Survival outcomes after breast-conserving surgery with radiotherapy versus mastectomy in apocrine carcinoma of the breast: a Surveillance, Epidemiology, and End Results-based retrospective cohort study with time-dependent analysis.

Background: Apocrine carcinoma (AC) of the breast is a rare subtype of breast cancer, with distinct biological features, often characterized by hormone receptor negativity and androgen receptor positivity. Because its behavior may differ from conventional triple-negative breast cancer, local treatment selection remains clinically important. Evidence comparing breast-conserving surgery combined with radiotherapy (BCS + RT) and mastectomy (MAST) remains limited. This study evaluated the survival impact of RT after BCS and compared BCS + RT with MAST, with attention to time-dependent effects. Methods: Patients with AC of the breast diagnosed between 2010 and 2022 were identified from the Surveillance, Epidemiology, and End Results (SEER) database using International Classification of Diseases for Oncology, Third Edition (ICD-O-3) code 8401/3. Patients with documented surgery type and available survival information were included; those with missing surgery type or follow-up information were excluded. Two cohorts were constructed: one compared BCS alone with BCS + RT, and the second cohort compared BCS + RT with MAST in early-stage, breast-conservation-eligible patients. Covariates included age, race/ethnicity, marital status, tumor grade, stage, tumor size, chemotherapy status, breast subtype, and axillary level I-II nodal burden. Follow-up was calculated from diagnosis to death or last follow-up. Overall survival (OS) and breast cancer-specific survival (BCSS) were analyzed using Cox and Fine-Gray models. Stabilized inverse probability of treatment weighting (IPTW) based on the full eligible cohort was used as the primary propensity score-based adjustment method, and propensity score matching (PSM) was performed as a sensitivity analysis. Landmark, continuous landmark, and restricted mean survival time (RMST) analyses were performed. A single-center cohort was included for descriptive clinicopathological supplementation. Results: A total of 1,199 patients were included, comprising 202 treated with BCS, 469 with BCS + RT, 400 with MAST, and 128 with MAST + RT. Most patients were aged ≥50 years, and 84.2% had stage I-II disease. The BCS + RT group had more early-stage and node-negative patients, whereas the MAST + RT group showed greater axillary nodal burden and higher-risk features. Among patients undergoing BCS, RT was associated with improved OS after IPTW adjustment [hazard ratio (HR) =0.608, 95% confidence interval (CI): 0.415-0.889], whereas its effect on BCSS was not statistically significant. In early-stage, breast-conservation-eligible patients, BCS + RT and MAST showed comparable long-term OS and BCSS after adjustment (IPTW HR for OS =0.785, P=0.20). Time-dependent analyses showed that the survival benefit of BCS + RT was mainly observed within 3 years (HR =0.377) and diminished over time. Conclusions: RT was associated with improved OS among patients undergoing BCS. In selected early-stage patients eligible for breast conservation, BCS + RT achieved survival outcomes comparable to MAST, suggesting that BCS + RT can be considered a reasonable survival-based local treatment option when breast preservation is clinically desired in this rare subtype. Clinicians may use these findings to support shared decision-making with early-stage AC patients who prioritize breast conservation.