Microdosimetric study on influence of low energy photons on relative biological effectiveness under therapeutic conditions using 6 MV linac

Purpose: Microdosimetry has been developed for the evaluation of radiation quality, and single-event dose-mean lineal energy y - 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 ȳ_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 μGy/min by decreasing the gun grid voltage of the linac. According to the microdosimetric kinetic model (MK model), the RBE_MK values for cell killing of the human salivary gland (HSG) tumor cells can be derived if the ȳ_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 ȳ_D values under the various conditions. Results: The changes of the ȳ_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 ȳ_D values inside the field and those outside the field. The maximum increase of approximately 50% in the ȳ_D value outside the field was obtained compared with those inside the field. The GEANT4 calculations showed that there existed a large relative number 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 approximately 8% inside the field. By using the MK model, the field size and the depth dependence of the RBE_MK values were less than approximately 2% inside the field. However, the RBE_MK values outside the field were 6.6% higher than those inside the field. Conclusions: The increase of the RBE_MK 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.