Observational studies suggest that the risk of cardiac toxicity increases with increasing radiation dose to the heart, without a threshold for this effect. Although a recent systematic review summarized heart doses in the published literature, less is known about the range of heart doses delivered in routine practice in the United States today. The goal of this study was to assess the current state of cardiac sparing in a large prospective observational cohort, and to determine how cardiac dose varies by practice setting, technique, and patient characteristics