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Dose-Volume Evaluation On Calculation Grid Size in Prostate VMAT Using the Gaussian Error Function

J Chow1*, R Jiang2 , D Markel1 , (1) Princess Margaret Cancer Centre, Toronto, ON, (2) Grand River Regional Cancer Centre, Kitchener, ON


(Sunday, 7/29/2018) 3:00 PM - 6:00 PM

Room: Exhibit Hall

Purpose: We carried out a dose-volume analysis in the dependence of calculation grid size on the prostate VMAT plan using the Gaussian error function (GEF).

Methods: Dose distributions of 10 prostate VMAT plans were calculated by the Eclipse treatment planning system using different grid sizes ranging from 1 – 5 mm. The VMAT plans were created using the 6 MV photon beams with prescription of 78 Gy/39 fractions. The dose-volume histograms (DVHs) of the targets and organs-at-risk (OARs) for each plan were modeled by the GEF to determine the dose-volume variation on the calculation grid size change. In addition, dose-volume parameters namely, conformity index (CI), homogeneity index (HI) and gradient index (GI) were calculated basing on DVHs of the targets and OARs using different grid sizes.

Results: For the GEF parameters of c� and c₂ reflecting the dose fall-off in the DVH, it is found that they decreased with rates of 7.8/mm and 6.8/mm when the grid size was increased for the PTV. However, there was no significant change in the CTV regarding the c� and c₂ with the grid size change. Percentage variation of dose-volume parameters (%ΔCI = 40.02%, %ΔHI = 12.45% and %ΔGI = 0.22%) for the PTV were also found larger than the CTV (%ΔCI = 13.55%, %ΔHI = 2.93% and %ΔGI = 0.06%). For OARs, the rectal wall showed larger %ΔHI than the rectum, while the bladder wall showed larger %ΔCI than the bladder. Both the left and right femur did not vary significantly on the grid size change.

Conclusion: Dose-volume variation was found more significant in the PTV than CTV on the calculation grid size change in the prostate VMAT plan. Moreover, dose distributions in the rectal and bladder wall were found more sensitive to the grid size change than the rectum and bladder.


Quality Assurance, Treatment Planning


TH- External beam- photons: Quality Assurance - VMAT

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