Pure secretory carcinoma in situ: a case report and literature review

Secretory breast carcinoma is an exceptionally rare type of breast carcinoma. Only 5 cases of pure secretory carcinoma in situ have been reported in English literature. Herein, we reported a rare case of pure secretory breast carcinoma in situ. The patient is a 38-year-old female with bloody discharge from the left nipple. Microscopically, the terminal-duct lobular units were enlarged and filled with tumor cells. The tumor cells were arranged in cystic, microcystic, solid and papillary pattern and formed a honeycomb-like appearance. The presence of intracellular and extracellular eosinphilic PAS-positive material was the most remarkable feature. Immunohistochemically, myoepithelial markers highlighted the complete presence of myoepithelial cells around the tumour nests. Tumour cells were strongly positive for S-100 and CK5/6, negative for ER, PR and HER2. Fluorescence in situ hybridization analysis showed ETV6-NTRK3 fusion. Secretory carcinoma in situ shares the same morphological, immunohistochemical and molecular features with invasive secretory carcinoma except that the papillary growth pattern is more common in the introductal components. Cautions should be taken to distinguish secretory carcinoma in situ from other introductal lesions. Our report is an important supplement to the morphology spectrum of secretory breast carcinoma.