Survival Outcomes of Metaplastic Breast Cancer Patients: Results from a US Population-based Analysis

Metaplastic breast cancer (MBC) is a rare histologic subtype needing further characterization. The aim of our study was to compare MBC to infiltrating ductal carcinoma (IDC) of the breast and to identify demographic, clinicopathologic, treatment, and survival differences. MBC and IDC patients were identified using the Surveillance, Epidemiology, and End Results (SEER) public-use data set. Disease-specific survival (DSS) differences were compared using the Kaplan-Meier method and log-rank tests. Univariate and multivariate Cox proportional hazard models were used to assess factors prognostic for DSS. To control for hormone receptor status, a subsequent planned analysis was completed for receptor-negative MBC and IDC. Lastly, a matched case-control analysis was conducted to minimize potential bias due to baseline demographic, clinical, and pathologic differences. The SEER data set included 1,011 MBC and 253,818 IDC patients diagnosed from 2001 to 2010. MBC patients had larger, higher grade tumors, had less frequent axillary nodal involvement, and were more likely to be treated with mastectomy. Five-year DSS rates were significantly worse for patients with MBC than for IDC patients (78 vs. 93 %, p < 0.0001) and for patients with receptor-negative MBC than receptor-negative IDC (77 vs. 85 %, p < 0.0001). The findings were confirmed on matched analysis. Prognostic factors identified on multivariate analyses included age, MBC histology, tumor grade, T stage, and axillary lymph node involvement. MBC patients have shorter DSS than IDC patients. Improved clinical and biological understanding of MBC may result in more effective therapy and better cancer outcomes.