Multi-species prostate implant treatment plans incorporating 192Ir and 125I using a Greedy Heuristic based 3D optimization algorithm

The goals of interstitial implant brachytherapy include delivery of the target dose in a uniform manner while sparing sensitive structures, and minimizing the number of needles and sources. We investigated the use of a multi-species source arrangement (192Ir with 125I) for treatment in interstitial prostate brachytherapy. The algorithm utilizes an "adjoint ratio," which provides a means of ranking source positions and is the criterion for the Greedy Heuristic optimization. Three cases were compared, each using 0.4 mCi 125I seeds: case I is the base case using 125I alone, case II uses 0.12 mCi 192Ir seeds mixed with 125I, and case III uses 0.25 mCi 192Ir mixed with 125I. Both multi-species cases result in lower exposure of the urethra and central prostate region. Compared with the base case, the exposure to the rectum and normal tissue increases by a significant amount for case III as compared with the increase in case II, signifying the effect of slower dose falloff rate of higher energy gammas of 192Ir in the tissue. The number of seeds and needles decreases in both multi-species cases, with case III requiring fewer seeds and needles than case II. Further, the effect of 192Ir on uniformity was investigated using the 0.12 mCi 192Ir seeds in multi-species implants. An increase in uniformity was observed with an increase in the number of 0.12 mCi 1921r seeds implanted. The effects of prostate size on the evaluation parameters for multi-species implants were investigated using 0.12 mCi 192Ir and 0.4 mCi 125I, and an acceptable treatment plan with increased uniformity was obtained.