OPTIMIZATION OF INTERSTITIAL VOLUME IMPLANTS

For interstitial applications of high dose rate (HDR) afterloading bra chytherapy, generally a single stepping iridium-192 source is used, en abling optimization of the dose distribution by optimization of the re lative time (dwell time) that the source remains at a certain position (dwell position). We analysed the effects of geometric optimization i n a regular volume implant, with strictly parallel catheters, and in a n irregular volume implant, such as an implant for tumours of the base of the tongue characterized by a non-parallel geometry and varying ca theter separations. In both examples the reference dose is specified a t 85% of the mean central dose (as is done in the Paris system for dos e specification) in the non-optimized as well as the optimized plan. T he irradiated volume, the dose uniformity, and the choice of the refer ence dose of optimized and non-optimized dose distributions were compa red. This was done by isodose plots for representative planes, volume dose histograms (distributed, contiguous, and natural), and dose non-u niformity ratios (DNRs). For the regular implant, optimization results in a 28% increase in the treated volume with a similar increase in th e overdosed volumes. In order to keep the treated volume comparable wi th the non-optimized dose distribution, 90-95% of the mean central dos e should be chosen as a reference dose or the range of active dwell po sitions should be shortened in case of optimization. In the case of th e irregular volume implant at the base of the tongue, the method for d ose specification should be kept unchanged after geometric optimizatio n as the volume enclosed by the reference isodose does not increase. I t is clear from the volume-dose histograms that there is a reduction o f the overdosed volume due to optimization. This is accompanied by an increase in the uniformity index and a decrease of the DNR. In conclus ion, geometric optimization appears to be an effective tool to improve the dose distribution of interstitial volume implants. Contiguous and natural volume dose histograms appear, apart from planar dose plots, valuable methods for evaluating the dose distribution of an implant.