HOW DOES POST-IMPLANT EDEMA IMPACT ON THE URETHRAL DOSE IN PROSTATE BRACHYTHERAPY?
M. Saiful Huq, Frank M. Waterman, and Adam P. Dicker
Supported in part by P30 CA 56036-03 (NCI)

Department of Radiation Oncology, Kimmel Cancer Center, Jefferson Medical College, Thomas Jefferson University Hospital, Philadelphia, PA 19107

Materials and Methods:
A pre-implant CT scan and 2 post-implant CT scans were obtained on 50 patients who received I-125 seed implants. The first post-implant CT scan was obtained on the day of the implant; the second typically 34 to 48 days later (mean: 46 + 23 days; range: 27-135 days). The urethra was outlined on each axial CT image between the base and the apex of the prostate. A Foley catheter identified the urethra in the pre- and first post-implant CT scans, but not in the second post-implant scan. The urethra in the second CT scan was localized from its location in the first post-implant CT scan by displaying the corresponding axial images simultaneously on the computer monitors of two adjacent work stations. The seeds were localized in both post-implant CT scans and a dose-volume-histogram (DVH) of the total dose delivered to the urethra was compiled from each scan. The total dose was calculated by first computing the initial dose rate based on the configuration of the seeds as imaged in the CT scan and then multiplying the initial dose rate by the average life of the radioisotope. The D₁₀ urethral dose, defined as the dose which encompasses only 10% of the urethra, was used to compare the urethral doses indicated by the two CT scans. The relative change in the volume of the prostate during the interval between the two post-implant CT scans was determined by outlining the prostate on each axial CT image and computing the volume of the prostate in each CT study.