To investigate prognostic value of post-mastectomy radiotherapy (PMRT) in stage T_1-2N₁M₀ triple negative breast cancer (TNBC).

According to the inclusion and exclusion criteria, this retrospective study analyzed the clinical data of 2 200 patients with stage T_1-2N₁M₀ TNBC who received mastectomy from January 1, 2010 to December 31, 2016 in the Surveillance, Epidemiology, and End Results (SEER) database. The patients were divided into PMRT group and non-PMRT group according to whether PMRT was given. Multivariate Cox proportional hazards model was used to assess the factors related to breast cancer-specific survival (BCSS) and overall survival (OS).

The age of 2 200 patients was 53 (43-62) years. There were 988 cases in PMRT group and 1 212 cases in non-PMRT group. The median follow-up was 34 months (range: 1-83 months). There were 389 death cases, including 319 cancer-related deaths. Multivariate Cox analysis showed that compared with non-PMRT group, radiotherapy did not improve the BCSS (HR=0.81, 95%CI: 0.64-1.02, P=0.077) and OS (HR=0.85, 95%CI: 0.69-1.05, P=0.129) in TNBC patients. The results of stratification analysis showed that PMRT improved the BCSS (HR=0.42, 95%CI: 0.25-0.69, P=0.001) and OS (HR=0.55, 95%CI: 0.37-0.82, P=0.003) in patients≥ 60 years; PMRT could not improve BCSS (HR=0.77, 95%CI: 0.59-1.00, P=0.050) in stage T₂N₁M₀ patients, but it improved OS (HR=0.78, 95%CI: 0.61-0.99, P=0.038). In the patients receiving no chemotherapy, PMRT was significantly associated with a higher OS (HR=0.11, 95%CI: 0.01-0.79, P=0.028).

Not all stage T_1-2N₁M₀ TNBC patients who underwent mastectomy benefit from PMRT. The patients with less systemic therapy or larger tumor size may benefit more from postoperative radiotherapy. Prospective clinical trials are needed for further validation.