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

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    Authors
    Boylan, Christopher J
    McWilliam, Alan
    Johnstone, Emily
    Rowbottom, Carl G
    Affiliation
    The Christie NHS Foundation Trust.
    Issue Date
    2012
    
    Metadata
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    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, 5head 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.
    Citation
    The impact of continuously-variable dose rate VMAT on beam stability, MLC positioning, and overall plan dosimetry. 2012, 13 (6):4023 J Appl Clin Med Phys
    Journal
    Journal of Applied Clinical Medical Physics
    URI
    http://hdl.handle.net/10541/263696
    DOI
    10.1120/jacmp.v13i6.4023.
    PubMed ID
    23149797
    Type
    Article
    Language
    en
    ISSN
    1526-9914
    ae974a485f413a2113503eed53cd6c53
    10.1120/jacmp.v13i6.4023.
    Scopus Count
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