Background: For well-selected elderly women who undergo segmental mastectomy for early-stage, ER+ breast cancer, hormonal therapy alone is emerging as an acceptable adjuvant therapy option since the initial publication of CALGB 9343 study in 2004 and update in 2013. The rate of adoption of adjuvant hormonal therapy alone in lieu of radiation therapy (RT) and its associated patterns of care is the subject of this study. Methods: The National Cancer Data Base (NCDB) is a oncology outcomes database, which captures 70% of all newly diagnosed cancer patients in the US. We utilized NCDB to identify women aged ³70 diagnosed with T1N0/T1Nx invasive breast cancer who underwent segmental mastectomy from 1998-2011. Only those who received hormonal therapy were included in this analysis. Univariate and multivariable exploratory analyses were performed. Results: Of the 182,115 patients who met inclusion criteria, 97,530 (53.6%) patients underwent hormonal therapy and were included in the analysis. The rate of utilization of RT in this subset decreased with time from 84.9% in 1998 to 75.1% in 2011 (p<0.001). Academic sites decreased RT utilization most rapidly over this time period where in 2011 the rate of RT utilization was 70.7% as compared to 75.0% and 77.0% in community and comprehensive community facilities respectively (p<0.001). From 2003 to 2005 (prior to and following the initial publication of CALGB 9343), RT utilization rate decreased from 88.2% to 78.4%. Multivariable analysis revealed that the factors associated with decreased use of RT include (in order of association): older age, later year of diagnosis, lack of insurance, low grade histology, treatment at academic site, race, rural location, greater comorbidity score, lower median income, and distance from center. Conclusions: This study comprehensively assesses the patterns of care associated with the omission of radiation therapy in elderly women with early-stage breast cancer who only received adjuvant hormonal therapy. Since the publication of major clinical trials, this strategy has been increasingly adopted. The strongest predictors of the use of this strategy included advanced patient age at presentation and low-grade disease.