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

The purpose of this work is to evaluate the EGS4/PRESTA electron multiple-s cattering (MS) algorithms for dose calculation in intravascular brachythera py (IVBT) using a Sr-90/Y-90 source. The small source size and the small vo lume of interest in NET require very fine spatial resolution, which may bre ak down the constraints of Moliere's MS theory as implemented in EGS4. The theory is accurate only when the electron step sizes are large enough to al low the number of collisions Omega(0) to he much greater than e = 2.7183. W hen step sizes are too small to allow at least 2.7183 collisions, as may be necessitated by the fine geometry, the algorithm may switch off MS, produc ing dosimetric artefacts. This study showed that switching off MS could pro duce a dose deviation of up to 6% when the half-thickness (d/2) of the dose scoring region is comparable with the Moliere minimum step size (t(min) = 2.7183). The effect of switching off MS is negligible if d/2 much greater t han t(min). For the case of Omega(0) > e, if the electron step sizes are ch osen to allow five to 40 collisions, with increasing step size, the doses s urrounding the source increase and the error decreases. On the other hand, when larger step sizes are chosen, the dose calculation voxel size must als o be increased in order for the calculations to converge. A good compromise between accuracy and applicability for IVBT simulation can be made, if the thickness of the scoring region is 0.1 mm and the electron step sizes are in the range allowing 10 to 30 collisions.