Cellular study of the energy delivered by electrons derived from 99mTc, 123I, 111In and 201Tl in a point-source model. Comparison with electromagnetic radiation

The analytical functions proposed by Prestwich and Kwok and modelizing energy loss by electrons were set to run on an IBM PC microcomputer. They were then applied to the main radioisotopes used for medical diagnostics and emitting single-energy electrons (99mTc, 123I, 111In and 201Tl). The curves representing energy deposition by electrons from a given isotope were drawn as a function of the distance from the point source, and revealed a large degree of heterogeneity of energy deposition along their paths. The energy absorbed in a sphere with a radius r around the point source was calculated, together with the ratio P (%) between this energy and the total energy emitted by the electrons. For a sphere with a radius of about 5μm (the size of a cell), P represents at least 14% of the energy emitted by the electrons for the isotopes studied. This represents a minimum of 2.6 keV per disintegration for 99mTc and a maximum of 11.3 keV for 201Tl in a volume similar to that of a cell; the contribution of photons was negligible