Moderated neutron detector for accumulated ambient dose equivalent in proton therapy

Abstract

The Varian ProBeam compact was installed at Her Royal Highness Princess Maha Chakri Sirindhorn Proton Center. Since 2017, the project has started at the King Chulalongkorn Memorial Hospital and after 4 years, during Covid19 pandemic, the 1st patient treatment received proton therapy on August, 2021. The radiation safety of the center was accomplished through firstly, the shielding calculation and secondary, the radiation survey at the milestone points such as cyclotron installation and 1st Beam ON at the treatment room. The only one WENDI-II dosimeter was used in radiation survey. The main purpose of this study is to validate the in-house neutron moderator based on CR-39 detector, Chulalongkorn University Neutron Moderator (CUMOD) through the ambient dose equivalent measurement, H*(10) and designed to use with proton pencil beam scanning, PBS of energy up to 250 MeV. The PHITS MC was used to simulate the neutron response function of CUMOD. The calibration of CUMOD was performed using 241AmBe source in the range of 100 to 1000 µSv. The H*(10) from CUMOD were compared with the one from WENDI-II. The variation of neutron fields was generated from various proton plans to cover most of the clinical scenarios. The linearly relationship between CUMOD and WENDI-II was observed with the factor of 0.64. The H*(10) per therapeutic dose was in the range of 0.018 to 0.105 mSv/Gy for 10 * 10 cm2 at 1 m distance. The advantage of CUMODs is the ability to measure H*(10) in various positions instantaneously.