Thorotrast kinetics and radiation dose: results from studies in thorotrast patients and from animal experiments

As a presupposition for estimating the mean tissue dose from intravascularly injected Thorotrast results of investigations on tissue distribution and steady state activity ratios of 232Th and daughters in Thorotrast patients were compiled and are presented as "best estimates". Special emphasis has been given to the non-uniformity of Thorotrast distribution on the organ and cellular level on the basis of results from animal experiments. Moreover, the variation widths of the mean tissue doses were calculated from the individual standard errors of the mean Thorotrast tissue distribution and activity ratios. According to the results of Thorotrast tissue distribution analyses about 97% of intravascularly injected colloidal ThO2 are retained by the organs of the reticulo-endothelial-system (RES) of the average Thorotrast patient (liver: 59%; spleen: 29%; bone marrow: 9%). Only 0.7 and 0.1% are distributed within the lungs and the kidneys, respectively. The fractional retention of 232Th in the marrow-free skeleton proved to be 2% on the average. Considering in addition the results on the steady state activity ratios between 232Th and its daughters and self-absorption of alpha-energy in Thorotrast agglomerates the mean annual tissue doses to the liver, spleen, red bone marrow, lungs (respiratory zone), and cells on bone surface, e.g., from 30 ml intravascularly injected Thorotrast are about 30 (10-70), 80 (30-200), 10 (4-27), 4.5 (1.8-11.3), and 15 (6-38) rad. The variation widths of the mean tissue doses given in brackets are based upon an average individual standard error of the mean Thorotrast tissue distribution and activity ratios of 150%. The data on mean tissue doses, however, do not include variations of the dose due to macroscopic inhomogeneities of Thorotrast distribution on the organ level, which in the liver may go up to a factor of 50. Contrary to the mean tissue dose the local annual dose, i.e., the dose to cells adjacent to the surface of 0.1-50 micron Thorotrast aggregates is between 40 and 40,000 rad.