Aims The prognosis of metaplastic breast cancer (MBC) is reportedly worse than that of triple-negative invasive ductal carcinoma (TN-IDC), but the determinants of poor prognosis are not yet known.

Methods Patients from two Korean cancer centres were included in this study (67 MBC and 520 TN-IDC). Characteristics of the two disease groups, including clinical parameters, histological features, chemoresponsiveness, disease recurrence and survival estimates, were evaluated.

Results MBC presented with larger tumours, more frequent distant metastasis and higher histological grade compared with TN-IDC (p<0.001). All but nine patients with MBC had triple-negative disease. Disease-free survival and overall survival (OS) of MBC were worse than TN-IDC (p<0.001). Multivariable analysis of disease-free survival revealed MBC type as an independent prognostic factor (HR 2.53; 95% CI 1.32 to 4.84) along with lymph node metastasis and implementation of breast conserving surgery. For OS, MBC type remained a significant prognostic factor (HR 2.56; 95% CI 1.18 to 5.54). Chemoresponsiveness of MBC and TN-IDC were similar in both neoadjuvant (p=1.000) and advanced disease settings (p=0.508). For a given MBC type, risk factors for disease recurrence included the presence of a squamous component (HR 4.0; 95% CI 1.46 to 10.99) and lymph node metastasis (HR 4.76; 95% CI 1.67 to 13.60); the risk factor for OS was initial distant metastasis (HR 10.77; 95% CI 2.59 to 44.76).

Conclusions MBC had worse survival outcomes compared with TN-IDC. Poor prognosis for MBC was likely caused by frequent recurrence with high initial stage and the unique biology of MBC itself.