Postmastectomy radiation therapy for patients with pT1-2N1 breast cancer over 2 decades in Korea: Korean Radiation Oncology Group 14-21.

PURPOSE: This study investigates patterns of postmastectomy radiation therapy (PMRT) use in patients with pT1-2N1 breast cancer treated with mastectomy in Korea over 2 decades. MATERIALS AND METHODS: Data from 700 patients treated at 16 institutions across three decades (1990, 2000, and 2010) were retrospectively reviewed. RESULTS: Overall, 10.0% of patients received PMRT, with utilization rates of 7.4%, 8.8%, and 11.1% in 1990, 2000, and 2010, respectively. The use of PMRT varied substantially among the hospitals (0%-87.5%). The two-dimensional radiotherapy was used in 1990, but the adoption of tangential beam three-dimensional conformal radiotherapy increased in subsequent decades. Axillary irradiation was performed in all patients in 1990 but was omitted in some cases starting in the 2000s (performed in 72.7% of cases in 2000, 84.1% in 2010). The inclusion of axillary levels, internal mammary nodes, and supraclavicular nodes decreased over time. The 10-year overall survival (OS) and recurrence-free survival (RFS) rates for all patients were 81.3% and 76.3%, respectively. Locoregional recurrence rates were significantly lower in the PMRT group (1.4%) compared to the no-PMRT group (8.1%, p=0.043). However, no significant differences were observed in 10-year OS, RFS, or distant metastasis-free survival rates between the two groups. CONCLUSION: PMRT was performed in approximately 10% of patients with pT1-2N1 breast cancer after mastectomy, with minimal changes in utilization rates over two decades. The use and extent of regional nodal irradiation has declined over time. Further research is needed to reflect the latest patterns of practice.