Mammary analogue secretory carcinoma of salivary gland origin: an update and expanded morphologic and immunohistochemical spectrum of recently described entity

Abstract

Mammary analogue secretory carcinoma of salivary gland origin (MASC) is a recently described tumor with ETV6 translocation. Akin to secretory breast cancer, MASC expresses S-100 protein, mammaglobin, vimentin, and harbors a t(12;15) (p13;q25) translocation which leads to ETV6-NTRK3 fusion product. Histologically, MASC displays a lobulated growth pattern and is often composed of microcystic, tubular, and solid structures with abundant eosinophilic homogeneous or bubbly secretions. Colloid-like secretory material stains positive for periodic acid-Schiff (PAS) with and without diastase and for Alcian blue. The cells of MASC are devoid of PAS-positive secretory zymogen granules. These features help to exclude the most important differential diagnostic considerations, namely acinic cell carcinoma, low-grade cribriform cystadenocarcinoma, cystadenocarcinoma (not otherwise specified), and low-grade mucoepidermoid carcinoma. To date the presence of the ETV6-NTRK3 fusion gene has not been demonstrated in any other salivary gland tumor than MASC. It is likely that MASC is more common than currently recognized and with further studies, the clinical need for molecular studies of the ETV6-NTRK3 fusion may diminish. However, molecular testing is recommended at this time to arrive at the diagnosis of MASC.

Fig. 1

MASCs are usually circumscribed but not encapsulated. Tumors often have a lobulated growth pattern divided by fibrous septa

Fig. 2

Perineural invasion can be sometimes present

Fig. 3

MASC is composed of microcystic/solid and tubular structures in most cases

Fig. 4

Presence of abundant eosinophilic homogeneous or bubbly secretory material is a typical finding in MASC

Fig. 5

MASCs may have follicular and macrocystic appearance with resemblance to thyroid gland colloid

Fig. 6

The tumor cells have low grade vesicular round-to-oval nuclei with finely granular chromatin and distinctive centrally located nucleoli

Fig. 7

Abundant bubbly secretion is present within microcystic and tubular spaces

Fig. 8

The secretory material stains positive for periodic acid-Schiff (PAS)

Fig. 9

MASCs show diffuse and strong expression of S-100 protein (a), STAT5a (signal transducer and activator of transcription 5a) (b), and mammaglobin (secretory material is also positive) (c) in all cases

Fig. 10

Fluorescence in situ hybridization (FISH) using LSI ETV6 (TEL) (12p13) Dual Color, Break Apart Rearrangement Probe (VYSIS/Abbott Molecular). Green and red arrows show split signals indicating break of ETV6 gene. Yellow arrows show nonaltered chromosome (Fig. 10a). Expression of ETV6-NTRK3 fusion transcript detected by RT-PCR. 1 marker, 2 positive MASC sample, 3 positive amplification control, 4 negative amplification control, 5 non-template control (Fig. 10b). Sequence analysis of ETV6-NTRK3 fusion transcript. Arrows show the translocation breakpoint (Fig. 10c)