Introduction

Mucoepidermoid carcinoma (MEC) of the breast is an exceptionally rare salivary gland-type malignancy with uncertain prognostic behaviour. This study compared the clinicopathologic characteristics, treatment patterns, and survival outcomes of MEC and invasive ductal carcinoma not otherwise specified (IDC-NOS) using Surveillance, Epidemiology, and End Results (SEER) data.

Material and methods

A retrospective cohort analysis of SEER data (1975–2022) identified cases with MEC (n = 42) and IDC-NOS (n = 50,881). Demographic, clinicopathologic, and treatment variables were compared using χ² and one-way analysis of variance tests. Overall survival (OS) and disease-specific survival (DSS) were analysed with Kaplan-Meier and Cox proportional hazards models.

Results

Mucoepidermoid carcinoma cases were older (≥ 70 years: 91.0% vs. 31.1%; p < 0.001) and predominantly White. Compared with IDC-NOS, MEC exhibited lower HER2 positivity (0.0% vs. 31.4%), oestrogen receptor/progesterone receptor expression (31.0% and 0.0% vs. 60.1% and 52.3%), and distant metastasis (0.0% vs. 20.1%) (all p < 0.001). Mucoepidermoid carcinoma cases were less likely to undergo surgery (85.7%) or receive systemic therapy (40.5%, p < 0.05). Only five deaths (OS) and three (DSS) occurred among MEC cases. Kaplan-Meier curves showed similar early survival, but MEC survival plateaued after 120 months, whereas IDC-NOS declined steadily. Multivariable Cox analysis revealed higher all-cause mortality for IDC-NOS (adjusted hazard ratios [HR] = 3.80, 95% CI: 1.18–12.22), with no significant difference in cancer-specific mortality (HR = 2.32, 95% CI: 0.71–7.58).

Conclusions

Mucoepidermoid carcinoma exhibits distinct clinicopathologic features, limited metastatic potential, and favourable long-term survival despite frequent triple negativity. Its indolent course supports conservative management for low-grade cases, warranting further molecular and prognostic studies.