Dosimetric dependence on the collimator angle in prostate volumetric modulated arc therapy

Purpose: The purpose of this study is to investigate the dose-volume variations of planning target volume (PTV) and organs-at-risk (OARs) in prostate volumetric modulated arc therapy (VMAT) when varying collimator angle. The collimator angle has the largest impact and is worth considering, so, its awareness is essential for a planner to produce an optimal prostate VMAT plan in a reasonable time frame. Methods : Single-arc VMAT plans at different collimator angles (0 o , 15 o , 30 o , 45 o , 60 o , 75 o and 90 o ) were created systematically using a Harold heterogeneous pelvis phantom. The conformity index (CI), homogeneity index (HI), gradient index (GI), machine monitor units (MUs), dose-volume histogram and mean and maximum dose of the PTV were calculated and analyzed. On the other hand, the dose-volume histogram and mean and maximum doses of the OARs such as the bladder, rectum and femoral heads for different collimator angles were determined from the plans. Results: There was no significant difference, based on the planned dose-volume evaluation criteria, found in the VMAT optimizations for all studied collimator angles. A higher CI (0.53) and lower HI (0.064) were found in the 45 o collimator angle. In addition, the 15 o collimator angle provided a lower value of HI similar to the 45 o collimator angle. Collimator angles of 75 o and 90 o were found to be good for rectum sparing, and collimator angles of 75 o and 30 o were found to be good for sparing of right and left femur, respectively. The PTV dose coverage for each plan was comparatively independent of the collimator angle. Conclusion: Our study indicates that the dosimetric results provide support and guidance to allow the clinical radiation physicists to make careful decisions in implementing suitable collimator angles to improve the PTV coverage and OARs sparing in prostate VMAT.