Matrix-Producing Carcinoma of the Breast: A Rare Triple-Negative, Vimentin-Positive Tumor With an Immune-Cold Microenvironment: A Case Report

Introduction: Matrix-producing carcinoma (MPC) is a rare subtype of metaplastic breast carcinoma characterized by a direct transition from epithelial carcinoma cells to a cartilaginous or osseous matrix. MPC typically exhibits a triple-negative immunophenotype and frequently expresses mesenchymal markers such as vimentin. However, data regarding its immune microenvironment remain limited. Case Presentation: A woman in her 70s presented with a palpable mass in the right breast. Imaging studies demonstrated peripheral enhancement with a central non-enhancing area on contrast-enhanced magnetic resonance imaging, along with partial hypointensity on T2-weighted images. She underwent breast-conserving surgery with sentinel lymph node biopsy, and pathological examination revealed stage IIA (T2N0M0) metaplastic carcinoma of the breast consistent with MPC. Immunohistochemically, the tumor showed a triple-negative phenotype with a high Ki-67 labeling index and strong vimentin expression. Stromal tumor-infiltrating lymphocytes were absent. Programmed death-ligand 1 expression was negative in tumor cells by the 22C3 assay (combined positive score < 1), whereas SP142 staining showed low programmed death-ligand 1 expression in immune cells (IC1, 1–4%). Adjuvant chemotherapy was not administered because of advanced age and patient preference. No recurrence has been observed 18 months after surgery. Conclusion: MPC is a rare breast cancer with distinctive pathological and radiological features. Although based on a single case with short follow-up, this report provides clinically relevant information on this uncommon carcinoma. The immune-cold tumor microenvironment observed in this case may have potential implications for immunotherapy, warranting further investigation.