Metaplastic breast carcinoma (MBC) is a rare, aggressive form of cancer comprising epithelial and mesenchymal elements. The purpose of this study was to use population-based data to review the clinicopathologic, molecular features, and outcomes of MBC.

Surveillance, Epidemiology, and End Results Program (SEER) data were used to identify MBC and invasive ductal carcinoma (IDC), no special type (NOS) between 2004 and 2015. Results from Oncotype DX’s 21-gene assay linked to SEER registries were included for hormone receptor (HR)-positive tumors. χ2 analysis was performed to determine the differences between MBC and IDC. Kaplan-Meier curves and multivariate Cox proportional hazards models were used for breast cancer specific death (BCSD).

Compared with IDC, NOS (n=509,864), MBC (n=3876) were more likely to present at an older age, be black, have negative lymph nodes, be >2 cm, grade 3, and triple negative (TN). All subtypes [HR-positive/human epidermal growth receptor 2 (HER2)-negative, HR-positive/HER2-positive, HR-negative/HER2-positive, and TN] had higher BCSD than IDC, NOS. 22.3% of MBC cases were HR-positive. HR-positive MBCs tested for a recurrence score (RS) 65% were high-risk compared with 16.8% of IDC, NOS. Within the MBC cohort, no significant differences in BCSD were identified with respect to different molecular subtypes. In a fully adjusted model, TN or HER2-positive status did not adversely affect BCSD compared with HR-positive MBC.

All molecular subtypes of MBC had a poorer prognosis compared with IDC, NOS. The different molecular subtypes of MBC did not affect the BCSD. HR-positive MBC patients had a significantly higher high-risk RS than IDC, NOS patients.