Answer: Acinic cell carcinoma

Histology:

Discussion: Acinic cell carcinoma is a salivary gland carcinoma that demonstrates serous acinar differentiation. Not surprisingly, the majority of these tumors arise from the serous dominant parotid gland. Even though these tumors may appear highly circumscribed and non-invasive, they have a real – albeit limited – capacity for local recurrence and metastatic spread. These are low-grade adenocarcinomas. The morphologic features of acinic cell carcinoma are highly diverse. Five different cell types are recognized: the acinic cell, the vacuolated cell, the clear cell, the intercalated ductal cell, and the non-specific glandular cell. Serous differentiation is best expressed in the acinar cell. These cells have abundant purple granules in their cytoplasm that correspond to the zymogen granules seen ultrastructurally (in contrast to the pink granules of oncoctyomas that correspond to mitochondria seen ultrastructurally). The diagnosis of acinic cell carcinoma is fairly straightforward when these cells are abundantly present. When these cells are sparse and the histologic picture is dominated by some other cell type, the diagnosis of acinic cell carcinoma is much more challenging.

Acinic cell carcinomas also demonstrate diversity at the architectural level. The four most common growth patterns are solid, microcystic, follicular and papillary cystic. Individual tumors can show any combination of these 4 patterns. Care must be taken not to dismiss the solid variant as normal parotid parenchyma, the follicular pattern as follicular thyroid carcinoma, or the papillary cystic pattern as a cystic mucoepidermoid carcinoma. Unlike the surrounding non-neoplastic parotid gland, the solid variant of acinic cell carcinoma does not maintain a lobular architecture, and it lacks striated and interlobular ducts. In the follicular variant, the follicles may be filled with a pink colloid-like material, but the follicles tend to show more size variability than encountered in follicular thyroid cancer and they are non-immunoreactive for thyroglobulin, TTF-1 and PAX8. The papillary variant lacks the mixed population of mucinous and epidermoid cells that characterizes cystic mucoepidermoid carcinoma.

Histologic grading is not very useful as a prognostic indicator. Patient outcome does not consistently correlate with a particular cell type or architectural pattern.