Primary large cell neuroendocrine carcinoma of the breast with metastasis in the lymph nodes: a case report and literature review.

Primary large cell neuroendocrine carcinoma (LCNEC) of the breast is an extremely rare and diagnostically challenging subtype. A 47-year-old female was admitted to Weifang People's Hospital (Weifang, China) with a left breast mass that was incidentally detected. The tumor demonstrated a unique triple-negative immunophenotype and presented significant diagnostic pitfalls, mimicking conventional triple-negative breast cancer (TNBC). Modified radical mastectomy was performed after neoadjuvant chemotherapy. Based on the clinical, imaging and pathological findings, she was diagnosed with primary LCNEC of the breast with lymph node metastasis. Accurate distinction from TNBC with focal neuroendocrine differentiation and metastatic neuroendocrine carcinoma is critical. Short-term follow-up (9 months) suggests favorable disease control, but long-term efficacy and prognosis remain undetermined due to limited follow-up. This case highlights the clinical significance of avoiding misdiagnosis, with immunohistochemistry providing key diagnostic clues.