Effect of post-implant edema on the rectal dose in prostate brachytherapy

Purpose: To characterize the effect of prostate edema on the determination of the dose delivered to the rectum following the implantation of I-125 or Pd-103 seeds into the prostate. Methods and Materials: From 3 to 5 post-implant computed tomography (CT) scans were obtained on 9 patients who received either I-125 or Pd-103 Seed implants. None of the patients received hormone therapy. The outer surface of the rectum was outlined on each axial CT image from the base to the apex of the prostate. The D-10 rectal surface dose, defined as the dose which encompasses only 10% of the surface area of the rectum, was determined from each CT scan by compiling a dose-surface histogram (DSH) of the rectal surface. The magnitude and half-life of the post-implant edema in each of these implants is known from the results of a previously published study based on the analysis of the serial CT scans. Results: As the prostate edema resolved, the distance between the most posterior implanted seeds and the anterior surface of the rectum decreased. As a result, the D-10 rectal surface dose increased with each successive post-implant CT scan until the edema resolved. The dose increased exponentially at approximately the same rate the prostate volume decreased. The D-10 rectal surface dose at 30 days post-implant ranged from 16% to 190% (mean 68 +/- 50%) greater than on day 0. The dose on day 30 was at least 50% greater in 6 of 9 cases. Conclusion: The rectal surface dose determined by analysis of a post-implant CT scan of an I-125 or Pd-103 prostate seed implant depends upon the timing of the CT scan. The dose indicated by the CT scan on day 30 is typically at least 50% greater than that indicated by the CT scan on day 0. Because of this difference, it is important to keep the timing of the post implant CT in mind when specifying dose thresholds for rectal morbidity, 0 1999 Elsevier Science Inc.