Calculation of the Transit Dose in HDR Brachytherapy Based on Monte Carlo Simulations

Abstract

The Monte Carlo method, which is the gold standard for accurate dose calculations in radiotherapy, was used to obtain the transit doses around a high dose rate (HDR) brachytherapy implant with thirteen dwell points. The midpoints of each of the inter-dwell separations, of step size 0.25 cm, were representative of the positions of the dynamic brachytherapy source, so the average dose rates were calculated when the geometrical centre of the moving source was at each of these points. The dose rates were then transformed into transit doses and our output compared to outputs from other methods.
There were especially high discrepancies between our results and the Sievert Integral results, ranging from –32 to –21% for the examples considered. The discrepancies could be much higher for implants of longer lengths where contributions from points near the longitudinal axis of the source become more important. These translate into errors of more than 4 % in the prescribed dose and could have significant impact on complication free tumour control. The Monte Carlo method is therefore recommended for the evaluation of transit doses around brachytherapy implants. Other methods which do not exhibit the same level of accuracy as the Monte Carlo method must be used with great caution.

Journal of the Ghana Science Association Vol. 10 (2) 2008: pp. 134-138