The Role of Post-Mastectomy Radiation Therapy in Patients with pT3N0 Breast Cancer and Low Genomic Recurrence Score

Abstract

<h3>Purpose/Objective(s)</h3> The role of post-mastectomy radiotherapy (PMRT) in patients with pT3 pN0 breast cancer remains controversial. We previously showed a correlation between PMRT and improved overall survival (OS) in pT3N0 patients who do not receive chemotherapy. However, it is unknown whether this correlation extends to patients with low genomic recurrence score (RS). In this analysis, we used the National Cancer Database (NCDB) to investigate the benefit of PMRT in pT3N0 patients with low RS. <h3>Materials/Methods</h3> We performed an analysis of NCDB patients with pT3N0 breast cancer who had low Oncotype DX RS (ORS) (< 25) and did not receive systemic chemotherapy. Patients were matched for baseline patient, tumor, and treatment characteristics using inverse probability of treatment weighting to minimize the impact of potential confounding variables. We stratified patients based on receipt of PMRT and compared OS using Kaplan-Meier estimates. Univariate and doubly-robust multivariable analyses were used to correlate PMRT and various baseline characteristics with OS. <h3>Results</h3> We identified 444 patients with pT3N0 breast cancer who had undergone mastectomy and had low-risk genomic assay scores who did not receive systemic chemotherapy. After IPTW matching, there were no significant differences in baseline patient characteristics. In the PMRT group, 44% received treatment to the chest-wall only, and 66% patients received chest wall and regional nodal irradiation. PMRT was associated with improved overall survival on both the univariate (HR=0.201; 95% CI=0.05-0.78; p=0.021) and doubly-robust multivariable analyses (HR=0.128; 95% CI=0.02-0.66; p=0.014). Receipt of endocrine therapy was also found to significantly improve overall survival (p=0.02). We attempted to repeat the analysis in patients with RS >25 who received chemotherapy, but we only identified a total of 78 such patients and could not perform a robust analysis. <h3>Conclusion</h3> This cohort analysis shows a strong correlation between PMRT and improved OS in pT3N0 patients with low ORS who do not receive chemotherapy. It is possible that with the de-escalation of chemotherapy and axillary surgery in this patient population in the modern era, the benefit of locoregional treatment with radiotherapy is magnified. Our analysis suggests that prospective data are needed before de-escalation of radiotherapy for these patients based on ORS.