  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 alpha radiation by micrometer sized portions of tissue placed at
a distance of 0-100 micrometers from a model source, that approximates a
compact thorium dioxide source in liver or spleen whose size is larger or equal
to 20 micrometers. 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 micrometer thick layer of tissue, adjacent to the source boundary.
Fluctuations where a target region with a volume of 1 cube micrometer is hit by
3-5 alpha particles in a day or in a shorter period of time are relevant in a
1-10 micrometer thick layer of tissue adjacent to the source boundary, where
their frequency is larger than the Poisson law prediction.
