Radiotoxicity of gadolinium-148 and radium-223 in mouse testes: Relative biological effectiveness of alpha-particle emitters in vivo

The biological effects of radionuclides that emit α particles are of considerable interest in view of their potential for therapy and their presence in the environment. The present work is a continuation of our ongoing effort to study the radiotoxicity of α-particle emitters in vivo using the survival of murine testicular sperm heads as the biological end point. Specifically, the relative biological effectiveness (RBE) of very low- energy α particles (3.2 MeV) emitted by ¹⁴⁸Gd is investigated and determined to be 7.4 ± 2.4 when compared to the effects of acute external 120 kVp X rays. This datum, in conjunction with our earlier results for ²¹⁰Po and ²¹²Pb in equilibrium with its daughters, is used to revise and extend the range of validity of our previous RBE-energy relationship for α particles emitted by tissue-incorporated radionuclides. The new empirical relationship is given by RBEα = 9.14 - 0.510 Eα, where 3 < Eα < 9 MeV. The validity of this empirical relationship is tested by determining the RBE of the prolific α-particle emitter ²²³Ra (in equilibrium with its daughters) experimentally in the same biological model and comparing the value obtained experimentally with the predicted value. The resulting RBE values are 5.4 ± 0.9 and 5.6, respectively. This close agreement strongly supports the adequacy of the empirical RBE-Eα relationship to predict the biological effects of α-particle emitters in vivo.