Time Trends And Predictors Of Heart Dose From Breast Radiation Therapy In A Large Consortium Of Community And Academic Practices

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. From 2012-2015, 3748 patients with breast cancer treated with lumpectomy and whole breast radiation therapy at 20 sites participating in a state-wide consortium were entered into a registry, with web-based transmission of clinical and treatment details, including dose-volume histograms (DVHs) for the heart. 2296 patients with left- and 1342 patients with right breast cancer had a heart DVH available. Mean dose to the heart was modeled using the following covariates: planned breast dose, IMRT versus 3D, use of deep-inspiration breath hold (DIBH), use of accelerated whole breast irradiation (AWBI), nodal treatment, academic institution, and year of RT. Separate models were constructed for treatment of patients with left and right breast cancer. The mean heart dose for patients with left breast cancer has decreased every year since 2012 (median 2.0, 1.8, 1.5, and 1.4 Gy respectively). The maximum heart dose has similarly decreased (median 23, 20, 14, 12 Gy respectively), P<0.0001 for both. Mean heart dose was estimated to be 1.2 Gy for a patient with the median planned breast dose of 47 Gy, and higher with IMRT use (+17%, P<0.0001), increasing planned breast dose (+2% per Gy over 47, P = 0.0003), boost (+23%,P<0.0001), SCV RT (+16%, P<0.0001), IMN RT (+42%, P<0.0001), academic (+13%, P<0.0001) center, and each year earlier than 2015 (+9%, P<0.0001). Mean heart dose was lower with DIBH (-20%, P<0.0001), AWBI (-7%, P<0.04), and prone position (-32%, P<0.0001). For patients with right breast cancer, heart dose was estimated to be 0.5 Gy higher with boost (+23%, P<0.0001), SCV (+15%, P = 0.01), IMN (+46%, P<0.0001), academic (+12%, P<0.0001) center. Mean heart dose was lower with AWBI (-13%, P<0.0001) and prone position (-25%, P<0.0001). Within a large, state-wide quality collaborative, cardiac dose is overall low and has decreased since 2012, coincident with increased focus on cardiac sparing in consortium meetings and in the research and popular media. Additional work will focus on identifying best practices to further minimize cardiac dose, particularly when IMRT is used.