Histological purity is independently associated with breast cancer-specific survival in mucinous breast carcinoma: a population-based analysis

Pure mucinous carcinoma (PMC) and mixed mucinous carcinoma (MMC) are morphologically distinct breast cancer subtypes routinely aggregated in population-based analyses. We aimed to evaluate whether histological purity is associated with breast cancer-specific survival (BCSS) in a large registry-based cohort using competing risk methodology. Female patients with first primary PMC ( n = 21,901), MMC ( n = 232), and invasive ductal carcinoma (IDC; n = 822,563) were identified from the SEER database (2000–2023; n = 844,696). Kaplan-Meier estimation, multivariable Cox proportional hazards models, and Aalen-Johansen competing risk analyses were performed. Five- and 10-year BCSS were 96.4%/93.2% for PMC, 88.8%/83.1% for MMC, and 89.9%/84.6% for IDC. In the fully adjusted Cox model, PMC demonstrated superior BCSS versus IDC (HR 0.792, 95% CI 0.748–0.838, p < 0.001) and MMC (HR 0.675, 95% CI 0.481–0.946, p = 0.023). The comparison of MMC versus IDC yielded HR 1.083 (95% CI 0.789–1.486, p = 0.622); this result is hypothesis-generating given only 41 MMC BCSS events and should not be interpreted as evidence of prognostic equivalence. Ten-year cumulative incidence of breast cancer death was 6.08% for PMC versus 14.61% for IDC; competing non-cancer mortality was 24.41% versus 13.73%. Time-stratified landmark analyses demonstrated PMC’s advantage across all periods (HR 0.438 to 0.778; all p < 0.001). These findings are exploratory and observational; clinical implications require confirmation in datasets with centralized pathology review and genomic profiling.