Matrix-producing carcinoma of the breast frequently exhibits amplification of the c-MYC gene.

Matrix-producing carcinoma (MPC) of the breast is an extremely rare but invasive subtype of metaplastic carcinoma, the fifth WHO breast tumor classification categorizes these tumors within "metaplastic carcinoma with heterologous mesenchymal differentiation", whose molecular characteristics remain incompletely understood. This study summarizes the clinicopathological, morphological, and molecular features of seven MPC cases using histomorphological evaluation, immunohistochemistry, and fluorescence in situ hybridization. Microscopically, the tumors displayed nodular or focally infiltrative growth patterns, with neoplastic cells uniformly distributed within a chondromyxoid matrix. Tumor cells showed moderate to marked atypia, prominent nucleoli, and readily identifiable mitotic figures. Tumor necrosis, tumor giant cells, and multinucleated giant cells were relatively common findings. Immunohistochemically, all cases exhibited a triple-negative phenotype, with most expressing basal-like markers CK5/6 and/or EGFR. A high proliferation index were observed in the majority of tumors (85.7%, 6/7). Fluorescence in situ hybridization revealed c-MYC gene amplification in six samples (85.7%). Follow-up ranged from 1 to 33 months: one patient died 15 months after diagnosis due to causes unrelated to breast cancer, and one developed bilateral lung metastases one month post-surgery. The remaining patients were alive and well at 16 to 33 months. MPC represents a distinct clinicopathological entity with frequent c-MYC amplification and may benefit from targeted MYC inhibition.