Effect of Foley catheters on seed positions and urethral dose in I-125 andPd-103 prostate implants

Purpose: To estimate the perturbation of seed position and urethral dose, s ubsequent to withdrawal of urethral catheters. Methods and Materials: A mathematical model based on the volume incompressi bility of tissues was used to compute seed positions and doses following re moval of the Foley. The model assumed that the central axis of the urethra remains stationary, and that prostate tissue and seeds move radially toward the center of the urethra to fill the void left by the catheter. Seed moti on has also been measured using transrectal ultrasound. Results: Based on the computations, seeds located originally close to the u rethra travel relatively large distances toward the urethra upon Foley remo val, whereas seeds located further away move substantially less. This seed motion leads to higher urethral doses than shown in a standard treatment pl an. Dose enhancements increase with catheter size, decrease,vith increasing prostate volume, are more pronounced for Pd-103 than for I-125, and range between 3.5% and 32.4%. Postimplant dosimetry is equally affected if images are taken with urethral catheters in plate, showing lower urethral doses t han actually delivered. Preliminary ultrasound based measurements of seed m otion agree with the theory. Conclusion: During the implantation procedure, 12 fr or smaller urethral ca theters are preferable to larger diameter catheters if urine drainage is su fficient. Treatment planners should avoid planning seeds at 5 mm or closer from the urethra. Special caution is indicated in prostates having about 20 cm(3) or smaller volumes, and when Pd-103 is used. Postimplant dosimetry i s susceptible to the same errors. (C) 2001 Elsevier Science Inc.