Excluding Upper Axillary Level 1 in Regional Nodal Irradiation Does Not Increase Axillary Recurrence Risk in Patients With Breast Cancer.

Purpose: The optimal extent of regional nodal irradiation (RNI) in postoperative radiation therapy for breast cancer, particularly regarding axillary level 1 (AXL1), remains uncertain. This study aimed to compare clinical outcomes between extensive RNI including the entire axilla and limited RNI excluding the upper AXL1 in patients with breast cancer.

Methods And Materials: A retrospective analysis included 1780 women with nonmetastatic unilateral breast cancer who underwent RNI during postoperative radiation therapy between 2007 and 2018. Patients were classified into extensive and limited RNI groups based on the upper AXL1 inclusion in the radiation field. Propensity-score matching yielded a cohort of 1020 patients. Noninferiority of limited RNI compared with extensive RNI was assessed with a defined margin of ≤2% in the 5-year axillary recurrence rate.

Results: After a median follow-up of 67.9 months, the 5-year axillary recurrence rates were similar between extensive and limited RNI groups (1.2% vs 1.6%; P= .790). Limited RNI demonstrated noninferiority with a 0.4% difference (95% confidence interval, -1.1% to 1.9%; P= .019). Disease-free survival (87.9% vs 91.5%; P= .122) and overall survival (94.1% vs 96.9%; P= .260) at 5 years were not significantly different between extensive and limited RNI groups. Multivariable analysis revealed that lymphovascular invasion (hazard ratio [HR], 5.17; P = .02) and negative hormone receptor status (HR, 11.73; P = .002) were associated with a higher risk of axillary recurrence, whereas limited RNI showed no significant association (HR, 1.35; P = .652). Subgroup analysis demonstrated that extensive RNI did not improve axillary control in patients with lymphovascular invasion, hormone receptor negativity, positive lymph node metastasis, or a small number of nodes removed.

Conclusions: Limited RNI, excluding the upper AXL1 from the radiation field, demonstrated axillary recurrence rates comparable with those of extensive RNI in patients with breast cancer. The study suggests that extensive RNI may not provide additional therapeutic benefits, whereas limited RNI appears to be a valid option for regional control.