RELATIVE BIOLOGICAL EFFECTIVENESS OF ALPHA-PARTICLE EMITTERS IN-VIVO AT LOW-DOSES

The therapeutic potential of radionuclides that emit alpha particles, as well as their associated health hazards, have attracted considerabl e attention. The Ra-224 daughters Pb-212 and Bi-212, by virtue of thei r radiation properties which involve emission of a and beta particles in their decay to stable Pb-208, have been proposed as candidates for radioimmunotherapy. Using mouse testes as the experimental model and t esticular spermhead survival as the biological end point, the present work examines the radiotoxicity of Pb-212 and its daughters. When Pb-2 12, in equilibrium with its daughters Bi-212, Po-212 and Tl-208, was a dministered directly into the testis, the dose required to achieve 37% survival (D-37) was 0.143 +/- 0.014 Gy and the corresponding RBE of t he mixed radiation field was 4.7 when compared to the D-37 for acute e xternal 120 kVp X rays. This datum, in conjunction with our earlier re sults for Po-210, was used to obtain an RBE-LET relationship for a par ticles emitted by tissue-incorporated radionuclides: RBE(alpha) = 4.8 - 6.1 x 10(-2) LET + 1.0 x 10(-3) LET(2). Similarly, the dependence of RBE on alpha-particle energy E(alpha) was given by RBE, = 22 E(alpha) (-0.73). These relationships, based on in vivo experimental data, may be valuable in predicting biological effects of alpha-particle emitter s.