Complete Pathological Response to Neoadjuvant Cisplatin, Etoposide, and Pembrolizumab in Small Cell Breast Carcinoma: A Case Report and Review of the Literature

Background: Small cell breast carcinoma (SCBC) is a rare, aggressive neuroendocrine breast cancer subtype comprising less than 1% of all breast malignancies. With no standardized treatment guidelines, management is typically extrapolated from small cell lung cancer (SCLC) or triple-negative breast cancer (TNBC) protocols. Immune checkpoint inhibitors (ICIs) have shown promise in both TNBC and SCLC, but their role in SCBC remains undefined. Case Presentation: We report the case of a 65-year-old woman diagnosed with early-stage, triple-negative SCBC, exhibiting high-grade features and a Ki-67 index >90%. She received neoadjuvant cisplatin and etoposide with pembrolizumab, followed by breast-conserving surgery and adjuvant pembrolizumab. Histopathology demonstrated a complete pathological response. The patient tolerated treatment well and remains disease-free on follow-up. Conclusion: This is the first reported case of SCBC successfully treated with neoadjuvant chemoimmunotherapy, achieving complete pathological remission. It highlights the potential role of ICIs in SCBC and supports future research into biomarker-driven strategies for this rare and aggressive.