Purpose: Metaplastic carcinoma of the breast (MCB) is a rare histological subtype of breast cancer with an incidence of less than 0.1%-0.5%. Due to its rarity, the clinical characteristics and prognostic significance of MCB compared with other common breast cancers (like infiltrating ductal carcinoma [IDC], and infiltrating lobular carcinoma [ILC]) are not clear, and controversial among different reports. Methods: We performed a collective comparison study of multi-institutional cases to evaluate the clinical characteristics and prognostic status of MCB to compare with IDC and ILC. A case control analysis was performed to minimize the bias from clinicopathologic factors between IDC and MCB. Disease free survival (DFS) and overall survival (OS) between groups were compared. Results: Forty-five MCB patients were enrolled from the 4 medical centers and compared with 1777 IDC and 53 ILC patients from the CCH cancer registry database comprise the current study. Compared with IDC, MCB was associated with older age, larger tumor size, a lesser lymph node positive rate, a higher likelihood of distant metastasis, higher tumor grade, lower ER-positive tumor, and higher triple negative breast cancer subtype (TNBC). MCB was associated with worse OS ( p=0.031) than IDC, but no difference in DFS ( p=0.071); however, MCB was not statistically different from ILC in both DFS and OS ( p=0.289 and 0.132, respectively). Compared with the case-controlled IDC group, MCB patients had poorer OS ( p=0.040), but no difference in DFS ( p=0.439). Conclusion: MCB is associated with poorer OS than IDC, and this was related to tumor behavior rather than clinicopathologic factors.
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