p16 Immunoexpression in sinonasal and nasopharyngeal adenoid cystic carcinomas: a potential pitfall in ruling out HPV‐related multiphenotypic sinonasal carcinoma

Aims Adenoid cystic carcinoma (AdCC) is frequent in the sinonasal region. The recently described human papilloma virus (HPV)‐related multiphenotypic sinonasal carcinoma (HMSC) histopathologically resembles solid AdCC, but has a better outcome. Thus, clinical and pathogenetic differences between HMSC and sinonasal AdCC necessitate their distinction. We conducted this study to assess p16 immunoexpression in previously diagnosed AdCC cases, and to identify HMSC cases in p16 immunopositive cases. Methods and results Cases diagnosed as sinonasal and nasopharyngeal AdCC were retrieved. Histomorphological features were reviewed. Immunohistochemistry (IHC) for p16 was performed. HPV testing was performed in p16‐positive cases by mRNA in‐situ hybridisation (mRNA ISH) and polymerase chain reaction (PCR) assay. MYB rearrangement was assessed by fluorescence in‐situ hybridisation. One hundred and two AdCC cases were retrieved. Six cases (5.9%) showed diffuse p16 positivity. HPV mRNA ISH and PCR were negative in p16‐positive cases. Two cases showed MYB rearrangement. p16‐positive cases were composed of basaloid cells demonstrating a cribriform pattern, at least focally. The predominant pattern was cribriform in three and solid in three cases. One case showed two distinct components: keratinising squamous cell carcinoma and cribriform AdCC. Other morphological patterns seen were tubular, reticular, epithelial‐myoepithelial carcinoma‐like, and glomeruloid, forming a minor component of the tumour area. Conclusions p16 staining alone, even when diffuse and strong, cannot be used as a surrogate for HPV testing to distinguish sinonasal AdCC from HMSC. p16 IHC should be accompanied by more specific methods, such as mRNA ISH, so as not to erroneously diagnose HMSC over sinonasal AdCC, bearing in mind the highly aggressive nature of the latter.