Electron dose reduction coefficients for seven radionuclides and cylindrical geometry

In determining internal radiation absorbed dose it is frequently not possible to assume a uniform distribution of radionuclide in an infinite, homogeneous, absorbing material. When the source-target geometry is such that the target is source-free, special considerations must be given to the dose contributions from electrons, since the conventional assumption of 100% absorption of non-penetrating radiations leads to overestimation of the absorbed dose. The absorbed dose from electrons determined at a point within the source-free region depends upon the depth within it. Electron dose reduction coefficients determined at appropriate distances from the source surface are multiplied by the electron doses determined from the general absorbed dose equation. This method of correcting for source-free regions has been applied to cylindrical and planar geometry, for spinal cord and nerve root dosimetry in cisternography. Utilising published scaled point kernels for mono-energetic electrons, electron dose reduction coefficients have been determined as a function of depth from the source surface for cylindrical source-free regions of radii 0.5, 0.05 cm and infinity, and seven gamma-ray-emitting radionuclides: 51Cr, 67Ga, 99Tcm, 111In, 113Inm, 169Yb and 203Pb. These values may be used in other internal radiation absorbed dose situations, and similar techniques applied to other source-target geometries.