OBJECTIVES: The American Society for Radiation Oncology (ASTRO) ‘Choosing Wisely’ (CW) recommendations note radiation interventions lacking evidence of benefit in cancer care. This study characterizes adherence to the 2014 CW recommendation to ‘not recommend radiation following hysterectomy in low-risk endometrial cancer’ (Stage I, Grade 1-2) using a novel linkage between the Western Washington Cancer Surveillance System (CSS) and claims from two large commercial insurance plans. No studies have previously evaluated adherence to this CW recommendation in a commercial insurance setting. METHODS:  CSS records for patients with first primary diagnosis of Stage I (Grade 1-2) endometrial cancer (2008-2015) were linked with Premera Blue Cross and Regence Blue Shield claims. Included cases had coverage for ≥12-months before/after diagnosis and hysterectomy 1 month before to 4 months after diagnosis. We used generalized estimating equations (GEE) clustered by institution to evaluate associations between demographic (age, race) and tumor characteristics (grade, dx year) and receipt of radiation therapy within 1 year of diagnosis. RESULTS: Among 2,227 identified endometrial cancer patients, 514 were Stage I & Grade 1/2, and 112 met all inclusion criteria. Mean age was 59.8, 91.9% were white, and 67.0% were Grade 1. Overall, 12.5% were non-adherent. Non-adherence was 10.7% in Grade 1, 16.2% in Grade 2, and was highest (15.8%) after the ASTRO CW recommendation (2014-2015). Most non-adherent cases (78.6%) received brachytherapy. In multivariate analyses, only age in years (continuous) was associated with non-adherence (Odds Ratio=1.05, p=0.01). CONCLUSIONS: More than 12% of low-risk endometrial cancer cases in our sample were non-adherent to ASTRO CW recommendations. Surprisingly, non-adherence was highest in the time after the ASTRO CW recommendation (2014-2015). This is an important finding because non-adherence may result in side effects and/or substantial cost without commensurate clinical benefit. Future studies should evaluate non-adherence in other regions and patient populations and evaluate strategies to curb overuse.