Investigations into the Optimization of Multi-Source Strength Brachytherapy Treatment Procedures

The goal of this project is to investigate the use of multi-strength and multi-specie radioactive sources in permanent prostate implant brachytherapy. In order to fulfill the requirement for an optimal dose distribution, the prescribed dose should be delivered to the target in a nearly uniform dose distribution while simultaneously sparing sensitive structures. The treatment plan should use a small number of needles and sources while satisfying the treatment requirements. The hypothesis for the use of multi-strength and/or multi-specie sources is that a better treatment plan using fewer sources and needles could be obtained than by treatment plans using single-strength sources could reduce the overall number of sources used for treatment. We employ a recently developed greedy algorithm based on the adjoint concept as the optimization search engine. The algorithm utilizes and ''adjoint ratio'', which provides a means of ranking source positions, as the pseudo-objective function. It ha s been shown that the greedy algorithm can solve the optimization problem efficiently and arrives at a clinically acceptable solution in less than 10 seconds. Our study was inclusive, that is there was no combination of sources that clearly stood out from the others and could therefore be considered the preferred set of sources for treatment planning. Source strengths of 0.2 mCi (low), 0.4 mCi (medium), and 0.6 mCi (high) of sup 1 sup 2 sup 5 I in four different combinations were used for the multi-strength source study. The combination of high- and medium-strength sources achieved a more uniform target dose distribution due to few source implants whereas the combination of low-and medium-strength sources achieved better sparing of sensitive tissues including that of the single-strength 0.4 mCi base case. sup 1 sup 2 sup 5 I at 0.4 mCi and sup 1 sup 9 sup 2 Ir at 0.12 mCi and 0.25 mCi source strengths were used for the multi-specie source study. This study also proved inconclusiv supposing a combination of two 0.12 mCi sup 1 sup 9 sup 2 Ir sources and sup 1 sup 2 sup 5 I sources or a combination of two 0.25 mCi sup 1 sup 9 sup 2 Ir sources and sup 1 sup 2 sup 5 I sources did not have better target and sensitive structures DVHs than that of the single specie sup 1 sup 2 sup 5 I 0.4 mCi base case. However, because of the high dose delivered by a sup 1 sup 9 sup 2 Ir source, the multi-specie treatment plan required fewer sources and needles and hence is less invasive than the treatment plan for the 0/4 mCi sup 1 sup 2 sup 5 I base case.