Background: Although trials have shown no survival advantage and only a modest improvement in local control from adjuvant radiotherapy after lumpectomy in older women with stage I, ER+ breast cancer, radiotherapy is commonly administered, raising concerns about overtreatment. Therefore, we sought to evaluate physician attitudes, knowledge, communication, and recommendations in this scenario. Methods: We mailed a survey to a national sample of 713 radiation oncologists and 879 surgeons between June to October 2015. Of these, 913 responded (57%). We assessed physicians’ attitudes, knowledge of pertinent risk information, and responses to clinical scenarios. Results: In patients age > = 70 with stage I, ER+ breast cancer treated with lumpectomy and endocrine therapy, omission of radiotherapy was felt to be unreasonable by 40% of surgeons and 20% of radiation oncologists (p < 0.001). Many surgeons (29%) and radiation oncologists (10%) erroneously associated radiotherapy in older women with improvement in survival. Similarly, 32% of surgeons and 19% of radiation oncologists tended to substantially overestimate the risk of locoregional recurrence in older women with omission of RT. In a scenario with an 81-year-old with multiple comorbidities, 31% of surgeons and 35% of radiation oncologists would still recommend radiotherapy. On multivariable analysis, erroneous attribution of a survival benefit to radiotherapy (OR 6.2; 95% CI 3.9-9.8) and overestimation of remaining life expectancy (OR 6.5; CI 4.2-9.9) were strongly associated with the opinion that radiotherapy omission is unreasonable. Conclusions: Many radiation oncologists and surgeons continue to consider omission of radiotherapy as substandard therapy. A sizeable proportion of surgeons overestimate radiotherapy’s benefits and consider omission of radiotherapy to be an unreasonable departure from the standard of care, suggesting that surgeon involvement in decisions about radiotherapy omission may be a key factor in reducing overuse of aggressive care in this setting.