Not All p16-Positive Head and Neck Carcinomas are HPV-related: A Correlation Between p16 Immunohistochemistry and HPV Testing

<title>Abstract</title> <p> <bold>Background:</bold> High-risk human papillomavirus (HR-HPV) is an important etiologic factor in head and neck squamous cell carcinoma (HNSCC), particularly in the oropharynx, where HPV-related tumors are associated with improved prognosis and treatment response. Although p16 immunohistochemistry (IHC) is widely used as a surrogate marker for HR-HPV infection, p16 expression does not always correlate with HPV status. We investigated the clinicopathologic features of p16-positive/HPV-negative head and neck carcinomas identified in our institutional practice. <bold>Design:</bold> Fifty-four malignant head and neck tumors with available p16 IHC and high-risk HPV <italic>in situ</italic> hybridization (HPV-ISH) results were retrospectively reviewed. Cases with discordant p16 IHC and HPV-ISH results, as well as p16 IHC-negative/HPV-ISH-negative cases, underwent re-review and rescoring of p16 staining. Based on p16 IHC and HPV-ISH results, tumors were classified into four groups: Group 1, p16 IHC-positive/HPV-ISH-positive; Group 2, p16 IHC-variable/HPV-ISH-negative; Group 3, p16 IHC-negative/HPV-ISH-negative; and Group 4, p16 IHC-negative/HPV-ISH-positive. <bold>Results:</bold> The cohort included 47 primary and 7 metastatic carcinomas, comprising 37 biopsy and 17 excision/resection specimens. The mean patient age was 62 years (range, 30–83 years), and 46 patients (85%) were male. Twenty-six tumors were concordantly p16 IHC-positive/HPV-ISH-positive, including 23 SCCs and three non-SCCs. Eighteen tumors demonstrated p16 IHC-variable/HPV-ISH-negative results and were further stratified according to the extent of p16 expression: Group 2A (≥70%, n=4), Group 2B (≥50% to &lt;70%, n=4), and Group 2C (&lt;50%, n=10). Eight tumors were p16 IHC-negative/HPV-ISH-negative, whereas two tumors were p16 IHC-negative/HPV-ISH-positive. Notably, all four tumors with positive p16 expression (≥70% of tumor cells showing moderate-to-strong nuclear and cytoplasmic staining) were HPV-ISH-negative, including a polymorphous adenocarcinoma (PAC) of the base of tongue and one SCC arising in the soft palate. Detailed demographic characteristics and clinicopathologic findings are summarized in Tables 1 and 2, respectively. <bold>Conclusions:</bold> p16 immunoreactivity should be interpreted in conjunction with tumor site, histomorphology, and clinical findings, as p16 positivity alone does not establish HPV-driven disease or fulfill the diagnostic criteria for HPV-related SCC. 2. In our cohort, p16 IHC demonstrated high sensitivity (92.9%) but only moderate specificity (84.6%) for identifying HPV-related head and neck carcinomas. </p>