immunity who showed symptoms of  Pneumocystis  pneumonia (PCP or PJP, the latter term recognizing that the causative agent is now called  Pneumocystis jirovecii ), a rare opportunistic infection that was known to occur in people with very compromised immune systems. [ 139 ]  Soon thereafter, researchers at the  NYU School of Medicine  studied gay men developing a previously rare skin cancer called  Kaposi's sarcoma  (KS). [ 140 ] [ 141 ]  Many more cases of PJP and KS emerged, alerting U.S.  Centers for Disease Control and Prevention  (CDC) and a CDC task force was formed to monitor the outbreak. [ 142 ]  The earliest retrospectively described case of AIDS is believed to have been in Norway beginning in 1966. [ 143 ]


In the beginning, the CDC did not have an official name for the disease, often referring to it by way of the diseases that were associated with it, for example,  lymphadenopathy , the disease after which the discoverers of HIV originally named the virus. [ 144 ] [ 145 ]  They also used  Kaposi's Sarcoma and Opportunistic Infections , the name by which a task force had been set up in 1981. [ 146 ]  In the general press, the term  GRID , which stood for  gay-related immune deficiency , had been coined. [ 147 ]  The CDC, in search of a name and looking at the infected communities, coined "the 4H disease", as it seemed to single out homosexuals, heroin users,  hemophiliacs , and  Haitians . [ 148 ] [ 149 ]  However, after determining that AIDS was not isolated to the  gay community , [ 146 ]  it was realized that the term GRID was misleading and  AIDS  was introduced at a meeting in July 1982. [ 150 ]  By September 1982 the CDC started using the name AIDS. [ 151 ]


In 1983, two separate research groups led by American  Robert Gallo  and French investigators  Françoise Barré-Sinoussi  and  Luc Montagnier  independently declared that a novel retrovirus may have been infecting AIDS patients, and published their findings in the same issue of the journal  Science . [ 152 ] [ 145 ] [ 153 ]  Gallo claimed that a virus his group had isolated from a person with AIDS was strikingly similar in  shape  to other  human T-lymphotropic viruses  (HTLVs) his group had been the first to isolate. Gallo admitted in 1987 that the virus he claimed to have discovered in 1984 was in reality a virus sent to him from France the year before. [ 154 ]  Gallo's group called their newly isolated virus HTLV-III. Montagnier's group isolated a virus from a patient presenting with swelling of the  lymph nodes  of the neck and  physical weakness , two classic symptoms of primary HIV infection. Contradicting the report from Gallo's group, Montagnier and his colleagues showed that core proteins of this virus were immunologically different from those of HTLV-I. Montagnier's group named their isolated virus lymphadenopathy-associated virus (LAV). [ 142 ]  As these two viruses turned out to be the same, in 1986 LAV and HTLV-III were renamed HIV. [ 155 ]


Another group working contemporaneously with the Montagnier and Gallo groups was that of  Jay A. Levy  at the  University of California, San Francisco . He independently discovered the AIDS virus in 1983 and named it the AIDS associated retrovirus (ARV). [ 156 ]  This virus was very different from the virus reported by the Montagnier and Gallo groups. The ARV strains indicated, for the first time, the heterogeneity of HIV isolates and several of these remain classic examples of the AIDS virus found in the United States. [ 157 ]


Both HIV-1 and HIV-2 are believed to have originated in non-human  primates  in West-central Africa, and are believed to have transferred to humans (a process known as  zoonosis ) in the early 20th century. [ 158 ] [ 159 ]


HIV-1 appears to have originated in southern  Cameroon  through the evolution of SIVcpz, a  simian immunodeficiency virus  (SIV) that infects wild  chimpanzees  (HIV-1 descends from the SIVcpz endemic in the chimpanzee subspecies  Pan troglodytes troglodytes ). [ 160 ] [ 161 ]  The closest relative of HIV-2 is SIVsmm, a virus of the  sooty mangabey  ( Cercocebus atys atys ), an  Old World monkey  living in littoral West Africa (from southern  Senegal  to western  Côte d'Ivoire ). [ 21 ]   New World monkeys  such as the  owl monkey  are resistant to HIV-1 infection, possibly because of a  genomic fusion  of two viral resistance genes. [ 162 ]


HIV-1 is thought to have jumped the species barrier on at least three separate occasions, giving rise to the three groups of the virus, M, N, and O. [ 163 ]


There is evidence that humans who participate in  bushmeat  activities, either as hunters or as bushmeat vendors, commonly acquire SIV. [ 164 ]  However, SIV is a weak virus, and it is typically suppressed by the human immune system within weeks of infection. It is thought that several transmissions of the virus from individual to individual in quick succession are necessary to allow it enough time to mutate into HIV. [ 165 ]  Furthermore, due to its relatively low person-to-person transmission rate, it can only spread throughout the population in the presence of one or more high-risk transmission channels, which are thought to have been absent in Africa prior to the 20th century.


Specific proposed high-risk transmission channels, allowing the virus to adapt to humans and spread throughout the society, depend on the proposed timing of the animal-to-human crossing. Genetic studies of the virus suggest that the most recent common ancestor of the HIV-1 M group dates back to  c.  1910 . [ 166 ]  Proponents of this dating link the HIV epidemic with the emergence of  colonialism  and growth of large colonial African cities, leading to social changes, including different patterns of sexual contact (especially multiple, concurrent partnerships), the spread of  prostitution , and the concomitant high frequency of  genital ulcer  diseases (such as  syphilis ) in nascent colonial cities. [ 167 ]  While transmission rates of HIV during vaginal intercourse are typically low, they are increased manyfold if one of the partners has a  sexually transmitted infection  resulting in genital ulcers. Early 1900s colonial cities were notable for their high prevalence of prostitution and genital ulcers to the degree that as of 1928 as many as 45% of female residents of eastern  Leopoldville (currently Kinshasa)  were thought to have been prostitutes and as of 1933 around 15% of all residents of the same city were infected by one of the forms of  syphilis . [ 167 ]


The earliest, well-documented case of HIV in a human dates back to 1959 in the  Belgian Congo . [ 168 ]  The virus may have been present in the United States as early as the mid- to late 1960s, as a sixteen-year-old male named  Robert Rayford  presented with symptoms in 1966 and died in 1969. [ 169 ]


An alternative and likely complementary hypothesis points to the widespread use of unsafe medical practices in Africa during years following World War II, such as unsterile reuse of single-use syringes during mass vaccination, antibiotic, and anti-malaria treatment campaigns. [ 165 ] [ 170 ] [ 171 ]  Research on the timing of most recent common ancestor for HIV-1 groups M and O, as well as on HIV-2 groups A and B, indicates that SIV has given rise to transmissible HIV lineages throughout the twentieth century. [ 172 ]  The dispersed timing of these transmissions to humans implies that no single external factor is needed to explain the cross-species transmission of HIV. This observation is consistent with both of the two prevailing views of the origin of the HIV epidemics, namely SIV transmission to humans during the slaughter or butchering of infected primates, and the colonial expansion of sub-Saharan African cities. [ 172 ]


Human papillomavirus  (HPV)-associated  oropharyngeal cancer  awareness and prevention  is a vital concept from a  public  and  community health  perspective.


