Treatment planning for prostate implants using magnetic-resonance spectroscopy imaging

Purpose: Recent studies have demonstrated that magnetic-resonance spectrosc opic imaging (MRSI) of the prostate may effectively distinguish between reg ions of cancer and normal prostatic epithelium. This diagnostic imaging too l takes advantage of the increased choline plus creatine versus citrate rat io found in malignant compared to normal prostate tissue. The purpose of th is study is to describe a novel brachytherapy treatment-planning optimizati on module using an integer programming technique that will utilize biologic -based optimization. A method is described that registers MRSI to intraoper ative-obtained ultrasound images and incorporates this information into a t reatment-planning system to achieve dose escalation to intraprostatic tumor deposits. Methods: MRSI was obtained for a patient with Gleason 7 clinically localize d prostate cancer. The ratios of choline plus creatine to citrate for the p rostate were analyzed, and regions of high risk for malignant cells were id entified. The ratios representing peaks on the MR spectrum were calculated on a spatial grid covering the prostate tissue. A procedure for mapping poi nts of interest from the MRSI to the ultrasound images is described. An int eger-programming technique is described as an optimization module to determ ine optimal seed distribution for permanent interstitial implantation. MRSI data are incorporated into the treatment-planning system to test the feasi bility of dose escalation to positive voxels with relative sparing of surro unding normal tissues. The resultant tumor control probability (TCP) is est imated and compared to TCP for standard brachytherapy-planned implantation. Results: The proposed brachytherapy treatment-planning system is able to ac hieve a minimum dose of 120% of the 144 Gy prescription to the MRS positive voxels using I-125 seeds. The preset dose bounds of 100-150% to the prosta te and 100-120% to the urethra were maintained. When compared to a standard plan without RIBS-guided optimization, the estimated TCP for the MRS-optim ized plan is superior. The enhanced TCP was more pronounced for smaller vol umes of intraprostatic tumor deposits compared to estimated TCP values for larger lesions. Conclusions: Using this brachytherapy-optimization system, we could demonst rate the feasibility of RMS-optimized dose distributions for I-125 permanen t prostate implants. Based on probability estimates of anticipated improved TCP, this approach may have an impact on the ability to safely escalate do se and potentially improve outcome for patients with organ-confined but agg ressive prostatic cancers. The magnitude of the TCP enhancement, and theref ore the risks of ignoring the MR data, appear to be more substantial when t he tumor is well localized; however, the gain achievable in TCP may depend quite considerably on the MRS tumor-detection efficiency. (C) 2000 Elsevier Science Inc.