量研学術機関リポジトリ「QST-Repository」は、国立研究開発法人 量子科学技術研究開発機構に所属する職員等が生み出した学術成果（学会誌発表論文、学会発表、研究開発報告書、特許等）を集積しインターネット上で広く公開するサービスです。 Welcome to QST-Repository where we accumulates and discloses the academic research results(Journal Publications, Conference presentation, Research and Development Report, Patent, etc.) of the members of National Institutes for Quantum and Radiological Science and Technology.
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Purpose: Microdosimetry has been developed for the evaluation of radiation quality, and single- event dose-mean lineal energy y_x1113088_D is well-used to represent the radiation quality. In this study, the changes of the relative biological effectiveness (RBE) values under the therapeutic conditions using a 6 MV linac were investigated with a microdosimetric method.
Methods: The y_x1113088_D values under the various irradiation conditions for x-rays from a 6 MV linac were measured with a tissue-equivalent proportional counter (TEPC) at an extremely low dose rate of a few tens of lGy=min by decreasing the gun grid voltage of the linac. According to the micro- dosimetric kinetic model (MK model), the RBEMK values for cell killing of the human salivary gland (HSG) tumor cells can be derived if the y_x1113088_D values are obtained from TEPC measurements. The Monte Carlo code GEANT4 was also used to calculate the photon energy distributions and to investigate the changes of the y_x1113088_D values under the various conditions.
Results: The changes of the y_x1113088_D values were less than approximately 10% when the field size and the depth in a phantom varied. However, in the measurements perpendicular to a central beam axis, large changes were observed between the y_x1113088_D values inside the field and those outside the field. The maximum increase of approximately 50% in the y_x1113088_D value outside the field was obtained compared with those inside the field. The GEANT4 calculations showed that there existed a large relative num- ber of low energy photons outside of the field as compared with inside of the field. The percentages of the photon fluences below 200 keV outside the field were approximately 40% against approxi- mately 8% inside the field. By using the MK model, the field size and the depth dependence of the RBEMK values were less than approximately 2% inside the field. However, the RBEMK values out- side the field were 6.6% higher than those inside the field.
Conclusions: The increase of the RBEMK values by 6.6% outside the field was observed. This increase is caused by the change of the photon energy distributions, especially the increase of the relative number of low energy photons outside the field.
Conclusions: The increase of the RBEMK values by 6.6% outside the field was observed. This increase is caused by the change of the photon energy distributions, especially the increase of the relative number of low energy photons outside the field.
Microdosimetric study on influence of low energy photons on relative biological effectiveness under therapeutic conditions using 6 MV linac
