The Comparative Toxicity of²²⁶ Ra,²³⁹ Pu,²²⁸ Th,²²⁸ Ra, and⁹⁰ Sr to Leukocytes of Beagles

The effectiveness of five internal emitters in depressing blood leukocytes in young adult beagles was studied during the first year postinjection. The radionuclides,²²⁶ Ra,²³⁹ Pu,²²⁸ Th,²²⁸ Ra, and⁹⁰ Sr, were given as a single intravenous injection, at 6-7 injection levels per radionuclide. The occurrence of dose-dependent depressions in leukocytes has been previously established. The greatest depression was found in polymorphonuclear leukocytes (pmns), monocytes, and eosinophils while lymphocytes were depressed to a lesser extent. The probit transform was used to linearly relate response to the logarithm of the injected activity. Injected activities required to produce a 50% leukocyte depression ( ED₅₀) were computed for eight time periods. Toxicity indices were also computed for four radionuclides relative to²²⁶ Ra as a standard and converted to hematological RBE, again relative to²²⁶ Ra. For pmns and lymphocytes median values of RBE estimated over the eight time periods were:${}^{228}{\rm Th}\sim 4$;${}^{239}{\rm Pu}\sim 4$;${}^{228}{\rm Ra}\sim 1.5$, and${}^{90}{\rm Sr}\sim 0.5$. Alpha emitters depositing on bone surfaces were more effective in reducing leukocytes than those which are volume seekers. The beta emitter,⁹⁰ Sr, was less effective than the alpha emitters rad for rad.