Case Report: Spindle cell metaplastic breast carcinoma.

Spindle cell metaplastic breast carcinoma (SpCC) is an uncommon subtype of metaplastic breast carcinoma. It often shows a triple-negative phenotype and can closely mimic malignant phyllodes tumor, primary breast sarcoma, and other spindle-cell lesions of the breast. A 56-year-old woman presented with a 5.4-cm right breast mass. Core needle biopsy showed high-grade spindle cell carcinoma. The tumor expressed CKAE1/AE3, CK8/18, EMA, vimentin, TRPS-1, and partial GATA3, and was negative for ER, PR, HER2, p40, p63, CK5/6, S-100, CD34, and ERG; the Ki-67 index was 85%. BRCA1/2 testing showed no detectable mutation. After six cycles of albumin-bound paclitaxel plus carboplatin, ultrasound and MRI showed marked tumor regression. Modified radical mastectomy revealed residual degenerated malignant cells and ductal carcinoma in situ, with Miller-Payne grade 4 response and no metastasis in 17 axillary lymph nodes. The patient received postoperative radiotherapy and low-dose capecitabine. At the latest follow-up on June 25, 2026, there was no evidence of locoregional recurrence or distant metastasis. This case supports a diagnosis-centered approach to SpCC, using spindle-cell morphology together with epithelial and breast-lineage markers and exclusion of key mimics. A platinum-taxane neoadjuvant regimen achieved a major pathological response in this patient, but evidence for SpCC remains limited and treatment should be individualized.