Adjuvant chemotherapy combined with pembrolizumab immunotherapy for primary triple-negative neuroendocrine carcinoma of the breast: a case report and literature review.

Neuroendocrine breast cancer (NEBC) is an exceedingly rare malignant tumor, with primary triple-negative NEBC representing an even rarer and more aggressive subtype lacking standardized treatment guidelines. Although pembrolizumab combined with chemotherapy has become the standard of care for high-risk early-stage triple-negative breast cancer (TNBC) following the KEYNOTE-522 trial and recent expert consensus recommendations, its role in the rare histologic subtype of primary NEBC remains largely unexplored. We report a 71-year-old postmenopausal woman who presented with a left breast mass discovered over three years earlier. Comprehensive preoperative staging excluded extra-mammary neuroendocrine primaries. Histopathology and immunohistochemistry confirmed primary NEBC (ER-, PR-, HER2-, CgA+, Syn+, CD56+, Ki-67 index 30%), PD-L1 positivity (CPS ≥10) and germline BRCA1/2 mutation. The patient underwent curative mastectomy with sentinel lymph node biopsy (pT2N0M0, stage IIA). Adjuvant therapy comprised paclitaxel (175 mg/m²) plus cisplatin (75 mg/m²) every 3 weeks for six cycles, combined with pembrolizumab (200 mg every 3 weeks). The regimen was well tolerated with only grade 1-2 toxicities. At 12-month follow-up, the patient remains disease-free with no evidence of recurrence. To our knowledge, this is one of the first reported cases of pembrolizumab combined with platinum-based chemotherapy in the adjuvant setting for primary triple-negative NEBC. This case provides hypothesis-generating evidence for chemo-immunotherapy in this rare, high-grade histologic subtype.