Case Report: Triple-negative metaplastic breast carcinoma with primary resistance to neoadjuvant chemoimmunotherapy, refractory tumor necrosis-related fever, and rapid postoperative disease progression.

Background: Metaplastic breast carcinoma (MpBC) is a rare and highly heterogeneous breast malignancy. Although most cases exhibit a triple-negative phenotype, MpBC differs from conventional triple-negative breast cancer in its biological behavior, treatment response, and metastatic pattern. Neoadjuvant chemoimmunotherapy has become an important strategy for high-risk early-stage triple-negative breast cancer; however, the evidence supporting its use in MpBC remains limited. Case presentation: A 54-year-old woman was initially diagnosed with locally advanced triple-negative breast cancer of the left breast with ipsilateral axillary lymph node metastasis. A pembrolizumab-containing neoadjuvant regimen was initiated, with paclitaxel plus carboplatin as the initial chemotherapy backbone and planned sequential doxorubicin plus cyclophosphamide. After two cycles of initial neoadjuvant therapy, she developed recurrent high-grade fever, with extensive negative infectious workup and poor response to broad-spectrum antibiotics. Incision of the breast lesion revealed extensive gray-white necrotic tissue rather than typical purulent discharge. Multidisciplinary evaluation suggested tumor necrosis-related fever, and a modified radical mastectomy was performed. Fever resolved rapidly after surgery, and inflammatory markers decreased. Postoperative pathology confirmed high-grade triple-negative metaplastic breast carcinoma with Miller-Payne grade 1 response, a Ki-67 index of approximately 75%, and no definite axillary lymph node metastasis. Despite postoperative systemic treatment adjustment, the patient developed lumbar and hip symptoms with suspicious bone and soft-tissue lesions and rapidly progressed to cachexia. She died of systemic failure 5 months after surgery. Conclusion: This case highlights that persistent fever during neoadjuvant chemoimmunotherapy in MpBC should prompt careful exclusion of infection, while tumor necrosis-related fever and immune-inflammatory amplification should also be considered. In patients with suspected or confirmed MpBC who develop early tumor progression, necrosis, ulceration, fever, or deterioration in performance status, timely pathological review, reassessment of treatment response, and multidisciplinary decision-making are essential. Earlier surgery may be necessary to establish definitive pathology, control the local inflammatory source, and preserve subsequent treatment opportunities.