Impact of the Use of Homogeneous and Heterogeneous Phantoms in Pretreatment Verification for Volumetric Modulated Arc Radiotherapy

The purpose of this study was to investigate the impact of quality assurance (QA) phantoms designed with different material to verify the accuracy of dose distribution for volumetric modulated arc radiotherapy (VMAT). Homogeneous and heterogeneous QA phantoms are developed to compare the pretreatment verification according to the different materials made in the phantom for routine VMAT of lung cancer. These phantoms have a quadrature rectangular shape with horizontal and vertical lengths of 300 mm and 315 mm, respectively, and with inserts of various slabs. The homogeneous QA phantom was constructed using only polymethyl methacrylate (ρ = 1.19 g/cm3). The heterogeneous QA phantom was constructed using a ceramic fiber board with a density of 0.3 g/cm3, which is similar to that of the lung in human thorax. Overall, the average gamma agreement values for the homogeneous and the heterogeneous QA phantoms were observed to be higher than 94.9% and 95.4% for two energies and the usual criteria. In addition, this study showed that the gamma agreement values obtained for both the homogeneous and the heterogeneous QA phantom agreed to within 2% for both the 2%/2 mm and the 3%/3 mm criteria. Furthermore, no statistically significant difference was observed in the gamma agreement values obtained using the homogeneous and the heterogeneous QA phantoms for both energies and the usual criteria (p > 0.05). Consequentially, this study found that patient specific QA results were not affected by phantoms made of the different materials.