Answer: Acinic cell carcinoma

Histology: This is a cellular lesion which is well delineated from the surrounding parotid gland. The lesion is comprised of small acini of cells with granular purple cytoplasm which closely resemble those of the normal acinar tissue of the parotid gland. However, these cells within the lesion form a solid mass without normal ducts or adipose tissue as seen in the normal parotid. The purple granules correspond to zymogen granules, which demonstrate serous (acinar) differentiation.

Discussion: The solid variant of acinic cell carcinoma, as exemplified by the current case, can be difficult to distinguish from normal parotid. The absence of normal ducts and adipose tissue within the lesion helps make this differential diagnosis. Oncocytoma would feature cells with granular eosinophilic cytoplasm. Metastastic thyroid carcinoma is a major differential diagnosis consideration in the follicular subtype of acinic cell carcinoma, not the solid variant. Immunohistochemistry for thyroglobulin or TTF could help make this distinction.