Mucinous carcinoma with micropapillary features is morphologically, clinically and genetically distinct from pure mucinous carcinoma of breast.

Micropapillary features are seen in pure mucinous carcinoma of breast (PMC), which is termed mucinous carcinoma with micropapillary features (MPMC). However, whether MPMC can be identified as a morphologically, clinically or genetically distinct entity from PMC remains controversial. In this study, a retrospective review of 161 cases of breast mucinous carcinoma was conducted to assess the clinicopathologic features, prognostic implications, and genomic alterations of MPMC and PMC. MPMCs were identified in 32% of mucinous carcinomas showing an excellent interobserver agreement (ICC = 0.922). MPMCs occurred at a younger age and exhibited higher nuclear grade, more frequent lymph nodal metastasis, lymphovascular invasion, and HER2 amplification compared with PMCs. Survival analyses revealed that MPMCs show decreased progression-free survival compared with PMCs in both unmatched and matched cohorts. A similar outcome of distant disease-free survival was observed only in the unmatched cohort. However, no statistical difference in recurrence score was observed between MPMC and PMC using a 21-gene assay. Notably, both MPMCs and PMCs displayed low mutation burden, common mutations affecting TTN, GATA3, SF3B1, TP53, recurrent 6q14.1-q27 losses, and 8p11.21-q24.3 gains. GATA3, TP53, and SF3B1 were recurrently mutated in MPMCs, while PIK3CA mutations were exclusively detected in PMCs. Moreover, MPMCs harbored 17q and 20q gains as well as 17p losses, while PMCs displayed gains at 6p. PI3K-Akt, mTOR, ErbB, and focal adhesion pathways were more frequently deregulated in MPMCs than in PMCs, which may responsible for the aggressive tumor behavior of MPMCs. Our findings suggest that MPMC is morphologically, clinically, and genetically distinct from PMC.