Microdosimetric Modeling of Relative Biological Effectiveness for Skin Reactions: Possible Linkage Between In Vitro and In Vivo Data.

Precise evaluation of the relative biological effectiveness (RBE) for skin reactions is indispensable for the treatment planning of particle therapy and boron-neutron capture therapy. The evaluation is also needed for the future radiation protection system for mixed radiation fields. Such RBE is often evaluated based on in vitro cell survival data, but its validity remains incompletely understood. This study aimed to develop a model for estimating RBE for skin reactions and dermal cell survival in the same framework and quantitatively discuss a possible link between them. The microdosimetric kinetic model, which was originally developed for estimating cell surviving fractions for various radiations, was improved to be capable of estimating the mean and uncertainty of RBE for skin reactions. The parameter used in the model was independently determined from in vitro measurements of dermal cell survival and in vivo measurements of skin reactions taken from 8 and 23 articles, respectively. In the parameter determination, the characteristics of the radiation fields employed in each measurement were reproduced in detail by the Particle and Heavy Ion Transport Code System. Our model quantitatively revealed that RBE for skin reactions tend to be higher than those for dermal cell survival. RBE of various monoenergetic radiations calculated from this model confirmed that the past evaluations made by the International Commission on Radiological Protection and the National Council on Radiation Protection and Measurements a few decades ago are still supported by recent experimental data. Our model can play important roles not only in medical physics for avoiding unnecessary skin reactions in particle therapy and boron-neutron capture therapy but also in radiation protection for future decision-making of the recommended RBE values.

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