放射線治療ビームにおける電離箱線量計のヒステリシス現象

概要

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Certain types of ionization chambers exhibit a response that depends on the length of time elapsed since the previous irradiation. This effect is called hysteresis. The effect of hysteresis on cylindrical and parallelplate ion chambers used in radiation therapy was measured in this study. The hysteresis of these chambers was examined for Co-60 γrays, x-rays(4-and 10MV), and electron beams(4, 6, 9, 12MeV). We found that this phenomenon was observed in all chambers and beams studied. Compared with the stable reading, the first reading was up to 0.5% lower in a cylindrical chamber and 1.0% lower in a parallel-plate chamber. In a cylindrical chamber, the dose rate dependence of hysteresis was also observed for the 4 MV x-ray beam. In contrast with photon beams, the first reading was lower for electron beams regardless of the type of ion chamber. The Markus chamber, a type of parallel-plate chamber, had a lower reading and a more gradual response curve in reaching a stable reading than the NACP parallel-plate chamber. This hysteresis phenomenon was confirmed not only when radiation was not used for several days but also when it was halted for a few minutes. As this phenomenon may increase errors on estimates of the effects of polarity, it is necessary to be aware of it in correcting these effects.
社団法人日本放射線技術学会の論文
2000-11-20