Measurement of Ambient Dose Equivalent in Compact Proton Therapy using In-house Neutron Moderator-based Poly Allyl Diglycol Carbonate

Purpose: The high-energy proton produces the unwanted dose contribution from the secondary neutron. The main purpose of this study is to report the validation results of in-house neutron moderator based on poly allyl diglycol carbonate (CR-39) detector, Chulalongkorn University Neutron Moderator (CUMOD) through the ambient dose equivalent, H*(10) measurement. Materials and Methods: The Particle and Heavy Ion Transport code System (PHITS) Monte Carlo code was used to simulate the neutron response function. The CUMOD was calibrated with 241AmBe source calibrator in the range of 100-1000 mu Sv. The variation of neutron fields was generated employing different proton treatment plans covering most of the clinical scenarios. The ambient dose equivalents, H*(10), evaluated employing CUMOD were compared to those obtained with WENDI-II dosimeter. Results: The linear relationship between CUMOD and WENDI-II responses showed an R-2 value close to 1. The H*(10) per Gy delivered dose was in the range of 22-105 mu Sv for a 10 cm x 10 cm field. Conclusion: The in-house CUMOD neutron moderator can expand the neutron detection dose range of CR-39 detector for ambient dose equivalent. The advantage of CUMODs is its capability to evaluate H*(10) in various positions simultaneously.