Answer: Acinic cell carcinoma

Histology: This parotid gland is involved by nodules of epithelial neoplasm with a prominent cribiform growth pattern. In some areas, it has a microcystic or cystic pattern. The cells are predominantly nonspecific glandular cells; but in foci well-formed acinic cells, with apical basophilic granules are recognized. The later feature is diagnostic of acinic cell carcinoma.

Discussion: Mucoepidermoid carcinoma may frequently have a papillocystic growth pattern; however, this lesion should contain intracytoplasmic mucin, not intracytoplasmic zymogen granules. Metastatic renal cell carcinoma is typically a clear cell lesion with a hypervascular stroma. Adenoid cystic carcinoma features both cyllindromatous and glandular differentiation, and again lacks the serous (acinar) differentiation, which is indicated by the presence of zymogen granules.

Acinic cell carcinoma comprises 2% of salivary gland neoplasms. It is a low grade carcinoma, as 20% recur, and 6% metastasize distantly. Stage is more crucial than histologic pattern for prognosis, but variations in the pattern causes diagnostic confusion. Acinic cell carcinomas are often associated with a lymphoid reaction, simulating lymph node involvement. Rarely, acinic cell carcinoma can have focal intracytoplasmic mucin, causing difficulty in the distinction from mucoepidermoid carcinoma. Again, the presence of serous (acinar differentiation), which can be confirmed by the presence of PAS-positive, diastase-resistant zymogen granules, supports classification as acinic cell carcinoma.