Experiments were performed to examine the influence of energy distribution at the cellular level on the relative biological effectiveness (RBE) of internally deposited alpha particle emitters. Cultured Chinese hamster ovary (CHO-K1) cells were irradiated in vitro by insoluble ceramic microspheres of zirconium oxide labelled with 239Pu. The absorbed dose and dose rate were held constant at 0.7 Gy and 0.17 Gy.h-1, respectively. The variation in local dose was achieved either by using stationary microspheres of two different specific energies or by gently rocking these spheres during irradiation. Relative biological effectiveness was evaluated using three endpoints: initial cell survival, mutation frequency, and primary DNA damage. RBEs for 239Pu alpha particles relative to 60Co gamma radiation varied from 1 to about 6, depending upon the local distribution of energy imparted at the cellular level. Cell death was greatest for irradiations by sources of low specific activity. Mutation frequency was greatest for cells exposed to agitated (more uniformly distributed alpha radiation) sources and least for irradiation by stationary high-specific-activity sources (“hot particles”). The amount of DNA damage increased as the alpha radiation was more uniformly distributed amongst the cells at risk.
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Fisher et al. (1985) studied this question.