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

BACKGROUND: The 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. METHODS: A 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 ≳20 μm. The biological depletion of the daughter nuclei of the thorium series is taken into account. The initial condition assumes chemically purified natural thorium. RESULTS: Most 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 μm(3) 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.