Evaluation of the dosimetric leaf gap for VMAT-SBRT in small target volumes

Abstract

Volumetric modulated arc therapy stereotactic body radiation therapy (VMAT-SBRT) is a highly efficient technique to treat inoperable patients. To implement the VMAT-SBRT technique in clinical applications, the commissioning process needs to be performed effectively. One of the most important steps in the commissioning process, the MLC modeling, depends on the MLC parameters of diametric leaf gap (DLG) and leaf transmission (LT) for Eclipse-based treatment planning. Inappropriate MLC parameters lead to poor agreement between the measured dose and the planned dose. For clinical application, many studies suggest that DLG adjustment must proceed carefully because it could compensate for the partial leaf transmission effect. This study aims to tune the measured DLG for the lung and prostate using the VMAT-SBRT technique by using electronic portal imaging (EPID). The experiment was conducted on a Varian True Beam equipped with EPID. The experiment was classified into five parts. First is the DLG measurement; the process was performed following the sweeping gap technique. Second, in the tuning direction, the dose distribution, which was optimized by measured DLG, was observed to determine the tuning direction. Third, in tuning DLG, the measured DLG was adjusted in the direction of the second step. Linear extrapolation was applied to calculate the optimal DLG, which determines the best gamma passing rate (GPR). In order to select the best DLG, a polynomial curve was plotted. Fourth, for verification, the measured DLG and the optimal DLG were compared to investigate the improvement of the gamma passing rate. Last, for validation, random retrospective cases were optimized by the optimal DLG in order to examine the performance of the optimal DLG for clinical application. The optimal DLG was 0.96 mm and 1.57 mm for 6FFF and 10FFF, respectively. Tuning the DLG is a time-consuming process because the optimal DLG could be used for prostate and lung VMAT-SBRT techniques. Therefore, EPID measurement is suggested to be applied practically because of its reliability and accessibility. Implication of the thesis: DLG tuning is a critical process that results in good agreement between measurement and plan. However, tuning DLG is a time-consuming and sophisticated process. According to the findings, EPID measurement should be done practically to save time. There are several tuning methods; this study recommends tuning from the gamma passing rate per arc to avoid dose washing out.