Measurement of absorbed dose, quality factor and dose equivalent in water phantom outside of the irradiation field in carbon-ion and proton radiotherapies

Purpose: Positive outcomes of carbon-ion and proton radiotherapies extend patients\u27 lives and allow younger patients to receive the radiotherapies, and thus undesired exposure in normal tissues far from the treatment volume has become a great concern. Such exposure is considerably lower than that near the treatment volume, but it is not negligible for estimating the risk of secondary cancer, especially for young patients. Organ-specific information on the absorbed dose and the biological effectiveness in the patient is essential for assessing the risk, but experimental dose assessment has seldom been done. In this study, absorbed dose, quality factor and dose equivalent in water phantom outside of the irradiation field were measured by using a commercial tissue equivalent proportional counter (TEPC) at passive carbon-ion and proton radiotherapy facilities.Methods: Measurements at 8 positions were carried out at the Heavy-Ion Medical Accelerator in Chiba of the National Institute of Radiological Sciences for 400 and 290 MeV/u carbon beams and the National Cancer Center Hospital East for a 235 MeV proton beam.Results: Dose equivalent per the treatment absorbed dose at the center of SOBP, H is lower as the position is farther from the beam axis and the upstream side of the phantom. The values of H are from 6.7 to 0.16 mSv/Gy for the 400 MeV/u carbon beam, from 1.3 to 0.055 mSv/Gy for the 290 MeV/u carbon beam, and from 4.7 to 0.24 mSv/Gy for the 235 MeV proton beam. The values of the dose-averaged quality factor, QD are from 2.4 to 4.6 for the 400 MeV/u beam, from 2.8 to 5.3 for the 290 MeV/u beam, and from 5.1 to 8.2 for the proton beam.Conclusions: We experimentally obtained the absorbed dose, dose-averaged quality factor and dose equivalent in water phantom outside of the irradiation field in passive carbon-ion and proton radiotherapies with TEPC. These data are very useful for estimating the risk of secondary cancer after receiving passive radiotherapies and for verifying the Monte Carlo calculations.PTCOG4