Abstract PS04-03: Survival benefit of regional nodal irradiation in clinically node-positive breast cancer following neoadjuvant chemotherapy and breast-conserving surgery

Abstract Introduction: In recent years, there has been increasing use of neoadjuvant chemotherapy (NAC) and pathologic complete response (pCR) rates for certain breast cancer phenotypes (40-70%). Although encouraging, this has resulted in uncertainties in traditional axillary management. Meanwhile, clinically node-positive (cN+) patients who have a low burden of disease downstaged to node-negative (ypN0) after NAC and get breast-conserving surgery (BCS) may avoid an axillary lymph node dissection (cALND) but still have regional nodal irradiation (RNI) recommended regardless of axillary surgery. Those who remain node-positive (ypN+) undergo cALND and RNI with an increased risk of lymphedema. In this study, we evaluated the survival benefit of RNI in cN+ patients after NAC and BCS. Methods: We reviewed (cN+) stage I-III non-inflammatory female breast cancer patients who underwent NAC followed by BCS between 2010 and 2020 in the National Cancer Database (NCDB). Patients having a history of another malignancy who received hormone therapy or radiotherapy before surgery or whose pathologic nodal status (ypN) was unknown were excluded. Overall survival (OS) from surgery was compared between those who did and did not receive RNI. Patient and tumor characteristics were compared across subgroups using chi-square and Wilcoxon rank sum tests. Weighted Kaplan-Meier curves and log-rank tests were used to assess the effect of RNI on OS. As a sensitivity analysis, analyses were stratified based on yPN status. Finally, multivariable logistic regression was used to determine predictors of receiving RNI. Results: The 8,250 cN+ patients had a mean age of 54.6±11.1 years. In total, 69.1% of patients were White, 23.9% were Black, and 10.2% were Hispanic. The mean number of removed and positive nodes were 10.0 ±7.8 and 2.3±3.8. The most common histology and phenotype were invasive ductal carcinoma (IDC) in 94.3% and hormone-positive (HR+) in 41.0%. Breast and nodal pCR rates were 25.9% and 34.7%, with nodal pCR achieved in 20.7%, 46.0%, and 44.4% of HR+, HER2+, and triple-negative tumors. RNI was performed in 52.1% of the patients (45.3% of the yPN0 and 55.9% of the yPN+). The mean number of nodes removed was 10.4±7.8 in the RNI+ and 9.6±7.7 in the RNI- groups (P 0.01). The mean number of positive nodes was 2.6±4.0 in the RNI+ and 1.9±3.6 in the RNI- groups (P 0.01). On multivariable analysis, the predictors of RNI administration included the number of positive nodes (p 0.01), cN stage (p 0.01), cT3 (p 0.01), and tumor phenotype (p 0.01). Grade I tumors (p=0.04) and Medicare beneficiaries (p=0.01) were less likely to get RNI. The survival rates were 79.9% and 81.9% in the RNI+ and RNI– groups, with a median follow-up of 62.1±31.4 and 62.8±32.2 in the RNI+ and RNI- groups. In the entire cohort, ypN0 patients had improved OS when compared with ypN+ patients (p 0.001), but in adjusted analyses, there was no difference in OS between those who did and did not receive RNI (p=0.6). There was also no difference in OS comparing those who did and did not receive RNI in the ypN0 (p=0.96) or ypN+ (p=0.76) groups. Conclusion: For breast cancer patients having a modest nodal burden of disease and BCS after NAC, traditional prognostic factors such as stage and nodal status, as well as phenotype, grade, and the presence or absence of pCR, impact overall survival. Although some such factors appear to bias practitioners into giving RNI for patients undergoing NAC and BCS, RNI after NAC and BCS does not improve OS, regardless of pCR status, even at a mean follow-up time of 5.5 years. Further studies evaluating the impact on local control and longer-term outcomes should be pursued to enable the creation of specific guidelines surrounding indications for RNI based on the initial burden of disease and the final response from NAC administration. Citation Format: Mahtab Vasigh, Richard Bleicher, Austin Williams, Allison Aggon, Mary Pronovost, Andrea Porpiglia, Matthew Pierotti, Christian Cruz Pico. Survival benefit of regional nodal irradiation in clinically node-positive breast cancer following neoadjuvant chemotherapy and breast-conserving surgery abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PS04-03.