Purpose or ObjectiveIn many studies that investigated radiation-induced cardiac toxicity of breast cancer radiotherapy, doses are described as those received by the entire heart and the mean heart dose is used as the reference dose for analyzing dose-response relationship. However the specific relationships between doses to cardiac substructures, in particular coronary arteries, and subsequent toxicity have not been well defined. Detailed individual dosimetry information for the heart and its sub-structures is required to better understand cardiac damage from radiation exposure. The aim of this dosimetric study was to analyze the distribution of individually-determined radiation exposure, in a population of breast cancer patients treated with three dimensional conformal radiation therapy (3D-CRT), and clarify whether mean heart dose is a good surrogate parameter for the dose to coronary arteries, in particular the left anterior descending artery.Material and MethodsPatients with left or right unilateral breast cancer (BC) treated with 3D-CRT between 2015 and 2017 were included (BACCARAT clinical study). Before RT, a coronary computed tomography angiography (CCTA) was performed. Registration of the planning CT and CCTA images allowed precise delineation of the coronary arteries on the planning CT images. Using the 3D dose matrix generated during treatment planning and the added coronary contours, dose distributions were generated for the following cardiac structures: whole heart, left main coronary artery (LMCA), 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.ResultsDose distributions were generated for 89 left-sided BC and 15 right-sided BC patients. The treatment schedule with tangential beams was either 50 Gy delivered in 25 fractions of 2 Gy or 47 Gy in 20 fractions of 2.35 Gy, with or without irradiation of regional lymph nodes. Additional beams to tumor bed (boost) were used, if clinically indicated. The mean heart dose (Dmean Heart) was 2.9 ± 1.5 Gy for left-sided BC and 0.5 ± 0.1 Gy for right-sided BC. For left-sided BC patients, the mean ratio Dmean LAD/Dmean Heart was around 5. All other ratios were below 1 except for RCA in right-sided BC patients (ratio=2.7). However, the coefficients of determination R² indicated that the proportion of the variance in Dmean LAD or Dmean RCA predictable from Dmean Heart was low (R²=0.45 and 0.36 respectively). For left-sided BC patients with lower exposure (Dmean Heart<3Gy), 56% of patients received doses > 40Gy to 20% of the LAD volume on average (V40Gy).ConclusionOur study illustrates that the predictive value of the mean heart dose was not good enough for coronary arteries, in particular for LAD, illustrating the importance of considering the distribution of doses within these cardiac substructures rather than just the mean heart dose to enhance knowledge on the risk of radiation-induced cardiotoxicity in breast radiotherapy.