A New Approach for Quantifying Radio-Biological Effects Using the Time Course of Mouse Leg Contracture

A digitization approach to the time course of radiation-induced mouse leg contracture was proposed for quantifying the radiation effect on an individual living mouse. The shortening of the mouse leg length can be easily measured with a caliper/ruler to offer a very simple digitalized index of the radiation effect. Left hind legs of mice were irradiated with single dose of 32 Gy of 290 MeV carbon-ion beam using 0, 50, or 117 mm binary filter (BF). The right legs were used as a control. The lengths of both hind legs of the mice were measured using a digital caliper before irradiation and every week after irradiation. The degree of leg contracture, ΔS_t, at the time point t was estimated by subtraction of the left irradiated leg length from the right control leg length. Equation was fitted on the daily time course of ΔS_t, and two parameters, ΔS_max and T_s, were estimated. ΔS_t=ΔS_max×(1-exp(t/T_s)), where ΔS_max is the maximum degree of leg contracture, and T_s is time of leg contracture. The effect of carbon-ion irradiation on a living mouse was quantified by ΔS_max and T_s of the leg contracture, and then compared to that of X-rays. By 32 Gy irradiation, ΔS_max was largest for the BF117 experiment, followed by X-ray~BF50>BF0. T_s was shortest for the BF50 experiment, while other irradiation conditions give similar T_s. A logarithmic function was successfully repurposed for the evaluation of radio-biological response.