Abstract Background: Comprehensive regional nodal irradiation (RNI) after either breast conserving surgery (BCS) or mastectomy (Mx) remains a standard of care for the majority of patients with breast cancer and axillary nodal macrometastases. However, for patients with nodal micrometastases (pN1mi), the benefit of RNI is less clear since these patients were not included in the historical trials. To date, little is known about RNI patterns of care or cancer control outcomes in patients with pN1mi disease. The SWOG 1007 (S1007) trial included hormone receptor-positive (HR+)/human epidermal growth factor receptor 2-negative (HER2-) patients with pN1mi disease. All patients randomized to the intervention (chemotherapy+endocrine therapy versus endocrine therapy alone) had Oncotype DX Score of 0-25. While RT was not directed by the protocol, data on RT use and targets was prospectively collected. Here, we report the patterns of care of RT use in patients with pN1mi disease. Materials/Methods: We reviewed the pathology reports of all patients enrolled on S1007 from 2011-2014, prior to an amendment that excluded patients with pN1mi disease. We identified patients with only pN1mi disease. We used the Radiation Therapy (RT) forms captured during the study to classify patients into 1 of 4 groups: 1) no RT, 2) Breast/Chestwall (CW) + Axillary RT, 3) Breast/CW RT, 4) RNI. Axillary RT was defined as irradiation of the level I and/or level II axilla without treatment of the supraclavicular (SCL) region. RNI was defined as inclusion of a SCL field. The analysis is primarily descriptive with use of chi-square test or t-test to compare groups. We also fit a multivariate logistic regression model to estimate the odds ratio (OR) for the associations of individual factors with RNI vs. other or no RT. Results: From 2486 patients enrolled prior to the amendment, we identified 599 patients (24%) with only pN1mi disease. Of these, 573 patients had RT forms available. The majority were postmenopausal (66%), most had undergone BCS (65%), the median number of axillary nodes removed was 3 (interquartile range (IQR), 2-6), and 87% (N=500) had pN1mi disease limited to 1 node. In addition, 61% of patients were treated with endocrine therapy (ET) alone and the median Oncotype DX score was 15 (IQR, 11-19). The overall distribution of RT use was 29% no RT, 10% Breast/CW+axillary RT, 47% breast/CW only, and 15% RNI. No patients received axillary RT without treatment of the breast/CW. Omission of RT was significantly more frequent in patients treated with Mx compared to BCS: 73% (148/202) vs. 4% (15/371), p0.001. Rates of RNI use were similar between Mx and BCS patients: 15.8% vs. 14.0%, p=0.56. On multivariable analysis, factors statistically significantly associated with RNI use (vs. no or other RT) included 1 node involved (OR=2.34, 95% CI 1.29-4.24, p=0.005) and T2-T3 tumors vs. T1 tumors (OR=1.93, 95% CI 1.20-3.11, p=0.007). Conclusion: We characterized RT patterns of care in a large cohort of patients with pN1mi disease who were enrolled on S1007. Nearly 75% of patients that underwent Mx did not receive RT and only 15% of all patients received RNI suggesting significant de-escalation of RT occurred in these patients. These variations in RT practice patterns should encourage enrollment of patients with pN1mi disease onto the ongoing TAILOR RT trial, which is investigating the role of RNI in patients with HR+/HER2-, pT1-2 disease with low volume axillary burden (including only pN1mi disease) and Oncotype RS≤25. Citation Format: J. G. Bazan, A. Meisner, K. Kalinsky, E. Connolly, W. E. Barlow, W. Woodward, A. Thompson, D. A. Lew, P. Sharma, L. Pusztai, R. Jagsi. Radiotherapy patterns of care in patients with nodal micrometastases: a secondary subgroup analysis of SWOG 1007 abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PD12-09.
Bazan et al. (Tue,) studied this question.