HPV is the  sexually transmitted   virus  that is known to be the cause of  genital warts . There are currently more than 100 different  strains  of HPV, half of which can cause genital infections. [ 1 ]  Although it is not usually the HPV strains that cause  genital warts  associated with the  oropharyngeal cancers , they have transmitted the same way through  oral-genital sexual contact , and consumers should protect themselves accordingly and adhere to routine health and dental screening schedule to monitor and maintain their health status. [ 2 ]  Many people with  HPV  do not develop any symptoms but can still infect others through sexual contact. Symptoms may include warts on the genitals or surrounding skin. There's no cure for the virus and warts may go away on their own. Treatment focuses on removing the  warts . A vaccine that prevents infection with the HPV strains most likely to cause  genital warts  and  cervical cancer  is recommended for  boys  and  girls . [ citation needed ]


Learning and utilizing self-examinations of your body, checking for changes or abnormalities in your mouth while performing your oral hygiene as well as routine checks of your genitals after showering can enhance your awareness of changes in your health status. According to the  Centers for Disease Control and Prevention  (CDC), most sexually active people are likely to acquire at least one  HPV  infection in their lifetime. [ 2 ]  It is important that consumers seek routine dental examinations as a part of their health care to allow screening of this type of cancer, as early detection can mean all the difference in treatments. Oral cavity and oropharyngeal  cancers  can occur anywhere in the mouth but occur most often in the tongue, tonsils, oropharynx (back of the throat), gums, and the floor of the mouth. [ 3 ]  According to the U.S. Preventative Services Task Force, "Oropharyngeal cancer is difficult to visualize and is usually located at the base of the tongue (the back third of the tongue), the soft palate (the back part of the roof of the mouth), the tonsils, and the side and back walls of the throat" [ 4 ]  and requires a thorough examination by a dental provider or preferably a specialist. Men are twice as likely as women to have oral cancer, regardless of race, as  African-Americans  and  Caucasians  are equally likely to develop  oral cancer . [ 3 ]


Many people do not realize that some  cancers  can be linked to viral illnesses like an HPV infection.  Oropharyngeal  cancer has been linked in some cases to HPV. The most common type (indicated in about 90%) of oral and oropharyngeal cancer is  squamous cell carcinoma . [ 5 ]  Approximately 63% of oropharyngeal squamous cell carcinomas each year are associated with HPV infection. [ 1 ]  Most cases of HPV go undetected and clear up on their own without the patient ever knowing they had contracted it. [ 1 ] [ 2 ]


There are 13 HPV strains that are known to potentially cause  cancer . [ 6 ]  According to the CDC, during an analysis of reported data from national cancer and epidemiological registries between 2008 and 2012, of 30,700 cases of cancer estimated to be caused by HPV, 24,600 (9500 oropharyngeal) were found to be caused by strains HPV 16 and HPV 18 (which are preventable through vaccination) and 3,800 (900 oropharyngeal) were found to be caused by 5 other preventable strains. [ 6 ]


Vaccination  is recommended for females and males alike, as early as age 9 and up to 26 years of age. [ 7 ]  Pre-teens can get their HPV vaccination at the same time as other scheduled vaccinations and boosters for convenience, as they can be safely administered together. Parents should speak to their child's  paediatrician  regarding their recommendations for vaccinations. Pregnant women are recommended to wait until after they deliver to receive the vaccination. [ 7 ]  Anyone with  allergy  concerns especially to  yeast  should consult their  physician . [ 7 ]


Initially,  Gardasil  (bivalent, quadrivalent, and 9-valent versions) and  Cervarix  brands were available; [ 7 ]  however, as of now only  Gardasil 9  (9-valent) is available for vaccination in the  U.S. [ 7 ]  (9-valent simply refers to the 9 strains that the vaccine helps to protect against that are known to be associated with cancers).


The World Health Organization  (WHO) says between 30% and 50% of cancer deaths could be prevented by avoiding  alcohol  and  tobacco  products, especially concurrent use. It also indicates there are about 529,000 new cases of cancers of the  oral cavity  and  pharynx  each year, and more than 300,000 deaths. [ 8 ]  "The  American Cancer Society 's most recent estimates for oral cavity and  oropharyngeal  cancers in the United States for 2017 are that about 49,670 people will get oral cavity or oropharyngeal cancer and an estimated 9,700 people will die of them." [ 3 ]


Human papillomavirus-positive oropharyngeal cancer  ( HPV-positive OPC  or  HPV+OPC ), is a  cancer  ( squamous cell carcinoma ) of the throat caused by the  human papillomavirus  type 16 virus (HPV16). In the past, cancer of the  oropharynx  (throat) was associated with the use of alcohol or tobacco or both, but the majority of cases are now associated with the HPV  virus , acquired by having oral contact with the  genitals  ( oral-genital sex ) of a person who has a genital HPV infection.  Risk factors include having a large number of sexual partners, a history of oral-genital sex or  anal–oral sex , having a female partner with a history of either an abnormal  Pap smear  or  cervical dysplasia , having chronic  periodontitis , and, among men, younger age at first intercourse and a history of  genital warts . HPV-positive OPC is considered a separate disease
from HPV-negative  oropharyngeal cancer  (also called HPV negative-OPC and  HPV-OPC).


HPV-positive OPC presents in one of four ways: as an asymptomatic abnormality in the mouth found by the patient or a health professional such as a dentist; with local symptoms such as pain or infection at the site of the tumor; with difficulties of speech, swallowing, and/or breathing; or as a swelling in the neck if the cancer has spread to local lymph nodes. Detection of a  tumour suppressor protein , known as  p16 , is commonly used to diagnose an HPV-associated OPC. The extent of disease is described in the standard cancer  staging system , using the  AJCC   TNM  system, based on the T stage (size and extent of tumor), N stage (extent of involvement of regional  lymph nodes ) and M stage (whether there is  spread of the disease  outside the region or not), and combined into an overall stage from I–IV. In 2016, a separate staging system was developed for HPV+OPC, distinct from HPV-OPC.


Whereas most  head and neck cancers  have been declining as smoking rates have declined, HPV-positive OPC has been increasing. Compared to HPV-OPC patients, HPV-positive patients tend to be younger, have a higher  socioeconomic status  and are less likely to smoke. In addition, they tend to have smaller tumours, but are more likely to have involvement of the cervical lymph nodes. In the United States and other countries, the number of cases of oropharyngeal cancer has been increasing steadily, with the incidence of HPV-positive OPC increasing faster than the decline in HPV-negative OPC. The increase is seen particularly in young men in  developed  countries, and HPV-positive OPC now accounts for the majority of all OPC cases. Efforts are being made to reduce the incidence of HPV-positive OPC by introducing  vaccination  that includes HPV types 16 and 18, found in 95% of these cancers, before exposure to the virus. Early data suggest a reduction in infection rates.


In the past, the treatment of OPC was radical surgery, with an approach through the neck and splitting of the  jaw bone , which resulted in morbidity and poor survival rates. Later,  radiotherapy  with or without the addition of  chemotherapy , provided a less disfiguring alternative, but with comparable poor outcomes. Now, newer  minimally invasive  surgical techniques through the mouth have improved outcomes; in high-risk cases, this surgery is often followed by radiation and/or chemotherapy. In the absence of   high-quality evidence  regarding which treatment provides the best outcomes, management decisions are often based on one or more of the following: technical factors, likely functional loss, and patient preference. The presence of HPV in the tumour is associated with a better response to treatment and a better outcome, independent of the treatment methods used, and a nearly 60% reduced risk of dying from the cancer. Most recurrence occurs locally and within the first year after treatment. The use of tobacco decreases the chances of survival.


