Answer: Mammary analogue secretory carcinoma

Histology: The carcinoma consists of a complex proliferation of eosinophilic cells arranged in a microcystic and micropapillary architecture. There are pink secretions in the glandular spaces. The tumor nuclei are uniform, with open chromatin and prominent nucleoli. Mitoses are rare, and necrosis is absent. Immunohistochemistry showed that the tumor is diffusely positive for S100 and mammaglobin. FISH was positive for ETV6 rearrangement.

Discussion: Mammary analogue secretory carcinoma (MASC) is a recently described salivary gland neoplasm that is characterized by the same ETV6-NTRK3 gene fusion that is found in secretory breast carcinoma as well as infantile fibrosarcoma and congenital mesoblastic nephroma. About two-thirds of MASCs have been reported in the parotid gland, while a quarter of cases have been in the oral cavity, and the remaining cases have arisen in submandibular glands. MASC has many histologic features that overlap with acinic cell carcinoma (ACC). Both tumors are cytologically low grade and show multiple architectural patterns (e.g., follicular, microcystic, papillary cystic). However, MASC does not have the basophilic cytoplasmic granules that characterize ACC.

Given its bland appearance, most investigators feel that MASC behaves in a low grade fashion (similar to ACC) but its clinical behavior is not yet known. Of note, Chiosea, et al. compared the behavior of MASC and ACC and found a trend (not statistically significant) toward increased lymph node metastases in MASC.

Reference(s):
– Skalova A, Vanecek T, Sima R, et al. AJSP 34(5):599-608.
Chiosea SI, Griffith C, Assaad, A, Seethala RR. AJSP 36(3):343-50.