Answer: Acinic cell carcinoma

Histology: The parotid and extraparotid soft tissues are infiltrative by a solid cellular proliferation. The tumor architecture varies from small uniform acini to microcysts to solid sheets. Cellular necrosis is present. The tumor cells are pleomorphic, and mitotic activity is high. In areas, the cytoplasm is finely granular. These purplish granules are PAS positive and diastase resistant.

Discussion: Acinic cell carcinoma is a tumor that demonstrates serous acinar differentiation. Indeed, the finding of acinar differentiation (as supported by the PAS/D stain) would exclude the diagnosis of adenocarcinoma, NOS. Not surprisingly, the majority of these tumors arise from the serous dominant parotid gland. The morphologic features of acinic cell carcinoma are incredibly diverse. Five different cell types are recognized: the acinic cell, the vacuolated cell, the clear cell, the intercalated ductal cell, and the non-specific glandular cell. Serous differentiation is best expressed in the acinar cell. These cells have abundant purple granules in their cytoplasm that correspond to the zymogen granules seen ultrastructurally.

By definition, acinic cell carcinomas are regarded as low grade carcinomas. In effect, subclassification of these tumors by cell type or architectural pattern of growth is not the least bit useful as these features have no bearing on prognosis. There are exceptional cases of high grade acinic carcinomas characterized by geographic necrosis, high mitotic activity and pleomorphism. These tumors are referred to as dedifferentiated or undifferentiated acinic cell carcinoma, but the term is somewhat of a misnomer in those tumors like the one presented here that retain serous acinar differentiation. These dedifferentiated areas portend a much more aggressive clinical course, and acinic cell carcinomas that harbor such areas should be treated in a more aggressive fashion.