Guidelines concur that postmastectomy radiation therapy (PMRT) in T1-2 tumors with one to three positive (+) lymph nodes (LNs) decreases locoregional recurrence (LRR) but advise limiting PMRT to patients at highest risk to balance against potential harms. In this study, we identify the risks of LRR after mastectomy in patients with T1-2N1 disease, treated with modern chemotherapy, and identify predictors of LRR when omitting PMRT.

Patients with T1-2N1 breast cancer undergoing mastectomy between 1995 and 2006 were categorized by receipt of PMRT. The Chi square test compared the clinicopathologic features between both groups, and Kaplan-Meier and Cox regression analysis was used to determine the rates of LRR, recurrence-free survival (RFS), and overall survival (OS).

Overall, 1087 patients (924 no PMRT, 163 PMRT) were included in the study, with a median follow-up of 10.8 years (range 0-21). We identified 63 LRRs (56 no PMRT, 7 PMRT), and 10-year rates of LRR with and without PMRT were 4.0% and 7.0%, respectively. Patients receiving PMRT were younger (p = 0.019), had larger tumors (p = 0.0013), higher histologic grade (p = 0.029), more positive LNs (p < 0.0001), lymphovascular invasion (LVI) (p < 0.0001), extracapsular nodal extension (p < 0.0001), and macroscopic LN metastases (p < 0.0001). There was no difference in LRR, RFS, or OS between groups. On multivariate analysis, age < 40 years (p < 0.0001) and LVI (p < 0.0001) were associated with LRR in those not receiving PMRT.

Consistent with the guidelines, 85% of patients with T1-2N1 were spared PMRT at our center, while maintaining low LRR. Age < 40 years and the presence of LVI are significantly associated with LRR in those not receiving PMRT.