HPV+OPC presents in one of four ways: as an asymptomatic abnormality in the mouth found by the patient or a health professional such as a dentist; with local symptoms such as pain or infection at the site of the tumor; with difficulties of speech, swallowing, and/or breathing; or as a swelling in the neck (if the cancer has spread to lymph nodes). These may be accompanied by more general symptoms such as loss of appetite, weight loss, and weakness. [ 3 ]


Most  mucosal   squamous cell   head and neck cancers , including  oropharyngeal cancer  (OPC), have historically been attributed to tobacco and alcohol use. However, this pattern has changed considerably since the 1980s. It was realised that some cancers occur in the absence of these risk factors and
an association between  human papillomavirus  (HPV) and various squamous cell cancers, including OPC, was first described in 1983. [ 4 ] [ 5 ]  Since then both  molecular  and  epidemiological  evidence has been accumulating, with the  International Agency for Research on Cancer  (IARC) stating that high-risk HPV types 16 and 18 are carcinogenic in humans, in 1995, [ 6 ]  and In 2007 that HPV was a cause for oral cancers. [ 7 ] [ 8 ]   Human papillomavirus  (HPV)-positive cancer (HPV+OPC)  incidence  has been increasing while HPV-negative (HPV-OPC) cancer incidence is declining, a trend that is estimated to increase further in coming years. [ 9 ]  Since there are marked differences in  clinical presentation  and  treatment   relative to HPV status, HPV+OPC is now viewed as a distinct biologic and clinical condition. [ 10 ] [ 11 ] [ 12 ]


Human HPV has long been implicated in the  pathogenesis  of several anogenital cancers including those of the  anus ,  vulva ,  vagina ,  cervix , and  penis . [ 13 ]  In 2007 it was also implicated by both  molecular  and  epidemiological  evidence in cancers arising outside of the anogenital tract, namely oral cancers. HPV infection is common among healthy individuals and is acquired through  oral sex . Although less data is available, the prevalence of HPV infection is at least as common among men as among women, with 2004 estimates of about 27% among US women aged 14–59. [ 8 ]


HPV oral infection precedes the development of HPV+OPC. [ 8 ] [ 5 ]  Slight injuries in the  mucous membrane  serve as an entry gate for HPV, which thus works into the  basal layer  of the  epithelium . [ 14 ] [ 15 ]   People testing positive for HPV type 16 virus (HPV16) oral infection have a 14 times increased risk of developing HPV+OPC. [ 14 ]   Immunosuppression  seems to be an increased risk factor for HPV+OPC. [ 5 ]  Individuals with  TGF-β1  genetic variations, specially T869C, are more likely to have HPV16+OPC. [ 16 ]  TGF-β1 plays an important role in controlling the immune system. In 1993 it was noted that patients with human papillomavirus (HPV)-associated anogenital cancers had a 4-fold increased risk of tonsillar squamous-cell carcinoma. [ 17 ]  Although evidence suggests that HPV16 is the main cause of OPC in humans not exposed to smoking and alcohol, the degree to which tobacco and/or alcohol use may contribute to increasing the risk of HPV+OPC has not always been clear [ 5 ]  but it appears that both smoking and HPV infection are independent and additive risk factors for developing OPC. [ 18 ]  The connection between HPV-infection and oropharyngeal cancer is stronger in regions of lymphoepithelial tissue (base of tongue and palatine tonsils) than in regions of stratified squamous epithelium (soft palate and uvula). [ 19 ]   Human herpesvirus-8  infection can potentiate the effects of HPV-16. [ 20 ]


Risk factors  include a high number of sexual partners (25% increase >= 6 partners), a history of  oral-genital sex  (125% >= 4 partners), or  anal–oral sex , a female partner with a history of either an abnormal  Pap smear  or  cervical dysplasia , [ 21 ]  chronic  periodontitis , [ 22 ] [ 23 ]  and, among men, decreasing age at first intercourse and history of  genital warts . [ 24 ] [ 25 ] [ 26 ] [ 27 ]


Cancers of the oropharynx primarily arise in lingual and palatine tonsil  lymphoid  tissue that is lined by respiratory  squamous   mucosal epithelium, which may be  invaginated  within the lymphoid tissue. Therefore, the tumour first arises in hidden crypts. OPC is graded based on the degree of squamous and  keratin  differentiation into well, moderate, or poorly (high) differentiated grades. Other pathological features include the presence of finger-like invasion,  perineural invasion , depth of invasion, and distance of the tumour from resection margins. Phenotypic variants include  basaloid squamous carcinoma , a high-grade form ( see  Chung Fig. 35-3(C) [ 28 ]  and illustration here). They are most commonly non-keratinising. HPV+OPC also differs from HPV-OPC in being focal rather than multifocal and not being associated with pre-malignant  dysplasia . HPV+OPC patients are therefore at less risk of developing other malignancies in the head and neck region, unlike other head and neck primary tumours that may have associated second neoplasms, that may occur at the same time (synchronous) or a distant time (metachronous), both within the head and neck region or more distantly. This suggests that the oncogenic alterations produced by the virus are spatially limited rather than related to a field defect. [ 29 ] [ 28 ] [ 30 ]


The  oropharynx , at the back of the  mouth , forms a circle and includes the  base of the tongue  (posterior third) below, the  tonsils  on each side, and the  soft palate  above, together with the walls of the  pharynx , including the anterior  epiglottis ,  epiglottic valleculae  and  branchial cleft  at its base.  The oropharynx is one of three divisions of the interior of the pharynx based on their relation to adjacent structures (nasal pharynx ( nasopharynx ), oral pharynx (oropharynx) and laryngeal pharynx ( laryngopharynx  – also referred to as the hypopharynx), from top to bottom). The pharynx is a semicircular fibromuscular tube joining the  nasal cavities  above to the  larynx  (voice box) and  oesophagus  (gullet), below, where the larynx is situated in front of the oesophagus. [ 31 ]


The oropharynx lies between the mouth (oral cavity) to the front, and the laryngopharynx below, which separates it from the larynx. The upper limit of the oropharynx is marked by the soft palate and its lower limit by the epiglottis and root of the tongue. The oropharynx communicates with the mouth, in front through what is known as the oropharyngeal isthmus, or  isthmus of the fauces . The  isthmus  (i.e. connection) is formed above by the soft palate, below by the posterior third of the tongue, and at the sides by the  palatoglossal arches . The posterior third of the tongue or tongue base contains numerous  follicles  of  lymphatic tissue  that form the  lingual tonsils . Adjacent to the tongue base, the lingual surface of the epiglottis, which curves forward, is attached to the tongue by median and lateral  glossoepiglottic folds . The folds form small troughs known as the epiglottic valleculae. The lateral walls are marked by two vertical pillars on each side, the pillars of the fauces, or palatoglossal arches. More properly they are separately named the palatoglossal arch anteriorly and the  palatopharyngeal arch  posteriorly. The anterior arch is named from the  palatoglossal muscle  within, running from the soft palate to the  tongue   ( glossus ), while the posterior arch similarly contains the  palatopharyngeal muscle  running from the soft palate to the lateral pharynx. Between the arches lies a triangular space, the  tonsillar fossa  in which lies the  palatine tonsil , another lymphoid organ.  [ 32 ]


The external pharyngeal walls consisting of the four constrictor muscles form part of the mechanism of  swallowing . The microscopic anatomy is composed of four layers, being from the  lumen  outwards, the  mucosa ,  submucosa , muscles, and the fibrosa, or fibrous layer. The mucosa consists of stratified squamous epithelium, that is generally non-keratinised, except when exposed to chronic irritants such as tobacco smoke. The submucosa contains aggregates of lymphoid tissue. [ 32 ] [ 33 ]


