Dosimetry considerations of bone-seeking radionuclides for marrow ablation

Marrow ablation by radionuclide therapy for patients requiring bone marrow transplantation is possible by injecting bone-seeking radiopharmaceuticals. For each radionuclide under consideration, one should determine the (1) amount of activity required to deliver a given radiation dose to 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 questions, dose estimates for several radionuclides of interest (32P, 90Y, 153Sm, 166Ho, 186Re, and 188Re) have been calculated. All doses are derived following the MIRD methodology. Biodistribution data of 153Sm-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 doses to other organs. Skeletal retention data measured from a diagnostic dose can be used to calculate the activity required to deliver a desired marrow ablation dose consistent with toxicity limits set by other critical organs.