Retention and Skeletal Dosimetry of Injected 226 Ra, 228 Ra, and 90 Sr in Beagles

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Abstract
Total-body retention was determined by in vivo counting in beagles injected with 226Ra (125 dogs), 228Ra (78 dogs) or 90Sr (96 dogs). The radionuclides were administered i.v. in a citrate buffer solution when the animals were about 1.4 yr of age. Retention measurements now extend to more than 14 yr after injection; future revision of these retention equations appears unlikely since the median lifespan in control beagles is about 12.7 yr. Dogs injected with about 10 .mu.Ci 226Ra/kg had a higher long-term retention than those receiving 0.04 to about 4 .mu.Ci 226Ra/kg. This was interpreted as a consequence of radiation damage to the bone remodeling processes at high 226Ra concentrations. Total-body biological retention [R(%)] at t days could be described by the following functions: 226Ra, for 1400 days after the injection of 10 .mu.Ci/kg: R(%) = 41e-0.56t + 10e-0.073t + 20e-0.0049t + 29e-0.00030t, 226Ra, for 5200 days after the injection of 0.04 to 4 .mu.Ci/kg: R(%) = 25e-0.98t + 21e-0.27t + 21e-0.016t + 18e-0.0020t + 15e-0.00015t, 90Sr for 5000 days after injection: R(%) = 36e-0.95t + 29e-0.12t + 10e-0.0091t + 12e-0.0019t + 13e-0.00015t. The 228Ra retention data were not analyzed numerically, but visual inspection of the curves prepared for individual dogs indicated that 228Ra retention could be described adequately by the retention equation derived for dogs given 0.04 to 4 .mu.Ci 226Ra/kg. The average skeletal radiation dose rate and cumulative radiation dose were calculated for each beagle injected with 226Ra or 228Ra or 90Sr using the appropriate retention equation, normalized to each dog''s individual retention measurements. Allowance was made in the 226Ra calculations for the changing skeletal retention of 222Rn and its radioactive daughters; in the 228Ra calculations for the diminution of 228Ra activity by radioactive decay as well as for the ingrowth and skeletal retention of 228Th and its radioactive daughters and in the 90Sr calculations for the diminution of 90Sr concentration by radioactive decay and the escape of .beta.-energy from bone. The half-time (4621 days) of the longest-lived component in the 90Sr retention equation was similar to that for the lower-level Ra dogs. This may reflect the net remodeling rate of cortical bone in the beagle skeleton.

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