Cancers arising in the tonsillar fossa spread to the  cervical lymph nodes , primarily the subdigastric (upper jugular) lymph nodes (level II), with secondary involvement of the mid (level III) and low (level IV)  jugular nodes  and sometimes the posterior cervical nodes (level V). Base of tongue cancers spread to the subdigastric and mid jugular nodes, and occasionally posterior cervical nodes but being closer to the midline are more likely to have bilateral nodal disease. Tonsillar cancers rarely spread to the contralateral side unless involving the midline. [ 34 ]


HPV-associated cancers are caused by high-risk strains of HPV, mainly HPV-16 and HPV-18. [ 35 ]  HPV is a small  non-enveloped   DNA virus  of the  papillomavirus  family. Its  genome  encodes the early (E)  oncoproteins  E5, E6 and E7 and the late (L)  capsid  proteins L1 and L2. The virus gains access to the mucosa through microlesions, where it infects the  basal layer  of cells, which are still able to proliferate. While the virus does not replicate in these cells,  expression  of its early genes stimulates proliferation and lateral expansion of the basal cells. As this moves the virus particles into the overlying suprabasal layers, late viral gene expression occurs, enabling replication of the circular viral genome ( see  figure) and structural proteins. As these are pushed into the most superficial mucosal layers, complete viral particles are assembled and released. [ 36 ]


An increased risk of HPV+OPC is observed more than 15 years after HPV exposure, [ 8 ]  pointing to a slow development of the disease, similar to that seen in cervical cancer. Relative to HPV-OPC, the  oncogenic  molecular progression of HPV+OPC is poorly understood. [ 28 ]   The two main viral  oncoproteins  of the high risk HPV types are E6 and E7. These are consistently expressed in malignant cell lines, and if their expression is inhibited the malignant  phenotype  of the cancer cells is blocked. Either of these oncoproteins can  immortalise  cell lines, [ 37 ]  but are more efficient when both are expressed since their separate molecular roles are  synergistic . [ 35 ] [ 36 ]  The E6 and E7  oncogenes  become integrated into host-cell DNA, and the oncoproteins they express interfere with a variety of predominantly  antiproliferative   cellular regulatory mechanisms . They bind to and inactivate the best known of these mechanisms, the  tumor suppressor proteins   p53  and  retinoblastoma protein  pRB (pRb) leading to genomic instability and then  cell cycle  deregulation ( see  Chung et al., 2016 Fig. 35.2). [ 28 ]  Further, yet to be elicited, mechanisms are required for the final steps of  malignant transformation  of HPV infected cells. [ 28 ]


HPV- and HPV+OPC are distinguishable at the molecular level. The naturally occurring ( wild type ) p53 is widely involved in  cellular processes , including  autophagy , response to DNA damage, cell cycle regulation, and  senescence ,  apoptosis  and the generation of  adenosine triphosphate  (ATP) through  oxidative phosphorylation . [ 38 ]  The gene encoding p53 is inactivated by E6 at the protein level and is found as the wild type in HPV+OPC but mutated in HPV-OPC. In HPV+OPC  p53  protein undergoes accelerated degradation by E6, drastically reducing its levels, while in HPV-OPC it undergoes  genetic mutation , which may result in  synthesis  of an abnormal p53 protein, that may not only be inactive as a tumour suppressor but can also bind and inactivate any non-mutated wild type p53, with an increase in oncogenic activity. [ 39 ]  Although p53 mutations occur in HPV+OPC, they are far less common than in HPV-OPC (26%  vs  48%), and do not appear to affect clinical outcome. [ 40 ]


The pRb protein is inactivated by E7 in HPV+OPC, but in HPV-OPC it is the p16 tumour suppressor part of the pRb tumour suppressor network that is inactivated. Also the pRb pathway is inactivated by E7 instead of  Cyclin D1  amplification. [ 8 ] [ 41 ]   CDKN2A  is a  tumour suppressor gene  that encodes a tumor suppressor protein,  p16  (cyclin-dependent kinase inhibitor 2A) and inhibits the  kinase  activity of the cyclin-dependent kinases  CDK4  and  CDK6 , which in turn induce cell cycle arrest. [ 38 ]  p16 expression is cell cycle-dependent and is expressed focally in only about 5–10% of normal squamous epithelium. Like most HPV+ cancers, HPV+OPC express p16 but the latter does not function as a tumour-suppressor, because the mechanism by which this is achieved, pRb, has been inactivated by E7. p16 is  upregulated  (over-expressed) due to E7-related loss of pRB with reduced negative feedback, [ 39 ] [ 42 ]  whereas it is downregulated in up to 90% of HPV-OPC. [ 43 ]  This diffuse over-expression in the tumour cells provides a diagnostic marker for HPV involvement. [ 44 ] [ 45 ]  Although HPV E6 and E7 reduce tumour suppressor activity, they do so less than genetic and  epigenetic  processes do in HPV-OPC. [ 46 ] [ 47 ] [ 11 ]


The tonsillar epithelia ( palatine  and  lingual ) share similar  nonkeratinization  characteristics with the  cervix , where HPV infection plays the major role in cases of  cervical cancer . [ 14 ] [ 48 ]  Also E6 and E7 may make HPV+OPC more  immunogenic  than HPV-OPC, since anti-E6 and E7  antibodies  may be detected in these patients. This, in turn, could restrict the malignant behaviour of HPV+OPC and the presence of antibodies has been associated with a better prognosis, while treatment may enhance the immunogenicity of the tumour, and hence improve response, although to what extent is not clear. [ 49 ] [ 11 ]  Outcomes are also associated with improved  adaptive immunity . [ 50 ]


Initial diagnosis requires visualisation of the tumour either through the mouth or  endoscopically  through the nose using a  rhinoscope , illustrated to the right, followed by  biopsy . [ citation needed ]


HPV+OPC is usually diagnosed at a more advanced  stage  than HPV-OPC, [ 8 ]  with 75–90% having involvement of regional lymph nodes. [ 51 ]  Furthermore, HPV+OPC is more likely to be poorly differentiated with nonkeratinized or basaloid cells. [ 52 ] [ 53 ]   [ 54 ]


Genetic signatures of HPV+ and HPV- OPC are different. [ 55 ] [ 56 ] [ 57 ] [ 58 ] [ 59 ]  HPV+OPC is associated with expression level of the  E6/E7 mRNAs  and of  p16 . [ 60 ]   HPV16 E6/E7-positive cases are  histopathologically  characterized by their  verrucous  or papillary ( nipple  like) structure and  koilocytosis  of the adjacent mucosa. Approximately 15% of HNSCCs are caused by HPV16 infection and the subsequent constitutive expression of E6 and E7, and some HPV-initiated tumors may lose their original characteristics during  tumor progression . [ 61 ]  High-risk HPV types may be associated with oral carcinoma, by  cell-cycle  control dysregulation, contributing to oral  carcinogenesis  and the overexpression of mdm2, p27 and cathepsin B. [ 62 ]


HPV+OPC is not merely characterized by the presence of HPV-16: only the expression of viral oncogenes within the tumor cells plus the serum presence of E6 or E7 antibodies is unambiguously conclusive for HPV+OPC. [ 14 ]


There is not a standard HPV testing method in head and neck cancers, [ 63 ]  both  in situ hybridization  (ISH) and  polymerase chain reaction  (PCR) are commonly used. [ 44 ] [ 64 ]  Both methods have comparable performance for HPV detection, however, it is important to use appropriate  sensitivity  controls. [ 65 ]   Immunohistochemistry  (IHC)  staining  of the tissue for p16 is frequently used as a cost-effective surrogate for HPV in OPC, compared to ISH or PCR [ 66 ] [ 67 ] [ 68 ]  but there is a small incidence of HPV-negative p16-positive disease accounting for about 5% of HPV-OPC. [ 66 ]


