Dose verification with Monte Carlo technique for prostate brachytherapy implants with (125)I sources.

The Monte Carlo technique has been implemented to generate the dose distributions in a model prostate patient, implanted with iodine-125 ((125)I) brachytherapy sources. The results of these calculations are also compared with the dose distributions calculated by a commercially available treatment planning system. The comparison shows that with the source strength suggested by the Monte Carlo technique, the current clinical planning system is found to provide 100% coverage of the prostate with the prescription dose for the same implant pattern. However, the dose-volume histogram of this investigation shows that the VariSeed treatment planning system has a 29% and 136% larger dose coverage for the 150% and 200% isodose lines, respectively, than the Monte Carlo simulation. These differences are attributed to the oversimplification of the current planning system using the point source approximation, and also to the interseed effects from multisources that are neglected in the conventional planning systems. The results of this study provide evidence that supports the use of the Monte Carlo technique in treatment planning systems to provide accurate dose calculations in brachytherapy implants.