The mucosal epithelium of the upper aerodigestive tract is the target of several, frequently concurrent carcinogenic events that contribute to the development of HNSCC. Exposure to tobacco-derived carcinogens, long-term alcohol use, and infection with oncogenic strains of the HPV, mainly HPV-16 and, less frequently, HPV-18, are the three main etiological pathways.[6] With a risk of tumor growth that is more than 35 times higher for tobacco users than non-users, tobacco use continues to be the most important modifiable risk factor for HPV-negative HNSCC. A complex combination of nitrosamines, including 4-(methylnitrosamine)-1-(3-pyridyl)-1-butanone, Polycyclic Aromatic Hydrocarbons (PAHs), and other mutagens mediate tobacco’s carcinogenic effects.[7] When these substances are metabolically activated, DNA adducts are created. If these adducts are not adequately repaired, they can cause point mutations and chromosomal instability. In tobacco-associated cancers, mutations in tumor suppressor genes including TP53 and CDKN2A are very common.[8]

Alcohol causes cancer in both independent and synergistic ways. It functions as a solvent for carcinogens produced from tobacco and increases mucosal permeability.[9] Additionally, acetaldehyde, a highly reactive substance that generates DNA adducts and worsens genomic instability, is produced as a result of alcohol metabolism.[10] Chronic alcohol consumption is also linked to increased oxidative stress and immune surveillance suppression, both of which aid in the development and spread of tumors.[11]

Unlike alcohol- and tobacco-related HNSCC, which is caused by mutagens, HPV-positive tumors, primarily oropharyngeal carcinomas, are caused by the integration of viral DNA into the host genome.[12] HPV integration in HNC does not occur at a single site but at multiple splice breakpoints; studies on HNC cell lines suggest that these breakpoints are associated with genomic instability, an important feature of virus-induced carcinogenesis. Integration further promotes cell transformation by the subsequent production of viral oncoproteins E6 and E7.[13] Both p53 and RB1 are functionally inactivated by these viral proteins, which promote unchecked growth and genomic instability. While HPV-positive HNSCCs are linked to a better prognosis, their rising prevalence highlights how crucial it is to comprehend virus-mediated carcinogenesis.

Field cancerization, a word originally used to describe the common occurrence of synchronous and metachronous cancers, is a unifying principle that underlies multifocal carcinogenesis in the head and neck area. The existence of genetically modified but histologically normal-looking epithelium broughtonby prolonged exposure to carcinogens is known as “field cancerization”.[14] Even in clinically unaffected mucosa, molecular investigations have shown that these preconditioned regions frequently contain early mutations in TP53, CDKN2A, epigenetic silencing of tumor suppressor sites, and abnormal methylation patterns. Molecular evidence of clonal linkages between anatomically different cancers and between tumors and nearby dysplastic epithelium supports this idea. Multiple primary cancers with similar or different genetic profiles can form more easily when mutant epithelial progenitors clonally expand, frequently in the presence of compromised DNA repair. According to the hypothesis of clonal expansion, genetically altered epithelial cells of the basal layer begin to grow in a horizontal direction, along the basal and parabasal layers, forming at first a focus (patch) and later expanding into a field, which will eventually mutate focally into cancer.[13] This process is further accelerated by chronic inflammation, especially in tissues exposed to alcohol and tobacco, which produces growth hormones, cytokines, and reactive oxygen species that promote immune evasion, stemness, and mutagenesis. The high occurrence of many primary tumors in HNSCC can be explained mechanistically by these interconnected carcinogenic pathways and the biology of field cancerization. They also support the clinical necessity of genetic surveillance and individualized risk assessment.