Effects of Inc parameter on quality of VMAT radiotherapy plans for right breast cancer based on dosimetric study

This study aimed to investigate the influence of gantry angle increment (Inc) parameters on the Monaco treatment planning system for volumetric modulated arc therapy (VMAT) in right breast cancer, and to determine the optimal Inc parameters. Twenty-three patients with right breast cancer at the Sir Run Run Shaw Hospital from August 2022 to August 2023 were retrospectively selected. Four VMAT plans were generated for each patient incorporating Inc of 10 degrees, 20 degrees (control group), 30 degrees, and 40 degrees, while the other optimization parameters and constraint functions remained constant. The D98 and conformity index were lower in the Inc30 and Inc40 groups than in the Inc20 group. As Inc increased from 10 degrees to 30 degrees, the irradiated dose to the ipsilateral lung, including the V20, V5, and Dmax, mean dose, gradually increased. There were no statistically significant differences in the irradiated dose to the contralateral breast, contralateral lung, heart, esophagus, and brachial plexus. The number of monitor units and control points decreased with increasing Inc. Smaller Inc values can achieve higher target coverage and lower irradiated dose to the ipsilateral lung, but will reduce delivery efficiency. Considering the plan quality and delivery efficiency, an Inc of 10 degrees or 20 degrees is recommended for right breast cancer VMAT plans.