Abstract: Background and purpose: Breast cancer is a common and heterogeneous disease. Mucinous carcinoma (MC) of the breast is often considered to has a better prognosis than invasive ductal carcinoma (IDC). National Comprehensive Cancer Network (NCCN) guidelines for optimal systemic control of MC are mostly extrapolated from data on the basis of IDC. In this study, we investigated the relationship between the breast cancer-specific survival (BCSS) and the significance of the available clinical and pathologic prognostic factors for patients with MC and IDC. Methods: Using the Surveillance, Epidemiology, and End Results (SEER) Program in the United States (2000-2009), we retrospectively analyzed the relative hazard ratio (HR) and absolute HR of the cumulative 10-year BCSS in 251 940 female operable breast cancer patients with IDC or MC, stratified by age, estrogen receptor (ER), progesterone receptor (PR) and lymph node (LN), treated with or without adjuvant chemotherapy or radiotherapy and adjusted for other prognostic factors. Results: The incidence of ER-positive breast cancer was 97.3% in MC and 76.0% in IDC (P<0.01). MC showed less aggressive behavior and had a better prognosis compared with IDC, and this favorable outcome was maintained after 10 years of follow up. The 10-year BCSS was 92.0% for MC and 84.0% for IDC, respectively (P<0.001). The 10-year BCSS was 93.0% and 87.0% for MC and IDC with ER- and PR-positive cancer, while the 10-year BCSS was 81.0% and 76.0% in ER- and PR-negative MC and IDC, respectively (P=0.022). Among MC patients with ER- negative and PR- negative cancer, the 10-year BCSS was 85.2% and 80.0% in pT ₂ N ₀ M ₀ and pT _3-4 N ₀ M ₀ respectively, and 94.5% in pT ₁ N ₀ M ₀ . Furthermore, the 10-year BCSS was 94.1% and 94.6% (P=0.87) in ER- and PR-negative MC pT ₁ N ₀ M ₀ with or without chemotherapy, respectively. Conclusion: Mucinous cancer of the breast has better prognosis. Systemic adjuvant chemotherapy may be avoided in the patients with pT ₁ N ₀ M ₀ , ER-and PR-negative mucinous breast cancer.

Key words: Breast carcinoma, mucinous adenocarcinoma, Clinicopathological characteristics, Survival analysis