Answer: Acinic cell adenocarcinoma

Histology: The tumor is arising in the serous-dominant parotid gland. It is sharply circumscribed, partially encapsulated and has broad and rounded borders. The tumor cells are growing as a solid sheet interrupted by variably sized microcystic spaces. The predominant cell type is characterized by abundant purple granular cytoplasm. The nuclei are round and uniform without pleomorphism or notable mitotic activity.

Discussion: Acinic cell adenocarcinoma is a tumor that demonstrates serous acinar differentiation. Not surprisingly, the majority of these tumors arise from the serous dominant parotid gland. Even though these tumors may appear highly circumscribed and non-invasive, they have a real – albeit limited – capacity for local recurrence and metastatic spread. These are low-grade adenocarcinomas. A benign counterpart (i.e. acinic cell adenoma) is not recognized. 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 (in contrast to the pink granules of oncoctyomas that correspond to mitochondria seen ultrastructurally). The diagnosis of acinic cell carcinoma is fairly straightforward when these cells are abundantly present. When these cells are sparse and the histologic picture is dominated by some other cell type, the diagnosis of acinic cell carcinoma is much more challenging. Acinic cell carcinomas also demonstrate diversity at the architectural level. The four most common growth patterns are solid, microcystic, follicular and papillary cystic. Care must be taken not to dismiss the solid variant as normal parotid parenchyma. Unlike the surrounding non-neoplastic parotid gland, the solid variant of acinic cell carcinoma does not maintain a lobular architecture, and it lacks striated and interlobular ducts.