67 An Institutional Experience With Mammary Analogue Secretory Carcinoma of the Salivary Gland

Mammary analogue secretory carcinoma of the salivary gland (MASC) is a newly identified entity genetically defined by harboring an unique cytogenetic features of t (12,15) (p13; q25) translocation with ETV6-NTRK3 fusion protein. This low-grade salivary gland tumor mimics salivary acinic cell carcinoma or low-grade adenocarcinoma morphologically and was not recognized until recently. Two cases of mammary analogue secretory carcinoma of the salivary gland have been recently identified in our institution, which was confirmed by ETV6 translocation fluorescence in situ hybridization study. To have a better understanding of the incidence and characteristics of this rare entity in our institution, we reviewed the cases with the diagnosis of acinic cell carcinoma or low-grade adenocarcinoma of salivary gland tumor from our archives. A total of 37 cases of salivary gland tumor with the diagnosis of salivary acinic cell carcinoma or low-grade adenocarcinoma were retrieved from our archives from between 2000 and 2016. The morphology of all the cases was reviewed. Microscopically, three cases are composed of circumscribed nodules with bland neoplastic epithelial cells arranged in microcytic, tubular, and solid growth pattern, characteristic for the description of typical MASC. All the cases were subjected to fluorescence in situ hybridization (FISH) study using dual-color breakapart rearrangement probes for ETV6. When two separate signals (a red and a green) were observed, ETV6 was considered split. The cutoff value for the ETV6 split was 10%. Seven cases were eliminated from the study due to technical difficulties for getting the signal. Result: A total of 30 cases with diagnosis of acinic cell carcinoma or low grade adenocarcinoma all show negativity for ETV6 breakapart FISH study. This includes 21 cases of acinic cell carcinoma, seven cases of polymorphous low-grade adenocarcinoma, and two cases of low-grade salivary gland adenocarcinoma. This result could be due to several possibilities. First, there might be some cases with possible positivity for ETV6 translocation within the seven cases with technical difficulties for getting the signal. Second, according to the literature, there is additional translocation in MASC involving ETV6, which may not be positive with the ETV6 FISH assay utilized in our study. A third possibility is that MASC is a rare tumor in our institution because of referral patterns. Our result shows that MASC is a very rare low-grade salivary gland tumor in our institution. Our study also confirms that true acinic cell carcinoma does not harbor ETV6 translocation.