Evaluation of relative biological effectiveness for diseases of the circulatory system based on microdosimetry

Abstract

In the next decade, the International Commission on Radiological Protection (ICRP) will issue the next set of general recommendations, for which evaluation of relative biological effectiveness (RBE) for various types of tissue reactions would be needed. ICRP has recently classified diseases of the circulatory system (DCS) as a tissue reaction, but has not recommended RBE for DCS. We therefore evaluated the mean and uncertainty of RBE for DCS by applying a microdosimetric kinetic model specialized for RBE estimation of tissue reactions. For this purpose, we analyzed several RBE data for DCS determined by past animal experiments and evaluated the radius of the subnuclear domain best fit to each experiment as a single free parameter included in the model. Our analysis suggested that RBE for DCS tends to be lower than that for skin reactions, and their difference was borderline significant due to large variances of the evaluated parameters. We also found that RBE for DCS following mono-energetic neutron irradiation of the human body is much lower than that for skin reactions, particularly at the thermal energy and around 1 MeV. This tendency is considered attributable not only to the intrinsic difference of neutron RBE between skin reactions and DCS but also to the difference in the contributions of secondary γ-rays to the total absorbed doses between their target organs. These findings will help determine RBE by ICRP for preventing tissue reactions.

Keywords: Particle and Heavy Ion Transport code System (PHITS); diseases of the circulatory system (DCS); microdosimetry; radiological protection; relative biological effectiveness (RBE).

Fig. 1

RBE calculated by Eqs. (3)–(5) in comparison with the corresponding experimental data [9–13] as a function of the reference radiation dose. The calculation was performed by setting r_d to and for DCS obtained from this study and those for skin reactions determined by our previous study [5]. The reference radiation of the experimental data taken by Broerse et al. [10] shown in Panel (A) was 300 kV X-ray instead of 200 kVp X-ray.

Fig. 2

Dependence of the calculated RBE for DCS on (A) (α/β)_c and (B) D_t for fission neutrons against ⁶⁰Co γ-rays. The experimental data are taken from Refs. [11, 12]. The values in the parentheses are R² obtained by analyzing all experimental data listed in Table 1.

Fig. 3

Calculated RBE for DCS and skin reactions for the isotropic irradiation of mono-energetic neutrons to the ICRP mesh-type adult male reference phantom. ⁶⁰Co γ-rays served as the reference radiation with the absorbed doses of (A) 0.5 and (B) 5 Gy in the target organs, which were blood and skin for DCS and skin reactions, respectively, for the data shown in this figure. The radiation weighting factor, w_R, assigned to the neutron is also plotted.

Fig. 4

Target-organ dependence of the calculated RBE for DCS for the isotropic irradiation of mono-energetic neutrons to the ICRP mesh-type adult male reference phantom. ⁶⁰Co γ-rays served as the reference radiation with the absorbed doses of (A) 0.5 and (B) 5 Gy in the target organs, which are specified in the parentheses in the legends.