Effects of Thorotrast in humans

Thorotrast, a patented radiological contrast medium that was widely used from 1930-1950, is a thorium dioxide colloid that has been neutralized and stabilized by protective colloids, which are decomposition products of starch, and which are now generally known as dextrans. The deposition of Thorotrast depends on its radiological use, its method of preparation, and the age of the preparation; however, the major site is the reticulo-endothelial system, where it is retained for long times. Some of its decay products, principally 228Ra and 224Ra, escape from the colloidal particles and deposit in the skeleton. The biological end-points that have been observed in the several human populations that are known to have received Thorotrast are Thorotrastomas, malignant hepatic neoplasms, and other neoplasms of the reticulo-endothelial system (RES), skeletal sarcomas, and leukemias in excess of the number expected. The question to be reviewed in this paper is whether irradiation, or chemical and mechanical effects on the reticulo-endothelial system, or a combination thereof, is the causative factor in the occurrence of these RES neoplasms.