Pleomorphic Adenoma With Cystic Change, Extensive Squamous Metaplasia and Positive Block-Like p16 Staining Pattern: A Sheep In Wolf’s Clothing

Abstract

Abstract Introduction/Objective Pleomorphic adenoma, the most common benign salivary gland neoplasm may show squamous metaplasia in up to 25% of cases, and less frequent cystic change. We report an unusual case of pleomorphic adenoma with cystic change, extensive squamous metaplasia and block-like p16 staining that was misdiagnosed as metastatic p16-positive squamous cell carcinoma in a limited core biopsy. Methods/Case Report A 78-year-old female with no history of malignancy presented with a right neck painless mass. PET-CT demonstrated a hypermetabolic ovoid mass adjacent to the right submandibular gland measuring 1.7 cm concerning for metastatic disease within a lymph node. Fine needle aspiration of lesion was hypocellular and showed cystic contents and rare atypical squamoid cells. Core biopsy of the lesion demonstrated sheets of squamoid cells with mild to moderate cytologic atypia in a hyalinized fibrotic stroma. Immunostain for p16 showed cytoplasmic and nuclear block-staining pattern. The biopsy was diagnosed as metastatic p16-positive squamous cell carcinoma. Subsequent human papilloma virus (HPV) in situ hybridization for low- and high-risk types was negative. The patient underwent right level 1B neck dissection revealing a pleomorphic adenoma arising at the periphery of the submandibular gland with cystic change and extensive central squamous metaplasia. Repeated p16 staining showed diffuse positivity in the area with squamous metaplasia and heterogenous staining in the non-metaplastic areas. Cobas HPV-PCR test covering 14 high-risk HPV subtypes was also negative. Results (if a Case Study enter NA) NA Conclusion We describe the unusual coexistence of extensive squamous metaplasia, cystic change and aberrant p16 staining pattern in an exophytic pleomorphic adenoma to raise awareness of how these changes individually, and more so in combination can mimic squamous cell carcinoma. The mechanism of p16 overexpression in metaplastic PAs appears to be independent of HPV infection.