Staging  is generally by the  UICC / AJCC   TNM  (Tumour, Nodes, Metastases) system. [ 68 ]  Staging is based on  clinical examination ,  diagnostic imaging , and pathology. On imaging, involved lymph nodes may appear  cystic , a characteristic of HPV+OPC. [ 69 ]


HPV+OPC has been treated similarly to stage-matched and site-matched HPV-unrelated OPC, but its unique features, which contrast smoking-related HPV-OPC head and neck cancers, for which patients' demographics, comorbidities, risk factors, and carcinogenesis differ markedly, suggest that a distinct staging system be developed to more appropriately represent the severity of the disease and its prognosis. [ 70 ]  Standard AJCC TNM staging, such as the seventh edition (2009) [ 71 ]  while predictive for HPV-OPC has no prognostic value in HPV+OPC. [ 72 ] [ 73 ] [ 67 ] [ 70 ]  The 8th edition of the  AJCC  TNM Staging Manual (2016) [ 74 ]  incorporates this specific staging for HPV+OPC. [ 75 ]  As of 2018, treatment  guidelines  are evolving to account for the different outcomes observed in HPV+OPC. Consequently, less intensive (de-intensification) use of radiotherapy or chemotherapy, [ 76 ]  as well as specific therapy, is under investigation, enrolling HPV+OPC in  clinical trials  to preserve disease control and minimise  morbidity  in selected groups based on modified TNM staging and smoking status. [ 77 ] [ 78 ] [ 79 ] [ 80 ] [ 81 ]


HPV+ cancer of the oropharynx are staged as (AJCC 8th ed. 2016): [ 75 ]
Tumour stage


Nodal stage


Clinical stage


However, the published literature and ongoing clinical trials use the older seventh edition that does not distinguish between HPV+OPC and HPV-OPC – see  Oropharyngeal Cancer – Stages . [ 82 ] [ 83 ]  The T stages are essentially similar between AJCC 7 and AJCC 8. with two exceptions. Tis ( carcinoma in situ ) has been eliminated and the division of T4 into substages (e.g. T4a) has been removed. The major changes are in the N stages, and hence the overall clinical stage. N0 remains the same, but as with the T stage, substages such as N2a have been eliminated. Extracapsular extension (ECE), also referred to as extranodal extension (ENE), which is invasion by the tumour beyond the capsule of the lymph node has been eliminated as a staging criterion. [ a ]


This results in an HPV+OPC tumour being given a lower stage than if it were HPV-OPC. For instance, a 5 cm tumour with one ipsilateral node involved that is 5 cm in size but has ECE would be considered T3N3bM0 Stage IVB if HPV- but T3N1M0 Stage II if HPV+. [ 75 ]


Prevention of HPV+OPC involves avoiding or reducing exposure to risk factors where possible. [ citation needed ]


About 90% of HPV+OPC carry HPV 16, and another 5% type 18. These two types are both targets of available vaccines.  HPV vaccines  given before exposure can prevent persistent genital infection and the consequent precancerous state. [ 11 ]  Therefore, they have a theoretical potential to prevent oral HPV infection. [ 8 ]  A 2010 review study has found that HPV16 oral infection was rare (1.3%) among the 3,977 healthy subjects analyzed. [ 84 ]


The goals of treatment are to optimise survival and locoregional disease control, and prevent spread to distant areas of the body ( metastasis ), while minimising short- and long-term  morbidity . [ 85 ]  There is no high quality  Level I evidence  from prospective clinical trials in HPV+OPC, therefore treatment guidelines must rely on data from treatment of OPC in general and from some retrospective unplanned  subsetting  of those studies, together with data for head and neck cancer in general. [ 68 ]  Treatment for OPC has traditionally relied on  radiotherapy ,  chemotherapy  and/or other systemic treatments, and surgical resection. Depending on the stage and other factors treatment may include a combination of  modalities . [ 86 ]  The mainstay has been radiotherapy in most cases. [ 67 ]  a pooled analysis of published studies suggested comparable disease control between radiation and surgery, but higher complication rates for surgery +/− radiation. [ 86 ] [ 87 ]  Ideally a single modality approach is preferred since triple modality is associated with much more toxicity and a multidisciplinary team in a large centre with high patient volumes is recommended. [ 68 ] [ 88 ] [ 12 ]


Differences in response to treatment between HPV-OPC and HPV+OPC may include differences in the extent and manner in which cellular growth-regulatory pathways are altered in the two forms of OPC. For instance, in HPV+OPC the HPV E6 and E7 oncogenes merely render the p53 and pRb pathways dormant, leaving open the possibility of reactivation of these pathways by  down-regulating  (reducing) expression of the oncogenes. This is in contrast to the mutant form of p53 found in HPV-OPC which is associated with treatment resistance. [ 11 ]  Furthermore, it is suggested that the effects of E6 and E7 on these pathways render the tumour more radiosensitive, possibly by interference with mechanisms such as  DNA repair , repopulation signalling, and cell-cycle redistribution. [ 89 ] [ 90 ]  The microenvironment is also important, with radiation increasing host  immune response  to viral  antigens  expressed on the tumour. [ 50 ] [ 49 ]  Also, there is an association between an increase in  tumour-infiltrating lymphocytes  and in circulating  white blood cells  in HPV+OPC patients and better prognosis. This implies a role for an  adaptive immune system  in suppressing  tumour progression . [ 91 ] [ 92 ] [ 90 ]


Historically, surgery provided a single approach to head and neck cancer. Surgical management of OPC carried significant morbidity with a transcervical (through the neck) approach, often involving mandibulotomy, in which the jawbone ( mandible ) is split. This is referred to as an open surgical technique. Consequently, surgical approaches declined in favour of radiation. In the United States, the use of surgery declined from 41% of cases in 1998 to 30% by 2009, the year that the  Food and Drug Administration  approved the use of the newer techniques. [ 93 ]


These improvements in surgical techniques have allowed many tumours to be  resected  (removed)  by transoral (through the mouth) surgical approaches (TOS), using transoral  endoscopic  head and neck surgery (HNS). [ 94 ]  Consequently, surgery became used more, increasing to 35% of cases by 2012. [ 93 ]  This approach has proven safety, efficacy and tolerability, and includes two main  minimally invasive  techniques,  transoral robotic surgery  (TORS) [ 95 ] [ 96 ] [ 97 ] [ 98 ] [ 99 ] [ 100 ]  and  transoral laser microsurgery  (TLM). [ 101 ] [ 102 ] [ 103 ]  No direct comparisons of these two techniques have been conducted, and clinical trials in head and neck cancer such as ECOG 3311 allow either. They are associated with substantial postoperative morbidity, depending on the extent of resection but compared to older techniques have a shorter hospital stay, faster recovery, less pain, and less need for  gastrostomy  or  tracheostomy , and less long-term effects, which are minimal in the absence of postoperative radiation (RT), or chemoradiation (CRT). [ 104 ] [ 105 ]  TORS has the practical advantage that angled telescopes and rotating robotic surgical arms provide better line of sight. Outcomes of minimally invasive procedures also compare favourably with more invasive ones. In early-stage disease, including involvement of neck nodes, TORS produces a 2-year survival of 80–90%. [ 106 ]  TLM similarly, is reported to have a five-year survival of 78% and local control rates of 85–97%. [ 107 ] [ 108 ]  In addition to early disease, minimally invasive surgery has been used in advanced cases, with up to 90% local control and disease-specific survival. [ 95 ] [ 108 ]  Postoperative swallowing was excellent in 87%, but long-term dysphagia was associated with larger (T4) cancers, especially if involving the base of the tongue. [ 108 ]   [ 12 ]


