Blood tritium level as an estimate of soft tissue dose

To test the blood tritium level as an estimate of free water tritium (FWT) and organically bound tritium (OBT) in human soft tissues, a tritiated amino acid mixture (TAA) and tritiated water (HTO) were injected or orally administered to mice. Relatively high tritium retention in the blood was found after TAA injection. This observation was explained by a low rate of clearance of non-volatile tritium components. The dose estimated for the soft tissues after TAA injection was larger than that estimated on the basis of tritium concentration in urine by a factor of 1.8-2.3. In both HTO and TAA injections, the blood tritium level gave a conservative estimate of the dose to soft tissues including active bone marrow. The accumulated dose to soft tissues after oral administration tended to be lower than that for intraperitoneal injection.