Dosimetric variations in calculation grid size in prostate VMAT: a dose-volume histogram analysis using the Gaussian error function

Background: Varying the calculation grid size can change the results of dose-volume and radiobiological parameters in a treatment plan, and therefore has an impact on the treatment planning quality assurance. Purpose: This study investigated the dosimetric influence of the calculation grid size variation in the prostate volumetric modulated arc therapy (VMAT) plan. Methods and materials: Dose distributions of 10 prostate VMAT plans were acquired using calculation grid sizes of 1-5 mm. Dose-volume histogram (DVH) analysis was carried out to determine the dose-volume variation corresponding to the grid size change using the Gaussian error function (GEF). At the same time, dose-volume points, dose-volume parameters and radiobiological parameters were calculated based on DVHs of targets and organs at risk (OARs) for each grid size. Results: Comparing percentage variations of GEF parameters between the planning target volume (PTV) and clinical target volume (CTV), GEF parameters of the PTV were found varied more significantly than the CTV. This resulted in larger variations of dose-volume (%Delta CI=40.02 versus 13.55%, %Delta HI=12.45 versus 2.93% and %Delta GI=0.22 versus 0.06%) and radiobiological parameters (%Delta TCP=0.61versus 0.25% and %Delta EUD=2.11 versus 0.26%) of the PTV compared with CTV. For OARs, the rectal wall showed a larger dose-EUD=2.11 versus 0.26%) of the PTV compared with CTV. For OARs, the rectal wall showed a larger dose-EUD=2.11 versus 0.26%) of the PTV compared with CTV. For OARs, the rectal wall showed a larger dose-volume variation than the rectum. However, similar dose-volume variation due to grid size change was not found in the bladder, bladder wall and femur. Conclusions: Knowing the dosimetric variation in this study is important to the radiotherapy staff in the quality assurance for the prostate VMAT planning.