A comparison of VMAT dosimetric verifications between fixed and rotating gantry positions

This study comparatively analysed dose distributions between the fixed and rotating gantry positions of volumetric-modulated arc therapy (VMAT) plans measured using different dosimetric techniques with the intent to provide pre-treatment quality assurance (QA). A total of 12 VMAT plans for the treatment of anatomical sites of various complexities were chosen. An ion chamber was used to measure the absolute central point doses, while EPID, Seven29, Matrixx and Delta4 were used to measure the dose distributions. With the exception of Delta4, all detectors were used in one of two different settings: the gantry was either fixed at 0°, or the gantry was rotating. The results were analysed using the γ-evaluation method. Regarding absolute central point doses, the ion chamber results were within 3% of the treatment planning system (TPS) calculated results. For the dose distributions measured by detectors and calculated by TPS, the γ pass rates, with 3% maximum dose and 3 mm γ criteria, were above 96% when the gantry was fixed at 0°. When the gantry was rotating, the pass rates decreased slightly but were still above 90%. The results obtained from the comparison between the measured and calculated doses demonstrated the reliability of four detectors associated with VMAT. However, the treatment delivery and detector response may impact the results when the gantry is rotating.