To evaluate clinicopathological features in women with mucinous breast cancer (MBC), distinguishing between pure (PMC) and mixed (MMC) subtype.

A retrospective analysis of all 358 women with MBC treated at Vietnam National Cancer hospital from June 2015 to December 2020. PMC was defined by ≥ 90% mucinous components.

We identified 358 women with MBC (245 PMC and 113 MMC) representing 2.7% of all 13,254 BC patients. The proportions of stage I, II, III and IV were 34.9%, 50.8%, 10.4% and 3.9% respectively. The rate of HER2 overexpression was 12%, and only 1.4% of patients was treated with anti-HER2. 193 patients (53.9%) had chemotherapy, including 55 patients (15.4%) treated in the neoadjuvant setting. Only 3 patients (5.5%) achieved pCR.

PMC patients were older (54.4 ± 13.3 vs 51.1 ± 13.1 years), had lower Ki67 expression, lower incidence of nodal metastasis (N +) (p values < 0.05). At a median follow-up of 58 months, the 5-year overall survival rate of non-metastatic patients was 86.6%. Multivariate analysis showed N + to be the most significant prognostic factor (HR = 3.3; 95%CI 1.5–7.1), followed by T-stage (HR = 2.9; 95%CI 1.4–6.3), HER2 + (HR = 2.5; 95%CI 1.2–5.3) and MMC subtype (HR = 1.9; 95%CI 1.0–3.9).

Poor prognostic factors of MBC include high T-stage, N-positivity, HER2 overexpression and MMC subtype. Given the low response rate to neoadjuvant CT, upfront surgery is appropriate for MBC patients.