DOSIMETRY CONSIDERATIONS OF BONE-SEEKING RADIONUCLIDES FOR MARROW ABLATION

Marrow ablation by radionuclide therapy for patients requiring bone ma rrow transplantation is possible by injecting bone-seeking radiopharma ceuticals. For each radionuclide under consideration, one should deter mine the (1) amount of activity required to deliver a given radiation dose to the marrow, (2) waiting period before reinfusion to limit the radiation dose to the transplant marrow to an acceptable level, and (3 ) dose to other critical organs. In an attempt to answer these questio ns, dose estimates for several radionuclides of interest (P-32 Y-90, S m-153, Ho-166, Re-186, and Re-188) have been calculated. All doses are derived following the MIRD methodology. Biodistribution data of Sm-15 3-labeled phosphates in the rat are used to estimate uptake of similar radiopharmaceuticals in humans. Typically the skeleton retains 50% of the injected activity and 50% is excreted rapidly through the urine, permitting delivery of ablation doses to the marrow, with tolerable do ses to other organs. Skeletal retention data measured from a diagnosti c dose can be used to calculate the activity required to deliver a des ired marrow ablation dose consistent with toxicity limits set by other critical organs.