A VERSATILE METHOD FOR PLANNING STEREOTAXIC BRAIN IMPLANTS

Our approach to planning stereotactic I-125 brachytherapy of brain tum ors has involved least-squares optimization of individual seed positio ns within the target contour, followed by repeated combining of seeds from nearest-neighbor catheters in order to achieve an acceptably low number of catheters and an acceptable separation of entry points. In o ne option, the catheters diverge from an extra-cranial point that can be close to the skull if all catheters are to be placed through a smal l craniectomy to treat a larger-diameter target. In another option, ca theters converge toward a point beyond the target, to facilitate perpe ndicularity at the skull surface if a separate opening is to be drille d for each catheter. In either case, the fact that seed orientations a re known, permits including anisotropy in dose calculations. Trial see d locations are constrained to a target region defined on a 1-mm mesh, both in the initial optimization of single-seed catheters and in subs equent combinations followed by tune-up optimizations. In the optimiza tion process, sum-of-squares contributions are weighted more heavily w hen the dose rate is lower than the target dose rate; the weighting im balance falls short of keeping all target points above the target dose rate and requires targeting on a dose rate about 25% higher than the desired minimum dose rate.