The details of the surgical approach depend on the location and size of the primary tumour and its N stage.  Neck dissection  to examine the draining lymph nodes may be carried out simultaneously or as a second staging procedure. For tumours of the tonsil and lateral pharyngeal wall, and clinically node-negative (N0) disease, dissection of the neck typically involves levels 2–4 ( see  diagram in Dubner 2017 )  ipsilaterally . Where nodes are involved clinically, dissection will depend on the location and size of the node or nodes. In the case of tongue base primaries, close to the  midline , bilateral dissection is recommended. [ 12 ]


An advantage of a primary surgical approach is the amount of  pathological  information made available, including grade,  margin status , and degree of involvement of lymph nodes. This may change the staging, as up to 40% of patients may have a different postoperative pathological stage compared to their preoperative clinical stage. In one study, 24% had their stage reduced (downstaged), which may impact subsequent decision making, including a reduction in intensity and morbidity. [ 109 ] [ 12 ]  In the United Kingdom, the  Royal College of Pathologists  (1998) [ 110 ] [ b ]  has standardised the reporting of surgical margins, with two categories, "mucosal" and "deep", and for each created groups based on the microscopic distance from invasive cancer to the margin, as follows: more than 5 mm (clear), 1–5 mm (close) and less than 1 mm (involved). [ 111 ]


Data on the use of postoperative  radiation therapy  (PORT) is largely confined to historical or retrospective studies rather than high quality  randomized clinical trials  and are based on the overall population of patients with head and neck cancer, rather than specific studies of HPV+OPC, which would have formed a very small proportion of the population studied. [ 12 ]  Despite surgical excision, in the more advanced cases local and regional recurrence of the cancer, together with spread outside of the head and neck region ( metastases ) are frequent. The risk of subsequent recurrent disease has been considered highest in those tumours where the pathology shows tumour at the margins of the resection (positive margins), multiple involved regional lymph nodes, and extension of the tumour outside of the capsule of the lymph node (extracapsular extension), based on historical experience with head and neck cancer. [ 112 ]  PORT was introduced in the 1950s in an attempt to reduce treatment failure from surgery alone. [ 113 ]  Although never tested in a controlled setting, PORT has been widely adopted for this purpose. [ 114 ]  In an analysis of surgical treatment failure at  Memorial Sloan-Kettering Cancer Center , patients treated with surgery alone between 1960 and 1970 had failure rates of 39 and 73% for those with negative and positive surgical margins respectively. These were compared to those who received PORT (with or without chemotherapy) from 1975 to 1980. The latter group had lower failure rates of 2% and 11% respectively. [ 115 ]  In addition, one randomised study from the 1970s (RTOG 73–03) compared preoperative radiation to PORT, and found lower failure rates with the latter. [ 114 ] [ 116 ]


The addition of another modality of treatment is referred to as  adjuvant  (literally helping) therapy, compared to its use as the initial (primary) therapy, also referred to as radical therapy. Consequently, many of these patients have been treated with adjuvant radiation, with or without chemotherapy. In the above series of reports of minimally invasive surgery, many (30–80%) patients received adjuvant radiation. However, functional outcomes were worse if radiation was added to surgery and even worse if both radiation and chemotherapy were used. [ 12 ]  Radiation dosage has largely followed that derived for all head and neck cancers, in this setting, based on risk. Historically only one randomised clinical trial has addressed optimal dosage, allocated patients to two dosage levels, stratified by risk, but showed no difference in cancer control between the low and high doses (63 and 68.4 Gy), but a higher incidence of complications at the higher doses. Consequently, the lower dose of 57.6  Gy  was recommended. [ 117 ] [ 118 ]  Because the authors used a fractionation scheme of 1.8 Gy per treatment, this dosage was not widely adopted, practitioners preferring a larger fraction of 2 Gy to produce a shorter treatment time, and a slightly higher dose of 60 Gy in 2 Gy fractions (30 daily treatments). [ 41 ]  Yet 57.6 Gy in 1.8 Gy fractions is equivalent (iso-effective dose) to only 56 Gy in 2 Gy fractions. [ 119 ]  60 Gy corresponds to the 63 Gy used as the low dose in the high-risk group. 60 Gy was also the dose used in RTOG 73–03. Subsequently, there was a tendency to intensify treatment in head and neck cancer, and many centres adopted a dose of 66 Gy, at least for those patients with adverse tumour features. [ 120 ]  The effectiveness of PORT in HPV+OPC receives some support from a  cohort study  (Level 2b), although the number of patients was low, and the number of events (recurrent disease or death) only 7%. [ 121 ]  Another retrospective population-level study (Level 4) of the  SEER  database (1998–2011) concluded that there was an overall survival but not disease-specific survival effect of radiation in 410 patients with a single lymph node involved, but used only  univariate  statistical analysis and contained no information on HPV status. [ 122 ]  A subsequent much larger study on a similar population in the National Cancer Database (2004–2013) of over 9,000 patients found a survival advantage but this was only in HPV-OPC, not in 410 HPV+OPC patients, [ 123 ]  and a subsequent study of 2,500 low and intermediate risk HPV+OPC patients showed similar overall survival whether PORT was given or not. [ 124 ]


While less studies have been completed examining deintensification (de-escalation) in this setting, than in primary radical radiation for this cancer (see below), it is an area of active investigation. [ 125 ]  In one single institution study, a decision was made to reduce the radiation dose in high-risk patients with HPV+OPC from 66 to 60 Gy, corresponding to the actual evidence, and follow-up has shown no decrease in cancer control. [ 120 ]  Current trials, both in North America and Europe (such as ECOG 3311 [ c ]  and PATHOS [ d ] ) use 50 Gy as the comparison arm. [ 127 ]  The comparator of 50 Gy was chosen on the grounds of (i) the exquisite sensitivity of HPV+OPC to radiation, both  in vitro  and  in vivo ; ECOG 1308 showing excellent disease control at 54 Gy; and data [ 128 ]  suggesting that 50 Gy in 1.43 Gy (iso-effective dose 43 Gy in 2.0 Gy) was sufficient to electively treat the neck. [ 126 ]  Other studies, such as MC1273 and DART-HPV have evaluated doses as low as 30–36 Gy. [ 129 ]  Lowering the radiation dose to 54 Gy was identified as one of the important Clinical Cancer Advances of 2018 by the
 American Society of Clinical Oncology , under the general theme of "Less Is More: Preserving Quality of Life With Less
Treatment". [ 130 ]
Chemotherapy has been used concurrently with radiation in this setting, as in primary treatment with radical radiation, particularly where pathological features indicated a higher risk of cancer recurrence. Several studies have suggested that this does not improve local control, but adds toxicity. [ 131 ]


Concerns over the morbidity associated with traditional open surgical en-bloc resection led to exploring alternative approaches using radiation. [ 121 ]  Intensity-modulated radiation therapy ( IMRT ) can provide good control of primary tumours while preserving excellent control rates, with reduced toxicity to salivary and pharyngeal structures relative to earlier technology. HPV+OPC has shown increased sensitivity to radiation with more rapid regression, compared to HPV-OPC. [ 132 ]  Generally, radiation can safely be delivered to the involved side alone (ipsilateral), due to the low rate of recurrent cancer on the opposite side (contralateral), and significantly less toxicity compared to bilateral treatment. [ e ] [ 134 ] [ 133 ]  IMRT has a two-year disease-free survival between 82 and 90%, and a two-year disease-specific survival up to 97% for stage I and II. [ 135 ] [ 136 ]


