Study on Dosimetry Technology of Absorbed Dose to Water for Proton

In proton therapy, precise energy transfer to tissues is crucial for cancer treatment, as accurate dose delivery directly influences tumor control efficiency and minimizes radiation damage to surrounding healthy tissues. The IAEA-TRS No. 398 report explicitly recommends that that the uncertainty in proton beam water absorption dose measurement should be less than 3% to ensure the accuracy of radiotherapy and treatment safety. Nevertheless, the metrological standard for proton absorbed dose to water has not been established. All existing proton beam water absorption dosimeters are traceable to the 60Co γ-ray standard However, the measurement results need to be corrected for the differences between the reference radiation quality and the actual proton beam, a process that increases the uncertainty of the measurement results. This study aims to develop an absolute measurement device based on the calorimetric method, establish reliable proton beam water absorbed dose measurement methods, and lay a solid technical foundation for constructing a metrological standard and quantity value transfer system. An absolute proton water calorimeter was developed using calorimetry, leveraging the principle that proton energy deposition in water is primarily manifested as temperature rise for dose calculation. Its core systems included a thermostatic insulation structure and a high-sensitivity temperature rise measurement system. The insulation structure adopted a three-layer design, maintaining a 4 °C quasi-adiabatic environment. The measurement system was equipped with a thin-walled calorimetric core with thermistors, paired with an AC Wheatstone bridge for μV-level signal readout. Pre-irradiation experiments with a total dose ≥4×103 Gy eliminated thermal defects caused by free radical reactions. Beam characteristic tests identified the 100 MeV proton beam spot center at (45 mm, -25 mm) and the Bragg peak at 35 mm depth. Formal measurements were conducted at SSD = 50 cm, with an 80 nA beam intensity and 60 s irradiation. Bridge signals were recorded continuously throughout the entire process. Six groups of experiments targeted a 120Gy dose. Radiation-induced voltage difference (ΔV) was derived via linear signal fitting and extrapolation, and dose was calculated incorporating thermodynamic (∏kt=1.02) and device (∏kc=0.96) correction factors. Normalized doses ranged from 118 Gy ~122Gy, with a relative standard deviation of 1.1%. Uncertainty components included ΔV measurement (1.1%), ohmic calibration (0.04%), temperature calibration (0.05%), thermodynamic correction (0.15%), and device correction (0.16%), resulting in a combined standard uncertainty of 1.1% and expanded uncertainty (k=2) of 2.3%, fully meeting IAEA requirements. This study successfully develops a high-precision proton water calorimeter. It provides critical technical support for establishing metrological standards, enhances the calibration accuracy of proton therapy dosimeters, ensures the precise implementation of proton therapy, and promotes the standardization and quality control of high-end proton radiotherapy, ultimately improving cancer treatment efficacy and reducing potential side effects for patients.