Comprehensive Next Generation Sequencing Reveals that Purported Primary Squamous Cell Carcinomas of the Parotid Gland are Genetically Heterogeneous

Squamous cell carcinoma (SCC) is one of the most common malignancies involving the parotid gland, but it has been recognized that the vast majority of parotid SCC represents metastases, especially from the ipsilateral facial skin. Bona fide primary SCC of the parotid is so rare that it is unclear whether it truly exists at all. We sought to molecularly characterize cases diagnosed as primary parotid gland SCC to see if they possess a unique genetic makeup. We identified cases in our archives which had been diagnosed as primary SCC of the parotid gland. In all cases, metastatic disease was excluded by a thorough history and physical examination. Cases with histologic evidence of a precursor neoplasm (e.g., carcinoma ex-pleomorphic adenoma) were also excluded. Targeted next-generation sequencing (NGS) was attempted on all cases. Six cases diagnosed as primary parotid SCC were identified, arising in 4 males and 2 females ranging from 8 to 73 years (mean, 51.8 years). All cases exhibited keratinization and unequivocal invasion. Four of 6 appeared to be arising from cystically dilated ducts. Five of 6 exhibited well-developed cellular atypia; the remaining case, while cytologically bland, demonstrated perineural invasion. Targeted NGS was successful in 5 of 6 cases. Two SCC harbored several mutations in a mutational profile reminiscent of SCCs seen in other organs. One case harbored YAP1::MAML2, a fusion previously reported in porocarcinoma and other neoplasms. One case harbored IRF2BP2::RUNX2, and presumably represents keratocystoma or SCC ex-keratocystoma. Finally, one case an increase of C > T mutations consistent with ultraviolet damage, suggesting that this case represented a cryptic metastasis from cutaneous SCC. Our analysis did not confirm a unifying genetic signature for purported primary parotid SCC. Indeed, our findings suggest that true primary parotid gland SCC is even rarer than already believed. In our 5 cases with results, NGS findings demonstrated that one was likely a keratocystoma, one a cryptic metastasis from a cutaneous SCC, and one a porocarcinoma, either metastatic or primary. The two remaining cases had complex genotypes reminiscent of SCCs from other sites. This may be the signature of genuine parotid primary SCC, but metastasis from an SCC from another organ cannot be excluded. Accordingly, a diagnosis of primary parotid gland SCC should be viewed with skepticism.