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

PURPOSE
Successful results in carbon-ion and proton radiotherapies can extend patients' lives and thus present a treatment option for younger patients; however, the undesired exposure to normal tissues outside the treatment volume is a concern. 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 doses, quality factors, and dose equivalents in water phantom outside of the irradiation field were determined based on lineal energy distributions measured with a commercial tissue equivalent proportional counter (TEPC) at passive carbon-ion and proton radiotherapy facilities.


METHODS
Measurements at eight positions in the water phantom 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 at the National Cancer Center Hospital East for a 235 MeV proton beam.


RESULTS
The dose equivalent per treatment absorbed dose at the center of the range-modulated region H/Dt, decreased as the position became farther from the beam axis and farther from the phantom surface. The values of H/Dt ranged 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/GV for the 235 MeV proton beam. The values of the dose-averaged quality factor QD ranged 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. The authors also observed differences in the distributions of H/Dt and QD between the carbon and proton beams.


CONCLUSIONS
The authors experimentally obtained absorbed doses, dose-averaged quality factors, and dose equivalents 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 Monte Carlo calculations.