Abstract
Thorotrast, a colloidal solution of thorium dioxide, is widely used as a radiological contrast medium for the vascular system, etc. Its radioactivity and retention by the reticulo-endothelial system make it potentially hazardous. 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, reinterpretations of published measurements of radioactivity of thorotrast patients are necessary. These are discussed in the light of recent results, and some measurements of concentrations of daughter-products in vivo are reported.