Comparisons of measured biological response and predictions from microdosimetric data applicable to brachytherapy

The site model of the theory of dual radiation action (TDRA) has come under question in recent years and has been replaced by the more general track model. The latter reduces to the former if the proximity function is a uniform distribution of energy transfer points and the probability of interaction between two 'sublesions' to produce a lesion is constant. The site diameter is usually assumed to be of the order of a few nanometres. This investigation tests if the conditions of the site model can be met for low LET radiation given a site diameter of 1 μm or more. The linear quadratic response of cells to radiation implies that for low dose and or low dose rate (if repair is viable) that single hit or linear response becomes the dominant process. Because of repair of 'sublesions', at dose rates in brachytherapy (1-7 Gy.h^-1), most cell lines are thought to exhibit linear response and, therefore, potentially offer a test of the TDRA site model if differences in linear energy exist. Single event spectra of ⁶⁰Co, ¹³⁷Cs, ¹⁹²Ir ¹⁶⁹Yb and ¹²⁵I were measured in propane tissue-equivalent (TE) gas at a simulated volume of 1.0 μm in diameter. The ȳ(d)s (and therefore the low dose predicted RBEs) relative to ⁶⁰Co are 1.2, 1.5, 1.7 and 2.0 for ¹³⁷Cs, ¹⁹²Ir, ¹⁶⁹Yb and ¹²⁵I, respectively. ¹⁹²Ir irradiation was selected as a test of predicted RBE from TDRA and cell survival experiments were performed on plateau phase CHO-K1 (repair proficient) and CHO-xrs-5 (repair deficient) cells at dose rates from 0.12 Gy.h^-1 to 4.54 Gy.min^-1. The xrs-5 line showed a linear response and no dose rate effect while the K1 line exhibited linearity at and below 0.3 Gy.h^-1. The RBEs (ratio of the survival curve slopes) of the K1 cells are consistent with those predicted by the site model of the TDRA.