Effect of random seed placement error in permanent transperineal prostate seed implant

Background and purpose: Random seed placement error may adversely effect dose distribution in transperineal prostate seed implants. In this study, we investigated the extent to which individual seed activity influences dose-distribution degradation due to random seed placement error. Patients and methods: Separate initial treatment plans were prepared for three prostate sizes, 27.3, 43.2 and 48.9 cc, using 0.35, 0.55 and 0.75 mCi iodine-125 seeds. All stated activities are understood to be apparent activities. The combinations produced a total of nine treatment plans. Each initial treatment plan was subjected to 1000 stochastic three-dimensional Gaussian perturbations of seed location, with a standard deviation of 4 mm for a total of 9000 treatment plans. The resulting plans were evaluated for target coverage and urethra involvement. Results: Satisfactory initial treatment plans were prepared for all prostate sizes and seed activities. All 9000 perturbed treatment plans showed acceptable target coverage under the D90/90 criterion. Some of the perturbed plans for the 27.3 and 43.2 cc prostates with 0.55 and 0.75 mCi seeds failed the V100/90 criterion. Some of the randomly perturbed seed distributions showed significantly increased doses to the urethra relative to the unperturbed treatment plan. This effect was more pronounced with greater seed activity. Conclusions: There may be a higher probability of unfavorable target coverage due to random seed placement error when performing transperineal iodine-1 25 prostate seed implants using seeds with activity greater than 0.35 mCi. There may also be a higher probability of unfavorable urethra involvement when using higher activity seeds. (c) 2006 Elsevier Ireland Ltd. All rights reserved.