Abstract
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 Delta4 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 2%/2 mm gamma level for prostates (p < 0.01). For the dual arc head‐and‐neck plans, CVDR delivery resulted in improved pass rates at all gamma levels (2%/2 mm to 4%/4 mm) for individual arc verifications (p < 0.01), 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, p < 0.01). 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 (p < 0.01). 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
Highlights
255 Boylan et al.: Dosimetric impact of continuously variable dose rate volumetric-modulated arc therapy (VMAT) can be achieved for a range of sites.[4,5,6] Just as significant are the advances in linear accelerator design — in the ability of linac control systems to reliably vary gantry speed, dose rate, and aperture shape simultaneously over the treatment arc.[7]
The Elekta VMAT solution only allowed the linac to select from fixed dose rate bins during delivery.[9]. The selection of dose rate bin and gantry speed for each control point is determined by the required change in multileaf collimator (MLC) shape and the number of monitor units to deliver
The purpose of this study is to investigate the impact of continuously variable dose rates (CVDR) on beam stability and MLC positioning accuracy, when compared to the binned dose rate (BDR) system
Summary
255 Boylan et al.: Dosimetric impact of continuously variable dose rate VMAT can be achieved for a range of sites.[4,5,6] Just as significant are the advances in linear accelerator design — in the ability of linac control systems to reliably vary gantry speed, dose rate, and aperture shape simultaneously over the treatment arc.[7]. The Elekta VMAT solution only allowed the linac to select from fixed dose rate bins during delivery.[9] The selection of dose rate bin and gantry speed for each control point is determined by the required change in multileaf collimator (MLC) shape and the number of monitor units to deliver. A more recent version of VMAT, packaged with the Integrity linac control software, allows for a much larger range of dose rates to be selected. The initial, and most prominent, impact of continuously variable dose rates (CVDR) is the much reduced treatment times. This is due to the linac being able to switch between smaller dose rate intervals, and maintain a higher gantry speed during treatment. A recent report by Bertelsen et al[13] has shown that CVDR VMAT provides good dosimetry and faster, smoother deliveries when applied to a number of clinical plans
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