Thorotrast and in vivo thorium dioxide: Numerical simulation of 30 years of α radiation absorption by the tissues near a large compact source

Abstract

BackgroundThe epidemiology of the slightly radioactive contrast agent named Thorotrast presents a very long latency period between the injection and the development of the related pathologies. It is an example of the more general problem posed by a radioactive internal contaminant whose effects are not noteworthy in the short term but become dramatic in the long period. A point that is still to be explored is fluctuations (in space and time) in the localized absorption of radiation by the tissues. MethodsA Monte Carlo simulation code has been developed to study over a 30-year period the daily absorption of α radiation by μm-sized portions of tissue placed at a distance of 0–100 μm from a model source, that approximates a compact thorium dioxide source in liver or spleen whose size is ≳20μm. The biological depletion of the daughter nuclei of the thorium series is taken into account. The initial condition assumes chemically purified natural thorium. ResultsMost of the absorbed dose is concentrated in a 25-μm thick layer of tissue, adjacent to the source boundary. Fluctuations where a target region with a volume of 1 μm3 is hit by 3–5 α particles in a day or in a shorter period of time are relevant in a 1–10 μm thick layer of tissue adjacent to the source boundary, where their frequency is larger than the Poisson-law prediction.