Dosimetric comparison of breath-hold and free-breathing techniques in left-sided breast cancer patients treated with proton pencil beam scanning

Background: To evaluate dosimetric differences in lung and heart doses in left-sided breast cancer patients treated with intensity-modulated proton therapy (IMPT) using active breathing control (ABC) and freebreathing (FB). Methods: Eight left-sided breast cancer patients undergoing IMPT were planned on both FB and ABC computerized tomography (CT) simulation scans and were robustly optimized with range uncertainty of 3.5% and setup uncertainty of 5 mm using a fast graphics-processing units (GPU) Monte Carlo optimization. The prescription for all patients was 50 Gy radiobiological equivalent (GyE) in 25 fractions. Dosimetric parameters for target coverage and dose to heart, left anterior descending (LAD) artery, ipsilateral and contralateral lungs, in addition to lung density and time on table for beam, were determined and compared using paired t-tests. Results: Volume of ipsilateral (34.9% vs. 29.1%, P<0.01) and contralateral (3.2% vs. 2.4%, P=0.04) lungs receiving 5 GyE and volume of ipsilateral lung receiving 20 GyE (13.9% vs. 10.4%, P<0.01) were all significantly higher with ABC than with FB. Maximum heart dose was lower with ABC than FB (24.8 vs. 35.8 GyE, P=0.03), but there were no statistically significant differences in any other heart or LAD artery volumetric endpoint between ABC and FB. Lung density was significantly lower with ABC than FB [-805 vs. -661 Hounsfield unit (HU), P<0.01]. Time on table for beam was significantly longer with ABC than with FB (13.8 vs. 7.6 min, P<0.01). Conclusions: ABC plans provided slightly higher lung dose and mostly similar cardiac dose in comparison with FB plans. Given the large increase in the time required for ABC treatment without obvious dosimetric or clinical benefit, FB techniques may be preferred when treating breast cancer with IMPT in order to maximize clinic workflow and minimize patient inconvenience, particularly in patients without cardiac risk factors.