Background: The National Comprehensive Cancer Network (NCCN) Guidelines incorporated omission of radiation therapy (RT) after breast-conservation surgery in woman age ≥ 70 years with stage I, estrogen receptor positive breast cancer who plan to receive endocrine therapy (ET). This guideline change was based on The Cancer and Leukemia Group B C9343 trial. A follow up study demonstrated that there is a wide variation in implementing this change across 13 different NCCN institutions. We evaluated the practice pattern at Johns Hopkins, and sought to construct an internal guideline. Methods: We identified women treated at our institution from 2009-2013 age ≥ 70 years at the time of diagnosis and met the C9343 inclusion criteria. RT omission rate was calculated for each year. We explored associations between RT omission and year, age, tumor size, race, nodal status and tumor type with t tests and Fisher’s exact tests. Results: A total of 544 women age ≥ 70 years sought treatment at our institution, and 98 (18%) were candidates for RT omission based on the NCCN guidelines. Mean age was 76.2 years (Range 70-95). Overall RT omission rate was 36/98 (37%), but varied greatly by year (Range 8-56%, p = 0.03). This variation in omission rate was still present after excluding women who did not tolerate ET (Range 9-67%, p = 0.02). Older age was associated with higher RT omission rate (mean age 78.7 vs. 74.8, p = 0.002). Women who did not undergo nodal evaluation had higher RT omission rate (68%) than women who had nodal evaluation (29%) even when the evaluated node(s) were negative (p = 0.003). The RT omission rate did not vary by race (Caucasians: 24/69, 35%; Non-Caucasians: 11/27, 40%; p = 0.64), tumor type (ductal: 27/72, 38%; non-ductal: 9/26, 35%; p > 0.99), or tumor size ( < 1cm: 17/37, 46%; 1-1.4cm: 10/34,29%; 1.5-2cm: 8/25, 32%; p = 0.35). Conclusions: The implementation of the NCCN guideline, which was based on category I evidence, is not consistent at our institution. Our results suggest that other tools should be used to apply the guidelines more consistently. To achieve this, we have developed a Quality Improvement Protocol that incorporates life expectancy estimate and a brief geriatric assessment to the treatment of all woman age ≥ 70 years at our breast centers.