10 Optimization of the angular resolution of the VMAT dose calculation for the Delta4DVH Anatomy software

Introduction It is now possible to compare the dose distribution actually delivered in the patient anatomy with the dose distribution computed by the Treatment Planning System (TPS). The Delta 4DVH Anatomy software option (Scandidos, Sweden) calculates the doses delivered (according to two algorithms: PBc Anatomy or TMM Anatomy) in the patient geometry taking into account its heterogeneities. However, the computations are long and can take several hours, which complicates the use of the option as a routine. For Volumetric Modulated Arc Therapy (VMAT), it is possible to modify the angular resolution of the control points for the calculation in the Delta 4DVH Anatomy software option with respect to that of the TPS. By increasing the value of this angle, it allows to accelerate the computation to the detriment of precision. The aim of this work is to find a compromise between the rapidity of computation of the doses delivered in the patient anatomy and the precision of the results. Methods This study was performed on 5 prostate cases, 5 brain cases and 5 head and neck cases treated on a Beam Modulator Synergy linac (Elekta AB, Sweden). After irradiation of the treatment plans on the Delta 4 phantom, dose distribution calculations in the patient anatomy were made repeatedly from the Delta 4DVH Anatomy software option by changing the angular resolution. The 2° angular resolution that is used for calculating VMAT plans in TPS Pinnacle v9.2 (Philips Medical System Inc., USA) was used as a reference in the software. For each of the angular resolutions selected in the software, the result of the calculations was compared to the angular reference resolution (2°) using the dose volume histograms (DVHs). Percentage dose difference (%DE) between DVHs were calculated for target volumes (D95%, Dmean, D2%) and organs at risk (Dmean, D2%). Both PBc and TMM algorithms were used for this study. Results We did not observe any significant differences between the two algorithms and we were able to determine an optimal angle of 5° for the brain and head and neck patients and 6° for the prostate patients (maximum angle for which the %DE are on the average close to one percent with a small standard deviation). On average, it has been possible to save two to three hours of computation by choosing these angles to perform the calculation. Conclusions The study will save considerable time in computation doses delivered in the patient anatomy by the software. It has, however, focused on a small sample of patients and, if it allows to conjecture an optimal angle for each location, it can’t constitute a reliable statistic. An in-depth study will be required to verify these results.