Reported  toxicities  include dry mouth ( xerostomia ) from  salivary gland  damage, 18% (grade 2); [ f ]  difficulty swallowing ( dysphagia ) from damage to the constrictor muscles, larynx and oesophageal sphincter, 15% (grade 2);  subclinical   aspiration  up to 50% (reported incidence of aspiration pneumonia approximately 14%);  hypothyroidism  28–38% at three years (may be up to 55% depending on amount of the thyroid gland exposed to over 45  Gy  radiation;  esophageal stenosis  5%;  osteonecrosis  of the  mandible  2.5%; and need for a  gastrostomy  tube to be placed at some point during or up to one year after treatment 4% (up to 16% with longer follow up). [ 12 ] [ 138 ] [ 136 ] [ 139 ] [ 140 ]  Concerns have been expressed regarding excessive short- and long-term toxicity, especially dysphagia, and xerostomia, [ 141 ] [ 142 ] [ 143 ]  and hence whether standard doses expose patients with better prognoses are being exposed to overtreatment and unnecessary side effects. [ 144 ] [ 90 ]


The probability of xerostomia at one year increases by 5% for every 1Gy increase in dose to the  parotid gland . Doses above 25–30 Gy are associated with moderate to severe xerostomia. Similar considerations apply to the  submandibular gland , but xerostomia is less common if only one parotid gland is included in the radiated field [ 145 ]  and the contralateral submandibular gland is spared (less than 39 Gy) [ 146 ]  In the same manner, the radiation dose to the  pharyngeal constrictor muscles ,  larynx , and  cricopharyngeal inlet  determine the risk of dysphagia (and hence the dependence on  gastrostomy  tube feeds). The threshold for this toxicity is volume-dependent at 55–60 Gy, [ 147 ] [ 148 ] [ 149 ] [ 90 ]  with moderate to severe impairment of swallowing, including aspiration, stricture and feeding tube dependence above a mean dose of 47 Gy, with a recommended dose to the inferior constrictor of less than 41 Gy. [ 150 ] [ 151 ]  Dose-toxicity relationships for the superior and middle constrictors are steep, with a 20% increase in the probability of dysphagia for each 10 Gy. [ 152 ]  For late dysphagia, threshold mean total constrictor doses, to limit rates of greater than or equal to grade 2 and 3 below 5% were 58 and 61 Gy respectively. For grade 2 dysphagia, the rate increased by 3.4% per Gy. [ 153 ]  Doses above 30 Gy to the thyroid are associated with moderate to severe hypothyroidism. [ 154 ]  Subjective, patient-reported outcomes of  quality of life  also correlate with radiation dose received. [ 142 ]


Altered  fractionation  schemes, such as RTOG 9003   [ g ] [ 141 ]  and RTOG 0129 [ h ]  have not conferred additional benefit. [ 155 ] [ 156 ]  Radiation dose recommendations were largely determined  empirically  in clinical studies with few HPV+OPC patients, and have remained unchanged for half a century, [ 90 ]  making it difficult to determine the optimum dose for this subgroup. A common approach uses 70 Gy bilaterally and anteriorly, such as RTOG 9003 (1991–1997) [ 141 ] [ 155 ]  and RTOG 0129 (2002–2005). [ 157 ] [ 156 ]  For lateralized tonsil cancer unilateral neck radiation is usually prescribed, but for tongue base primaries bilateral neck radiation is more common, but unilateral radiation may be used where tongue base lesions are lateralised. [ 12 ]


Concerns have been expressed regarding excessive short- and long-term toxicity, especially dysphagia and xerostomia, [ 141 ] [ 142 ] [ 143 ]  and hence whether standard doses expose patients with better prognoses to overtreatment and unnecessary side effects. [ 144 ] [ 90 ]  Current toxicities have been described as "not tolerable", [ 158 ]  and hence an intense interest in de-escalation. [ 127 ]


While comparison with historical controls has limited value compared to randomised clinical trials ( phase III ),  phase II  studies using reduced doses of radiation compared to the historical standard of 70 Gy have been carried out. A study using 54–60 Gy (a 15–20% reduction, stratified by response to initial induction chemotherapy) demonstrated comparable levels of disease control with much lower complication rates, [ 90 ]  when compared to similar studies, using 70 Gy, such as ECOG 2399. [ 159 ] [ 160 ]  The percentage of patients alive after 2 years was 95% at the higher dose and 98% at the lower dose. Similarly for the percentage free of disease (86 and 92%). Toxicities were greatly reduced from an incidence of grade 3 or greater dysphagia and mucositis of 54 and 53% respectively, to 9%. A lower incidence and severity of dysphagia also means that fewer patients require gastrostomy feeding. [ 90 ]  A similar comparison can be made with the pooled data from two RTOG studies which utilized 70 Gy (0129 and 0522). [ 161 ]


No new guidelines dealing specifically with HPV+OPC have yet been developed, outside of clinical trials. Indirect data suggests the efficacy of less intense treatment. A retrospective analysis of advanced (N+) HPV+OPC suggested 96% 5-year local control with de-intensified radiation of 54 Gy and concurrent  cisplatin  based chemotherapy. [ 162 ]  The conclusions of the above pair of similar phase II trials have been supported by several other phase II trials. A prospective trial ( ECOG  1308) demonstrated similar locoregional control with 54 Gy, [ 144 ]  and another study, a high pathological complete response rate at 60 Gy. [ 163 ]  The Quarterback trial [ i ]  showed comparable outcomes between 56 and 70 Gy. [ 164 ]  and was followed by Quarterback 2, comparing 50 to 56 Gy. [ j ]  Similarly, the Optima trial showed good disease control with doses between 45 and 50 Gy. [ 165 ]  Ongoing studies, following the experience of the  Mayo Clinic  trial (MC1273), [ 129 ]  such as that the  Memorial Sloan Kettering Cancer Center  are exploring doses as low as 30Gy. [ k ]  These studies all used well below the previous standard dose of 70 Gy. Since long-term toxicity is associated with radiation dose, determining the efficacy of lower and hence less morbid doses of radiation is a priority, since many HPV+ patients can be expected to have long-term survival. [ 12 ]


Radiation is commonly utilised in combination with chemotherapy but also may be used as a single modality, especially in earlier stages, e.g. T1-T2, N0-1, and its use in later stages is being explored in clinical trials such as RTOG 1333 which compares radiation alone to radiation with reduced chemotherapy, in non or light smokers. [ 12 ]


