Increasing Utilization of Regional Nodal Irradiation for Patients with Low-Risk Node-Positive Breast Cancer

Abstract

<h3>Purpose/Objective(s)</h3> For patients with biologically low-risk, N1 breast cancer, the benefit of regional nodal irradiation (RNI) and post-mastectomy radiotherapy (PMRT) is unclear and is the subject of ongoing clinical trials. Population-level data of rates of RNI receipt is lacking in this cohort, and may be increasing despite its uncertain benefit. The purpose of this study was to determine if there was increasing use of RNI and PMRT for low-risk, N1 breast cancer. <h3>Materials/Methods</h3> This was a population-based study of all patients diagnosed between 2005 to 2014 in the [state/province] of [blinded], who underwent breast conserving surgery (BCS) or mastectomy for breast cancer. We based our definition of low-risk on the original inclusion criteria of MA.39/TAILOR RT trial, which is a non-inferiority clinical trial examining whether the omission of RNI is non-inferior. We included: pT1-2 pN1 (macroscopically node-positive) breast cancer. To define a biologically low-risk population, we included patients with a Luminal A subtype. This was approximated by: ER Allred 6-8/8, PR Allred 6-8/8, HER2-negative, and Grade 1-2. Patients who underwent a BCS and mastectomy were analyzed separately. The primary outcome was RNI receipt for patients who had a BCS, and PMRT receipt for those who had a mastectomy. We performed a multivariate, logistic regression to see whether year of diagnosis predicted for receipt of radiation. Other variables included in the multivariate model were defined a priori, based on factors known to influence RNI and PMRT receipt in the literature. <h3>Results</h3> There were 637 women who had BCS and 532 who had mastectomy in this cohort. For patients who had BCS, the rates of RNI receipt by years were: 2005-2008 68%, 2009-2011 79%, 2012-2014 89%. For patients who had a mastectomy, the rates of PMRT receipt by years were: 2005-2008 67%, 2009-2011 68%, 2012-2014 80%. During this time period, decreasing rates of ALND were observed: 2005-2008: 92%, 2009-2011 74%, 2012-2014 42%. On multivariate analysis of patients who had a BCS, RNI receipt was associated with number of involved macroscopic nodes (p=0.002) and date of diagnosis (p<0.001), but not age (p=0.2), presence of LVI (p=0.2), ALND (p=0.08), or chemotherapy receipt (p=0.9). For patients who had a mastectomy, PMRT receipt on multivariate analysis was associated with number of involved macroscopic nodes (p=0.02) and chemotherapy receipt (p=0.004), but not age (p=0.09), LVI (p=0.3), or diagnosis date (p=0.1). <h3>Conclusion</h3> There was an increased rate of RNI receipt, and a trend for increased receipt of PMRT for patients with low-risk, node-positive breast cancer between 2005 to 2014. This population-level data shows the large proportion of women who may be spared RNI if the MA.39 non-inferiority trial determines that omission of RNI is safe.