Transdifferentiation of Secretory Carcinoma to Metaplastic Carcinoma at Metastatic Site

Secretory carcinoma is a rare breast tumor driven by <i>ETV6::NTRK3</i> fusion. It has characteristic morphologic features and identification of these tumors is critical as these patients may benefit from TRK inhibitors. Morphologic shift upon treatment has not been reported in secretory carcinoma or other <i>NTRK</i>-driven tumors. Herein we describe a patient with secretory carcinoma showing transdifferentiation into metaplastic carcinoma (spindle cell and squamous cell carcinoma) at the metastatic site following treatment. Identification of these morphologic shifts is crucial for accurate classification and identification of potential resistant mechanisms.