Evaluation of Cardiac Conduction System Exposure with Breast Volumetric Modulated Arc Therapy and Intensity Modulated Proton Therapy.

PURPOSE/OBJECTIVE(S) Conduction disorders and arrhythmias are reported radiation-induced adverse events. Exposition of cardiac conduction system during breast radiotherapy has never been studied, despite increasing use of intensity modulated radiotherapy which expose larger volumes to low dose bath. We evaluated conduction node exposure during breast irradiation with volumetric modulated arc therapy (VMAT) and estimated the potential dosimetric benefit with intensity modulated proton therapy (IMPT). MATERIALS/METHODS Atrioventricular (AVN) and sinoatrial (SAN) nodes were retrospectively delineated according to published guidelines on the simulation CT scans of twelve breast cancer patients having undergone conserving surgery and adjuvant locoregional VMAT. According to these guidelines, the SAN was delineated by a 2 cm-diameter sphere, tangent to the external wall of the right atrium, centered at the height of the ascending aorta origin and the AVN was delineated by a 2 cm-diameter sphere centered at the junction between the four cardiac chambers, 1 cm above the last slice where the left atrium is visible. IMPT treatment was re-planned on the simulation CT scans for left-sided breast cancer patients. Mean and maximum doses to the SAN and the AVN were retrieved and compared. RESULTS Average mean doses to the SAN and to the AVN were 2.8 Gy and 2.3 Gy respectively for left-sided irradiation and 9.6 Gy and 3.6 Gy respectively for right-sided irradiation. Average maximum doses to the SAN and to the AVN were 3.5 Gy and 2.8 Gy respectively for left-sided irradiation and 13.1 Gy and 4.6 Gy respectively for right-sided irradiation. There were no significant differences between mean and maximum dose to the AVN and to the whole heart between right- and left-sided irradiations, while the SAN was significantly more exposed for right-sided irradiation. For left-sided breast cancer patients, IMPT significantly reduces mean dose to the SAN from 2.8 Gy to 0.0 Gy and to the AVN from 2.3 Gy to 0 Gy, and maximum dose to the SAN from 3.5 Gy to 0.2 Gy and to the AVN from 2.8 Gy to 0 Gy (P < 0.01). CONCLUSION SAN and AVN can be substantially exposed during breast VMAT, especially for right-sided irradiation, while IMPT delivers virtually no dose to the SAN or the AVN. Cardiotoxicity studies evaluating conduction node exposure might define dose constraints and criteria for additional cardiac sparing techniques, such as respiratory techniques or proton therapy, which could be beneficial to patients with underlying rhythmic or conduction disorders.