Abstract
Thorotrast, a colloidal solution of thorium dioxide, has been widely used as a radiological contrast medium for the vascular system etc. Its radioactivity and its retention by the reticulo-endothelial system make it potentially hazardous and its study is important in assessing the effect of radiation on human beings. Because of the radiometric properties of the thorium series, and the tendency for the longer-lived daughter-products to be treated by the bloodstream and the body according to their different chemical properties, several reinterpretations of published measurements of radioactivity of thorotrast patients have become necessary. These are discussed in the light of recent results, and some measurements of concentrations of daughter-products in vivo are reported.