Characteristics study of a large-area parallel plate ionization chamber for proton radiotherapeutic dosimetry

BackgroundThe depth-dose distribution of proton beams, as a key parameter in the dose calculation of treatment planning systems, directly affects the quality and accuracy of proton radiotherapy.PurposeThis study aims to develop and evaluate a large-area Plane-Parallel Ionization Chamber (PPIC) with an effective sensitive area diameter of 16 cm for precise measurement of proton beam depth-dose distributions.MethodsThe PPIC was systematically tested using a 60Co gamma-ray field at the Shanghai Institute of Measurement and Testing Technology and the proton beam of the proton therapy facility at the Shanghai Ruijin Hospital Proton Therapy Center. In the 60Co gamma-ray field, the dose rate was 450 mGy·min-1. For the proton beam tests, measurements were performed in the fixed-beam treatment room of the Ruijin Proton Center under medical proton beam conditions. The ionization charge generated was collected and quantified using an electrometer, and key performance parameters of the PPIC including leakage current, saturation characteristics, repeatability, dose linearity, and energy dependence were evaluated.ResultsTest results show that the leakage current of the PPIC does not exceed 53 fA, with a standard deviation of less than 6.4 fA. At an operating voltage of 400 V, the recombination correction factor (ks) is 1.000 09, and the charge collection efficiency reaches 99.9%. In the 60Co gamma-ray radiation field, the ionization chamber exhibits a repeatability of 0.15%, with dose response deviations ranging from -0.08% to 0.04%. Under proton beam irradiation, the repeatability is 0.18%, the dose response deviations fall within -0.06% to 0.09%, and energy response deviations ranged from -0.72% to 0.95% relative to the mean measured value.ConclusionsThese test results in this study fully comply with the performance requirements specified in the IEC 60731 standard​for therapy-level parallel plate ionization chambers, and the PPIC demonstrated good energy consistency across the medical proton beam energy range. This indicates that the ionization chamber has the potential for application in proton beam depth-dose measurements.