Answer: Acinic cell carcinoma

Histology: The tumor consists of multiple nodules of varying sizes infiltrating into the surrounding salivary gland. The tumor shows basophilic cells arranged in solid and microacinar patterns. At higher magnification, the cells vary from having amphophilic to granular cytoplasm. The nuclei are enlarged, moderately pleomorphic, and show visible nucleoli. Mitotic figures are rare. The tumor is associated with a lymphoid infiltrate.

Discussion: Acinic cell carcinoma accounts for 1-3% of all salivary gland tumors, with parotid being the most common site. There are various histological patterns, including solid, microcystic, papillary cystic and follicular. The cells can also show varied morphology, with clear cell or vacuolated features. Acinic cell carcinoma is commonly associated with a dense lymphoid infiltrate that can be mistaken for a lymph node with metastatic carcinoma.

The differential diagnosis is usually not difficult, especially when the zymogen granules are present. Adenoid cystic carcinoma consists of islands of epithelial cells typically with a very prominent cribriform pattern. The cribriform acini are more well formed than acinic cell carcinoma, and contain eosinophilic or basophilic secretions within the lumina. Perineural invasion is usually prominent. Mucoepidermoid carcinoma can show solid pattern, but the tumor usually shows a mixture of four different cell types and would not show the granular cytoplasm seen in acinic cell carcinoma. Salivary duct carcinomas resemble adenocarcinomas of the breast and show a greater degree of pleomorphism than acinic cell carcinomas.