The impact of continuously‐variable dose rate VMAT on beam stability, MLC positioning, and overall plan dosimetry

A recent control system update for Elekta linear accelerators includes the ability to deliver volumetric‐modulated arc therapy (VMAT) with continuously variable dose rate (CVDR), rather than a number of fixed binned dose rates (BDR). The capacity to select from a larger range of dose rates allows the linac to maintain higher gantry speeds, resulting in faster, smoother deliveries. The purpose of this study is to investigate two components of CVDR delivery — the increase in average dose rate and gantry speed, and a determination of their effects on beam stability, MLC positioning, and overall plan dosimetry. Initially, ten VMAT plans (5 prostate, 5 head and neck) were delivered to a [Formula: see text] dosimetric phantom using both the BDR and CVDR systems. The plans were found to be dosimetrically robust using both delivery methods, although CVDR was observed to give higher gamma pass rates at the [Formula: see text] gamma level for prostates ([Formula: see text]). For the dual arc head‐and‐neck plans, CVDR delivery resulted in improved pass rates at all gamma levels ([Formula: see text] to [Formula: see text]) for individual arc verifications ([Formula: see text]), but gave similar results to BDR when both arcs were combined. To investigate the impact of increased gantry speed on MLC positioning, a dynamic leaf‐tracking tool was developed using the electronic portal imaging device (EPID). Comparing the detected MLC positions to those expected from the plan, CVDR was observed to result in a larger mean error compared to BDR (0.13 cm and 0.06 cm, respectively, [Formula: see text]). The EPID images were also used to monitor beam stability during delivery. It was found that the CVDR deliveries had a lower standard deviation of the gun‐target (GT) and transverse (AB) profiles ([Formula: see text]). This study has determined that CVDR may offer a dosimetric advantage for VMAT plans. While the higher gantry speed of CVDR appears to increase deviations in MLC positioning, the relative effect on dosimetry is lower than the positive impact of a flatter and more stable beam profile. PACS numbers: 87.56.bd; 87.55.km; 87.55.Qr