BACKGROUND: Current national guidelines encourage consideration for treating the at-risk regional nodes (axillary, supraclavicular, and/or internal mammary) with directed regional nodal irradiation (RNI) during post-lumpectomy radiotherapy for high risk N0 (node negative) and N1 (1-3 nodes positive) breast cancer patients. This recommendation is based on the results of several randomized trials published over the last decade. Evidence regarding translation of these trials into clinical practice in the United States has been limited to date. In this study, we sought to characterize the temporal changes in and clinical factors associated with the utilization of RNI during post-lumpectomy radiotherapy for N0-N1 breast cancer across a contemporary, statewide consortium of radiation oncology practices.

METHODS: Within a statewide radiation oncology quality consortium, 12,170 breast cancer patients were consecutively enrolled between 1/1/2013 and 10/31/2019 in both academic (teaching) and community (non-teaching) facilities. Data on receptor status, adjuvant systemic therapy, age, TNM stage, extent of axillary surgery, race, body mass index (BMI), type of treating facility, and year completing radiotherapy (RT) were collected. Eligibility for the present analysis was limited to patients with N0 and N1 disease not receiving neoadjuvant systemic therapy and receiving adjuvant radiotherapy after lumpectomy for non-metastatic breast cancer. Multiple variable logistic regression models were separately fit to explain the use of directed RNI (to the axilla, supraclavicular region, and/or internal mammary region) for the N0 and N1 populations separately and described using odds ratios (OR), with significant ORs (p<0.05) reported.

RESULTS: A total of 8,468 patients from 29 treating facilities met the inclusion criteria: 6,929 (81.8%) with N0 and 1,539 (18.2%) N1 disease. RNI was performed in addition to whole breast radiation in 95 (1.4%) and 908 (59%) patients in the N0 and N1 cohorts respectively. For the N0 cohort, significant correlates of RNI on multivariable analysis (MVA) were receipt of adjuvant chemotherapy (OR 2.7), higher T-stage (OR 1.9 for T2 vs T1 and 27.3 for T3/T4 vs T1), axillary surgery [compared to sentinel node biopsy (SLN) alone : no axillary surgery (OR 14.5), axillary lymph node dissection (ALND) with 10+ nodes removed (OR 15.1) ALND after SLN (OR 2.7)], and underweight BMI (OR 4.9 compared to overweight, which was the reference as the largest BMI category). For the N1 cohort, MVA suggested adjuvant chemotherapy (OR 1.8) and larger tumors (OR 1.6 [T2 vs T1]) were significantly associated with use of RNI. The year completing RT was also significantly associated with RNI use, with 22% and 15% increases per year from 2013 to 2019 in the N0 and N1 cohorts, respectively. Lastly, receiving treatment in an academic facility compared to a community facility was significantly associated with receipt of RNI in both the N0 (OR 1.8) and N1 (OR 2.2) cohorts.

CONCLUSION: In this large cohort, selective use of RNI added to post-lumpectomy whole breast radiotherapy is estimated to have increased over time, suggesting growing implementation of recent trial data and current clinical practice guidelines. Patient, treatment, and tumor characteristics appear to factor into the decision to treat with RNI, but differences in use between academic and community practices suggest opportunities for improving the consistency of care across care delivery settings. Ongoing trials seeking to identify subgroups of N1 patients in whom RNI can safely be omitted may be especially important to inform decisions, given the almost even split (59% receiving, 41% not) in practice observed in this large American cohort.

Citation Format: Derek Bergsma, Kent Griffith, Reshma Jagsi, Michael Dominello, Thomas Boike, Joshua Dilworth, Amit Bhatt, Frank Vicini, Maggie Grubb, Jean Moran, James Hayman, Lori Pierce, on behalf of MROQC, the Michigan Radiation Oncology Quality Consortium. Contemporary practice patterns for the use of regional nodal irradiation during post-lumpectomy radiotherapy for patients with N0/N1 breast cancer [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PD4-07.