Regional nodal irradiation (RNI) in breast cancer patients with residual isolated tumor cells or micrometastatic nodal disease after neoadjuvant chemotherapy

Background/Purpose: The optimal management of residual micrometastases and isolated tumor cells (ITC) in patients with invasive breast cancer who undergo neoadjuvant chemotherapy (NAC) followed by definitive surgery is not well-studied. We evaluated the role of regional nodal irradiation (RNI) in clinically node-positive (cN1) breast cancer patients with residual low-volume nodal disease following NAC.Methods/Materials: We queried the National Cancer Database (NCDB) and included patients with cN1 invasive breast cancer diagnosed from 2004 to 2016 who were treated with NAC and definitive surgery and had residual micrometastases (ypN1mi) or ITC (ypN0i+). We used univariable (UVA) and multivariable (MVA) Cox regression analyses to determine prognostic factors and Kaplan-Meier (KM) methods to evaluate overall survival (OS). We used inverse probability treatment weighting (IPTW) to reweight data to account for confounding factors.Results: Our final cohort included 1980 patients, including 527 patients with ypN0i + disease and 1453 patients with ypN1mi disease. 1101 patients (45.0%) received RNI in the overall cohort with a higher proportion of ypN1mi patients receiving RNI (56.5%) compared to 53.1% of ypN0i + patients. There was no significant dif-ference in OS between ypN0i + and ypN1mi patients. RNI had no significant effect on OS in the overall cohort using Cox MVA and KM methods. With separate subset analysis of ypN0i + and ypN1mi patients, there was no significant effect of RNI on OS. This was confirmed with IPTW.Conclusions: In a national hospital-based study of cN1 invasive breast cancer patients with residual ITC or micrometastases after NAC, RNI did not have a significant effect on OS.