Molecular Subtype Dictates Survival Outcomes in Encapsulated Papillary Carcinoma of the Breast: A Propensity Score-Matched SEER Analysis Supporting Treatment De-Escalation.

BACKGROUND: Encapsulated papillary carcinoma (EPC) is a rare breast malignancy. Controversy persists over its classification and whether its prognosis stems from histology or biology. This study clarified the role of molecular subtypes in EPC prognosis and evaluated treatment de-escalation feasibility.

PATIENTS AND METHODS: EPC and contemporary invasive ductal carcinoma (IDC) patients (2010-2019) were identified from the surveillance, epidemiology, and end results (SEER) database. A 1:1 propensity score matching (PSM) balanced baseline characteristics, including age, grade, stage, and molecular subtype. Breast cancer-specific survival (BCSS) and overall survival (OS) were analyzed using Kaplan-Meier and Cox regression.

RESULTS: We included 165 EPC and 247,581 IDC patients. EPC patients were significantly older with lower grade tumors, less nodal involvement, and higher HR+/HER2- proportions (89.7% vs. 71.1%, P < .001). Before matching, EPC showed superior BCSS (P = .044). After PSM (n = 165 per group), no significant differences were observed in BCSS (P = .207) or OS (P = .733) between groups. Subgroup analysis of HR+/HER2- patients yielded consistent results. Chemotherapy usage was low (10.3%) in EPC with excellent outcomes; radiotherapy was an independent protective factor for survival.

CONCLUSION: EPC's favorable prognosis is primarily driven by its molecular profile (HR+/HER2-) rather than histology. After subtype adjustment, EPC survival is equivalent to IDC. Findings support chemotherapy omission for most EPC patients while emphasizing radiotherapy for local control.