Tissue Dosage from Thorotrast in the Body

Abstract

DURING the past twenty-five years a large number of people have been injected with thorotrast for radiographic purposes. Thorotrast is a colloidal solution of thorium oxide which, after injection, is rapidly taken up by the reticulo-endothelial system, particularly the liver and spleen, where the thorium remains practically indefinitely, and only some of its products may be excreted. In the course of time the action of the radiations may give rise to certain clinical symptoms¹, and this fact offers an opportunity for studying the correlation between the clinical effects of radiations and the dosage delivered to tissue. The amount of thorotrast in the given organ is often estimated from a measurement of the γ-ray activity, mainly due to thorium-C″, observed with a Geiger–Müller or scintillation counter². Since, however, more than 90 per cent of the total energy dissipated in tissue is carried by the α-particles, which have a very short range, it is clear that a quantitative study of the radiation dosage requires a technique employing microscopic analysis. Furthermore, it is known³ that thorotrast in the body forms aggregates of size similar to, or even greater than, the range of the α-particles; some of the energy of the α-particles is thus absorbed in the thorotrast itself, and in order to determine the energy dissipated in tissue this self-absorption has to be taken into account.