Answer: Acinic cell carcinoma with high grade transformation

Histology: The parotid parenchyma is highly infiltrated by rounded tumor nodules. Perineural invasion is identified. Architecturally, the tumor cells form acinar structures. The mitotic rate is very high (5-10 mitosis per 1 hpf). On close inspection, the tumor cells have a fine to course basophilic granularity. The granules are PAS positive and diastase resistant. By immunohistochemistry, the tumor cells are positive for DOG-1 (not shown).

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. Acinic cell carcinoma is generally considered a low grade carcinoma. They are often highly circumscribed, cytologically bland, and exhibit little or no mitotic activity. There are very rare cases of acinic cell carcinomas that show geographic necrosis, high mitotic activity and pleomorphism. These 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 past, when these high grade histologic features were present and well developed, these tumors were sometimes referred to as “dedifferentiated” acinic cell carcinomas.

“Dedifferentiation”, however, is not an appropriate term. As demonstrated in the present case, serous acinar differentiation as evidence by the presence of zymogen granules (basophilic cytoplasmic granules) is often retained even in the high grade areas. Accordingly, the designation of acinic cell carcinoma with high grade transformation is more fitting.

DOG1 staining is a marker of salivary acinar and to a lesser extent intercalated duct differentiation. Strong staining can be used to support the diagnosis of acinic cell carcinoma. In our limited experience, DOG1 staining is retained in those acinic cell carcinomas that undergo high grade transformation; and the presence of strong staining can be helpful in making a diagnosis in those tumors that do not exhibit a conventional low grade component.