Utility of Point Dose Verification Using Measured Ionization Amount Ratio of Reference Irradiation to Volumetric Modulated Arc Therapy

Abstract

In the point dose verification of intensity-modulated radiation therapy (IMRT), we compared the commonly used method of measuring absolute dose (absolute method) with the measurement method by the American Association of Physicists in Medicine Task Group 119, which describes the point dose verification for IMRT using the ratio of reference irradiation and measured ionization. The target was 66 plans for head and neck cancer, 46 plans for lung cancer with 6 MV X-ray, and 31 plans for prostate cancer with 10 MV X-ray. They were treated with volumetric-modulated arc therapy (VMAT). Each plan was evaluated by the absolute method and the TG119 method using 3D-array. The average and 2SD of the verification results for head and neck cancer, lung cancer, and prostate cancer were 0.129±2.185%, 0.963±2.125%, and 0.259±2.019% by the absolute method, and 0.952±2.039%, 1.704±2.080%, and 0.524±1.274% by the TG119 method. The ratio between the average of the TG119 method and the absolute method corresponded to the error of reference irradiation dose. Considering that the measurement method is simple, the TG119 method enables more stable point dose verification of VMAT.