Inverse planning for HDR prostate brachytherapy used to boost dominant intraprostatic lesions defined by magnetic resonance spectroscopy imaging.

PURPOSE: To dose escalate selected regions inside the prostate without compromising the dose coverage of the prostate and the protection to the urethra, rectum, and bladder for prostate cancer patients treated with high-dose-rate brachytherapy. METHODS AND MATERIALS: Magnetic resonance imaging combined with magnetic resonance spectroscopy imaging was used to differentiate between normal and malignant prostate and define cancer-validated dominant intraprostatic lesions (DIL) on 10 patients. The DILs were then contoured on the planning scans (CT or MRI based, 5 patients each), and our inverse planning dose optimization algorithm (called IPSA) was used to generate dose distributions for 3 different boost levels. Dose-volume histograms of the target and each organ at risk were compared with optimized plans without DIL boost.
RESULTS: Combined MRI/magnetic resonance spectroscopic imaging identified 2 DILs in 8/10 of the 10 patients studied and a single DIL in the remaining 2 patients. The average prostate dose coverage V100 was 97% (sigma = 1.0%). When the minimum DIL dose requested was 120% of the prescribed dose, the average DIL V120 was 97.1% (sigma = 1.8%). For a boost value of 150%, the average V150 ranged from 77.8% to 86.1%, depending on the upper limit of the dose constraints. The bladder V50 increased by 1%, independently of the boost levels. The absolute increases in V50 for the rectum varied from 1% to 3%, depending on the boost level. The urethra V120 were increased by 13.4% and 32.5% for the lowest and highest boost levels, respectively.
CONCLUSION: The DIL dose can be escalated to a minimum of 120% while the entire prostate is treated simultaneously, without increasing the dose to surrounding normal tissues. Higher boost levels between 150% and 170% are feasible, but with slightly larger doses delivered to the rectum and urethra.