Mammary analog secretory carcinoma of the thyroid gland: A rare cancer harboring TRK fusion.

Mammary analog secretory carcinoma (MASC), or secretory carcinoma of the thyroid is an extremely rare disease harboring ETV6-NTRK3 gene fusion with TRK activation. Here we report the twelfth case of MASC of the thyroid worldwide. A 36-year-old female was diagnosed with poor-differentiated thyroid carcinoma (PDTC). Pathology consultant and immunochemical workups showed the tumor cells were negative for TTF1, TG, PAX8, positive for S100, Vimentin, GATA-3, and focally positive for mammaglobin. Fluorescence in situ hybridization (FISH) assay using a dual-color break-apart probe showed ETV6 translocation t(12p13) (ETV6) was present and established the diagnosis of MASC. Next-generation sequencing (NGS) of a 47-gene panel identified exon 1-5 of ETV6 gene were fused with exons 15-19 of NTRK3 gene. The patient experienced three loco-regional recurrences within 12 months and eventually developed inoperable local disease as well as bilateral lung metastasis. She is currently receiving anti-TRK treatment with a follow-up time of 33 months. A literature review of MASC in the thyroid was also conducted.