Evaluation of EGS4/PRESTA multiple-scattering algorithms for 90Sr/90Y intravascular brachytherapy dosimetry

Abstract

The purpose of this work is to evaluate the EGS4/PRESTA electronmultiple-scattering (MS) algorithms for dose calculation in intravascularbrachytherapy (IVBT) using a 90Sr/90Y source. The small sourcesize and the small volume of interest in IVBT require very fine spatialresolution, which may break down the constraints of Molière's MS theory asimplemented in EGS4. The theory is accurate only when the electron step sizesare large enough to allow the number of collisions Ω0 to be muchgreater than e = 2.7183. When step sizes are too small to allow at least2.7183 collisions, as may be necessitated by the fine geometry, the algorithmmay switch off MS, producing dosimetric artefacts. This study showed thatswitching off MS could produce a dose deviation of up to 6% when thehalf-thickness (d/2) of the dose scoring region is comparable with theMolière minimum step size (tmin = 2.7183). The effect of switchingoff MS is negligible if d/2>>tmin. For the case of Ω0>e,if the electron step sizes are chosen to allow five to 40 collisions, withincreasing step size, the doses surrounding the source increase and the errordecreases. On the other hand, when larger step sizes are chosen, the dosecalculation voxel size must also be increased in order for the calculations toconverge. A good compromise between accuracy and applicability for IVBTsimulation can be made, if the thickness of the scoring region is 0.1 mm andthe electron step sizes are in the range allowing 10 to 30 collisions.