The Relative Biological Effect of High-Z, High-LET Charged Particles for Spermatogonial Killing

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Abstract
The testis weight loss at 28 days after irradiation was used to assess the relative biological effectiveness [RBE] of high-energy heavy ions for cell killing of mouse spermatogonia. The testis weights are corrected for the radiation-insensitive fraction. With this model the data were found to fit an exponential survival curve with no shoulder. For all ions the extrapolation number is 1.0. For 60Co radiation the extrapolation number is 1.0 and the D0 [mean lethal dose] is 107 rad. All ions have killing effectiveness higher than 60Co radiation except nonstopping He ions. The radiation modalities used were 228 MeV/amu [atomic mass unit] He ions and 400-670 MeV/amu C, Ne and Ar ions. The RBE for spermatogonial killing was a maximum of 3.0 for plateau Ar ions. Both Ne and Ar ions had reduced effectiveness if the LET [linear energy transfer] exceeded 100 keV/.mu.m. One of the most significant observations made in these studies is that LET alone does not predict the effectiveness of the various ions. Large difference in effectiveness are seen as a function of atomic number of the particle, even though LET is held approximately constant.