As with the radiotherapy data, most of the available knowledge on the efficacy of chemotherapy derives from the treatment of advanced head and neck cancer rather than specific studies of HPV+OPC. Since 1976, many clinical studies have compared CRT to RT alone in the primary management of locally advanced head and neck cancers and have demonstrated an advantage to CRT in both survival and locoregional control. [ 166 ] [ 167 ]  Cisplatin is considered the standard agent, and a survival advantage was seen for those patients who received radiation with concurrent cisplatin. [ 168 ]  Despite this no trials directly comparing cisplatin with other agents in this context have been conducted. The other agent that is widely used is  Cetuximab , a  monoclonal antibody  directed at the  epidermal growth factor receptor  (EGFR). A 10% survival advantage at three years was noted when cetuximab was given concurrently with radiation (bioradiation). [ 169 ]  Cetuximab trials were completed before knowledge of HPV status. [ 170 ]  Laboratory and clinical studies on the utility of cetuximab in this context are conflicting.  The main toxicity is an  acneiform  rash, but it had not been compared directly to cisplatin in HPV+OPC, until RTOG 1016 ( see Talk ) addressed this question. [ 12 ] [ 164 ]  Analysis of the results three years after the trial was completed demonstrates that cetuximab is inferior to cisplatin. [ 171 ]  Concurrent chemotherapy is also superior to chemotherapy alone ( induction chemotherapy ) followed by radiation. [ 166 ] [ 12 ]  Cetuximab shows no advantage when added to cisplatin in combination with radiation. [ 143 ]  Although chemoradiation became a treatment standard based on clinical trials and in particular,  meta-analyses , a subsequent  population-based study  of patients with OPC, indicated no advantage to the addition of chemotherapy to radiation in either HPV+OPC or HPV-OPC, [ 172 ]  and significant concerns about added toxicity. [ 173 ]


Chemotherapy also has a role, combined with radiation, in the postoperative setting (adjuvant therapy). [ 174 ]  Generally it is used where the  pathology  of the resected specimen indicates features associated with a high risk of locoregional recurrence (e.g. extracapsular extension through involved lymph nodes or very close margins). It has shown improved disease-free survival and locoregional control in two very similar clinical trials in such high-risk patients,  EORTC  22931 (1994–2000) [ 112 ]  and RTOG 9501 (1995–2000). [ l ] [ m ] [ n ] [ 175 ] [ 176 ] [ 177 ]  However, for HPV+OPC patients, such extracapsular spread does not appear to be an adverse factor [ 178 ] [ 179 ] [ 180 ]  and the addition of chemotherapy to radiation in this group provided no further advantage. [ 179 ]  Since the  sample size  to detect a survival advantage is large, given the small number of events in this group, these studies may have been  underpowered  and the question of the utility of adding chemotherapy is being addressed in a randomized clinical trial (ADEPT) with two-year locoregional control and disease-free survival as the endpoint. [ o ]  The addition of chemotherapy to radiation increases acute and late toxicity. In the GORTEC trial, chemotherapy with  docetaxel  provided improved survival and locoregional control in locally advanced OPC but was associated with increased mucositis and need for feeding by gastrostomy. [ 181 ]  Chemotherapy and radiation are associated with a risk of death of 3–4% in this context. [ 182 ]  It is unclear whether the added toxicity of adding chemotherapy to radiation is offset by significant clinical benefits in disease control and survival. [ 12 ]


It is thought that HPV+OPC patients benefit better from radiotherapy and concurrent  cetuximab  treatment than HPV-OPC patients receiving the same treatment, [ 183 ]  and that radiation and cisplatin induce an immune response against an  antigenic  tumour which enhances their effect on the cancer cells. [ 49 ]  Although the incidence of HPV positivity is low (10–20%), an advantage for HPV+OPC was seen in trials of both cetuximab and  panitumumab , a similar anti-EGFR agent, but not a consistent interaction with treatment, although HPV+OPC appears not to benefit to the same extent as HPV-OPC to second line anti-EGFR therapy, possibly due to lower EGFR expression in HPV+OPC. [ 170 ]


In the absence of high-quality evidence comparing a primary surgical approach to other modalities, decisions are based on consideration of factors such as adequate surgical exposure and anatomically favourable features for adequate resection, post-treatment function, and  quality of life . Such patient selection may enable them to avoid the morbidity of additional adjuvant treatment. In the absence of favourable surgical features, the primary treatment of choice remains radiation with or without chemotherapy. Tumor characteristics that favour a non-surgical approach include invasion of the base of the tongue to the extent of requiring resection of 50% or more of the tongue, pterygoid muscle involvement, extension into the  parapharyngeal fat  abutting the  carotid , involvement of the mandible or  maxilla  or invasion of the  prevertebral space . [ 12 ]


The adequacy of surgical  resection  is a major factor in determining the role of postoperative adjuvant therapy. In the presence of a  positive margin  on pathological examination, most radiation oncologists recommend radiation to the primary site, and concurrent chemotherapy. A negative margin is more likely to be treated with lower doses and a smaller treatment volume. Also, the removal of a bulky tumour may allow reduced dosage to adjacent uninvolved pharyngeal structures and hence less effect on normal  swallowing . [ 76 ] [ 12 ]


The cancer outcomes (local control, regional control, and survival) for transoral resection followed by adjuvant therapy are comparable to primary chemoradiation, [ 102 ] [ 98 ] [ 139 ]  so that treatment decisions depend more on treatment-related morbidity, functional outcome, and quality of life. Patient factors also need to be taken into account, including general baseline functionality, smoking history, anesthesia risk, oropharyngeal function, swallowing and airway protection, and potential for rehabilitation. Patient preference is equally important. Many clinical trials are underway focussing on deintensification, often with risk  stratification , e.g. Low, Intermediate and High Risk ( see Fundakowski and Lango, Table I ). [ 12 ] [ p ]


Clinical decisions also take into account morbidities, particularly if cancer outcomes are comparable for instance surgery is associated with a risk of bleeding between 5–10%, and a 0.3% risk of fatal postoperative haemorrhage. [ 103 ] [ 184 ] [ 99 ] [ 100 ]  Surgery may also be complicated by  dysphagia , and while most patients can tolerate a diet on the first postoperative day, long-term use of a feeding tube has been reported as high as 10%. [ 108 ] [ 99 ] [ 100 ]  Patients with larger tumours, involvement of base of the tongue and requiring postoperative adjuvant therapy are more likely to require a long-term feeding tube. [ 185 ] [ 186 ]  Overall, function and quality of life appear relatively similar between surgery with postoperative radiation, and primary chemoradiation, [ 187 ] [ 188 ] [ 12 ]  but HPV+OPC patients tend to have a better quality of life at diagnosis than HPV-OPC but may sustain greater loss following treatment. [ 189 ]


Anatomical considerations may also dictate a preference for surgical or non-surgical approaches. For instance  trismus , a bulky tongue, limited extension of the neck, prominent teeth,  torus mandibularis  (a bony growth on the mandible), or limited width of the mandible would all be relative contraindications to surgery. [ 101 ]  Tumour-related considerations include invasion of the mandible,  base of the skull , and extensive involvement of the larynx or more than half of the base of the tongue. [ 102 ]  Technical considerations in offering surgery as a primary modality include the presumed ability to achieve adequate margins in the resected specimen and the degree of resulting defect, since close or positive margins are likely to result in subsequent adjuvant therapy to achieve disease control, with resultant increased morbidity. Costs are difficult to estimate but one US study, based on estimates of 25% of all OPC patients receiving surgery alone and 75% surgery followed by adjuvant therapy, using the criteria of the  NCCN , found that this approach was less expensive than primary chemoradiation. [ 190 ] [ 191 ] [ 192 ]


Early stage disease [ q ]  is associated with a relatively favourable outcome, for which single modality therapy is recommended, the choice depending on tumour location and accessibility. For instance, unilateral tonsil or tongue base tumours will generally be treated with transoral resection and selective ipsilateral neck dissection. On the other hand, a large midline tongue lesion would require bilateral neck dissection, but in the absence of what are considered adverse pathology (positive margins, extracapsular extension) will likely be treated by surgery alone or radiation including ipsilateral or bilateral neck radiation fields, with surgery for those instances where the likelihood of adjuvant therapy is low. [ 12 ]