21-gene recurrence score in predicting the outcome of postoperative radiotherapy in T1-2N1 luminal breast cancer after breast-conserving surgery.

Background: In those with one to three positive lymph nodes (N1) breast cancer (BC), the 21-gene recurrence score (RS) classification can be referred for decision-making on adjuvant chemotherapy. This study aimed to investigate the effect of RS in predicting the survival benefit of postoperative radiotherapy (PORT) in T1-2N1 BC with estrogen receptor-positive and human epidermal growth factor receptor 2-negative disease after breast-conserving surgery (BCS).

Methods: We included patients with BC and available RS data from the Surveillance, Epidemiology, and End Results Oncotype DX database. The chi-square test, Kaplan-Meier method, propensity score matching (PSM) as well as multivariable Cox proportional hazard analyses were used for statistical analyses.

Results: We included 6509 patients in the analysis. Of these patients, 5302 (85.5%) were treated with BCS + PORT, and 207 (15.5%) had BCS alone. There were 1419 (21.8%), 4319 (66.4%), and 771 (11.8%) patients being low-, intermediate-, and high-risk RS, respectively. After PSM, PORT was significantly associated with a 5-year overall survival (OS) advantage (95.1% vs. 90.5%, P < 0.001) compared to those without PORT, which similar breast cancer-specific survival (BCSS) was found between the treatment arms (P = 0.126). The sensitivity analyses showed that PORT was not associated with a better BCSS (P = 0.472) and OS (P = 0.650) than those without PORT in the low-risk RS cohort. However, PORT was associated with a better BCSS (P = 0.031) and OS (P < 0.001) compared to those without PORT in the intermediate/high-risk RS cohorts.

Conclusions: Our study highlights the possible role of the RS in predicting the outcome of PORT in T1-2N1 luminal BC patients undergoing BCS.