Triple-Negative Metaplastic Breast Carcinoma With Squamous and Glandular Differentiation Treated With Neoadjuvant Chemoimmunotherapy: A Case Report

Metaplastic breast carcinoma with squamous and glandular differentiation is a rare and aggressive subtype of breast cancer that frequently exhibits a triple-negative phenotype and lacks standardized treatment guidelines. We report the case of a 69-year-old female who presented with a progressively enlarging left breast mass and a palpable axillary lymph node. Imaging demonstrated a suspicious 4 cm breast mass and an enlarged axillary lymph node with cortical thickening. Core needle biopsy revealed a high-grade invasive carcinoma with squamous and focal glandular differentiation, consistent with metaplastic breast carcinoma. Immunohistochemistry demonstrated a triple-negative phenotype with expression of CK5/6 and p63, supporting squamous differentiation and the diagnosis of metaplastic breast carcinoma. Axillary lymph node biopsy was negative for metastatic disease. The patient was initiated on neoadjuvant chemoimmunotherapy according to the KEYNOTE-522 regimen, with plans for breast-conserving surgery. This case highlights the diagnostic challenges and evolving treatment strategies for this rare breast cancer subtype.