Answer: Acinic cell carcinoma

Histology: The tumor involves the parotid as a rounded circumscribed mass. Architecturally, the tumor is comprised of cells growing as sheets and ribbons with prominent central cellular necrosis. The cells themselves have abundant cytoplasm that varies from clear to pink and finely granular to purple and coarsely granular. The nuclei are enlarged with prominent nucleoli. In areas the mitotic rate is very high.

Discussion: Acinic cell carcinoma is a salivary gland carcinoma that demonstrates serous acinar differentiation. Not surprisingly, it most commonly arises from the serous predominant parotid gland. Its cellular composition can be highly variable. Indeed, up to 5 distinct cell types are recognized: the acinar cell, the intercalated ductal cell, the vacuolated cell, the clear cell, and the non-specific glandular cell. The hallmark cell that defines this entity is, of course, the serous acinar cell microscopically recognized by the presence of purplish cytoplasmic granules. These purple dots represent the zymogen secretory granules seen at the ultrastructural level. As in the present case, the diagnosis of acinic cell carcinoma is generally straightforward when these acinic cells are abundantly present. Conversely, the diagnosis can be problematic when these cells are not readily appreciated and the histologic picture is dominated by the other cell types.

Acinic cell carcinoma is generally considered a low grade carcinoma. There are very rare cases of acinic cell carcinomas that show geographic necrosis, high mitotic activity and pleomorphism. When these histologic features are present and well developed, as in the present case, the tumor is sometimes referred to as dedifferentiated or undifferentiated acinic cell carcinoma. 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. In the present case, the patient developed multiple lung metastases shortly following resection of the primary tumor in the parotid.