title: Relation between lineal energy distribution and relative biological effectiveness for photon beams according to the microdosimetric kinetic model.
abstract: Our cell survival data showed the obvious dependence of RBE on photon energy: The RBE value for 200 kV X-rays was approximately 10% greater than those for mega-voltage photon beams. In radiation therapy using mega-voltage photon beams, the photon energy distribution outside the field is different with that in the radiation field because of a large number of low energy scattering photons. Hence, the RBE values outside the field become greater. To evaluate the increase in RBE, the method of deriving the RBE using the Microdosimetric Kinetic model (MK model) was proposed in this study. The MK model has two kinds of the parameters, tissue-specific parameters and the dose-mean lineal energy derived from the lineal energy distributions measured with a Tissue-Equivalent Proportional Counter (TEPC). The lineal energy distributions with the same geometries of the cell irradiations for 200 kV X-rays, (60)Co γ-rays, and 6 MV X-rays were obtained with the TEPC and Monte Carlo code GEANT4. The measured lineal energy distribution for 200 kV X-rays was quite different from those for mega-voltage photon beams. The dose-mean lineal energy of 200 kV X-rays showed the greatest value, 4.51 keV/µm, comparing with 2.34 and 2.36 keV/µm for (60)Co γ-rays and 6 MV X-rays, respectively. By using the results of the TEPC and cell irradiations, the tissue-specific parameters in the MK model were determined. As a result, the RBE of the photon beams (y(D): 2~5 keV/µm) in arbitrary conditions can be derived by the measurements only or the calculations only of the dose-mean lineal energy.
authors.affiliation: Department of Energy Sciences, Tokyo Institute of Technology, Kanagawa, Japan. hiokamot@ncc.go.jp
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Okamoto
authors.forename: Tatsuaki
authors.initials: T
authors.lastname: Kanai
authors.forename: Yuki
authors.initials: Y
authors.lastname: Kase
authors.forename: Yoshitaka
authors.initials: Y
authors.lastname: Matsumoto
authors.forename: Yoshiya
authors.initials: Y
authors.lastname: Furusawa
authors.forename: Yukio
authors.initials: Y
authors.lastname: Fujita
authors.forename: Hidetoshi
authors.initials: H
authors.lastname: Saitoh
authors.forename: Jun
authors.initials: J
authors.lastname: Itami
authors.forename: Toshiyuki
authors.initials: T
authors.lastname: Kohno
mesh_terms: D003198:Computer Simulation; D007700:Kinetics; D018499:Linear Energy Transfer / Q000502:physiology*; D008954:Models, Biological; D017785:Photons / Q000627:therapeutic use; D011829:Radiation Dosage; D011874:Radiometry / Q000379:methods*; D012062:Relative Biological Effectiveness; D012542:Scattering, Radiation