Radiotherapy (RT) is a major component of breast cancer treatment and advanced RT techniques allowed reducing irradiation of healthy tissue. However, the heart often remains partially exposed. Detailed individual heart dosimetry information is required to better understand radiation-induced cardiac damage.

To analyze the distribution of individually-determined radiation dose to the heart and its substructures, in particular coronary arteries, after RT in breast cancer patients from the BACCARAT study.

BACCARAT is a monocentric prospective cohort study that included unilateral breast cancer patients treated with RT between 2015 and 2017 and followed for 2 years with repeated cardiac imaging examinations, including coronary computed tomography angiography. Using the 3D dose matrix generated during RT treatment planning and the added coronary contours, dose distributions were generated for the following cardiac structures: whole heart, left ventricle (LV), left main coronary artery (LM), left anterior descending artery (LAD), left circumflex artery (LCX) and right coronary artery (RCA). A descriptive analysis of the physical doses in Gray (Gy) was performed.

Dose distributions were generated for 59 patients (50 left-sided breast cancer, 9 right-sided) who all received a treatment of 50Gy to the breast. The mean heart dose was 2.98Gy for left sided patients and 0.42Gy for right sided and mean LV doses were respectively 6.23Gy and 0.09Gy. For left-sided patients, mean dose to LM (D_LM)=1.29Gy, D_LAD=16.32Gy, D_LCX=1.59Gy and D_RCA=0.67Gy, whereas corresponding doses for right-sided patients were D_LM=0.35Gy, D_LAD=0.11Gy, D_LCX=0.14Gy and D_RCA=1.10Gy. For left sided patients, the most exposed part of the LAD could receive doses>45Gy.

Our study illustrates the wide range of doses experienced by the heart substructures and thus the poor significance of the mean heart dose as a radiation damage indicator.