Squamoglandular Variant of Acinic Cell Carcinoma: A Case Report of a Novel Variant.

While salivary gland tumors have considerable plasticity, juxtaposition of the morphologies of two named tumor types is rare. Tumors with both mucoepidermoid and serous acinar components, dubbed "mucoacinar" carcinomas were recently characterized, and based on morphologic and molecular features, considered variants of mucoepidermoid carcinoma. Here we describe a unique case of a 59-year-old male with a 0.9 cm right parotid mass with a similar blend of mucoepidermoid-like and acinar elements that instead has a molecular phenotype of acinic cell carcinoma, essentially the reverse of mucoacinar carcinoma. The tumor was fairly well circumscribed with a prominent tumor associated lymphoid response. It consisted of a predominant bland but basaloid squamoid proliferation with scattered pockets of serous acinar differentiation as well as rare mucous cells and tubules. The tumor showed diffuse cytokeratin and DOG1 reactivity as well as p40 expression in the squamoid components. Immunostaining for NR4A3 was diffusely positive, and an NR4A3 rearrangement was noted on fluorescence in situ hybridization, while testing for MAML2 and MSANTD3 rearrangements were negative. Based on these findings, this tumor is best considered a "squamoglandular variant of acinic cell carcinoma." Morphologic and clinical evidence argues against this representing a form of high-grade transformation. While overall bland, the differential diagnosis may include various basaloid tumors in the parotid gland, both primary and metastatic.