Mackeprang, Paul-Henry; Volken, Werner; Terribilini, Dario; Frauchiger, Daniel; Klippel, Norbert; Zaugg, Kathrin; Aebersold, Daniel; Fix, Michael; Manser, Peter (June 2016). SU-F-T-412: Spatiotemporal Distribution of Dose Rates in VMAT Plans. Medical physics, 43(6), p. 3558. American Association of Physicists in Medicine AAPM 10.1118/1.4956597
PURPOSE
To assess dose rates in volumetric modulated arc therapy (VMAT) plans.
METHODS
For dose rate analysis, using Monte Carlo methods, dose distributions are calculated in sectors of consecutive DICOM control points and each multiplied with the MU-rate obtaining dose rate distributions for every voxel at every time-point during a treatment fraction. Dose rate distributions were calculated for a clinical head and neck, cranial (2 Gy fraction dose) and a SBRT liver treatment case (5 Gy per fraction). To assess optimizer influence on dose rate distributions, five conventionally fractionated head and neck cases were optimized using the progressive resolution optimizer (PRO) versions 2 and 3 (Varian Medical Systems, Palo Alto, CA).
RESULTS
Firstly, histograms of dose rates over the course of treatment delivery in volumes of interest were generated for the PTV and spine or brainstem of the clinical treatment plans. Secondly, dose rates were assessed as color wash overlay on top of CT slices, allowing the user to explore the dose rate variation over time. For the cases considered, dose rates were spread out over a continuous spectrum with a mean value in the lower half. PRO3 generated plans showed higher maximum dose rates compared to PRO2 (699.80 cGy/min vs. 543.46 cGy/min) and modulation of MU rates in blocks as opposed to gradual modulation.
CONCLUSION
Calculation and visualization of dose rate distributions in VMAT have been performed. Dose rates in clinical relevant VMAT cases are spread out over a continuous spectrum with a mean in the lower part for both conventional and SBRT test cases. Different optimization algorithms may lead to substantially different dose rate distributions in the same cases. The development of the Swiss Monte Carlo Plan framework has been partially supported by Varian Medical Systems.
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