A SEMIEMPIRICAL APPROACH TO THE EVALUATION OF THE RELATIVE BIOLOGICALEFFECTIVENESS OF THERAPEUTIC PROTON-BEAMS - THE METHODOLOGICAL FRAMEWORK

We address the problem of the evaluation of the relative biological ef fectiveness (RBE) of therapeutic proton beams for cell inactivation. W e consider a general approach to the evaluation of the lethal effect o f protons which is applicable to different situations, including those in which an extended Bragg peak is obtained via a modulated beam. Our approach combines two kinds of information: (1) the experimental resu lts available in the literature for the response of Chinese hamster V7 9 cells to monoenergetic beams and (2) the energy spectrum of the beam in the target volume, computed through a Monte Carlo algorithm. We ha ve applied this method to a simple Bragg peak produced by a broad-fiel d proton beam of about 70 MeV initial energy, such as those that, afte r attenuation, are typically used for treatment of ocular tumors. We h ave found that the RBE increases with depth, even beyond the Bragg pea k, up to a value close to 2, when evaluated at the same surviving frac tion as that resulting after exposure to 2 Gy of X rays. (C) 1997 by R adiation Research Society.