Effect of edema associated with 131Cs prostate permanent seed implants on dosimetric quality indices

This study was designed to investigate the effect of prostatic edema on various dosimetric quality indices following transperineal permanent 131Cs seed implant. Thirty-one patients with early prostate cancer, who received 131Cs permanent seed implant, were included in this study. Each patient received a prescribed dose of 115 Gy from the implant. Transrectal ultrasound (U.S.) was used to measure the preimplant prostate volume and pre- and postneedle implant volumes, and postimplant CT images were used to obtain postimplant prostate volumes at days 0, 14, and 28 for all patients. The magnitude of edema was determined by comparing the preneedle and postimplant prostate volumes, which was used to compute the half life of the edema using the least-squares method. Dose volume histograms were generated for each set of volumes to determine the percentage of the prostate volume that received a dose equal to or greater than the prescribed dose to compute the quality index (V100) and fractional D90 (FD90). There were no statistically significant differences between the postneedle and postimplant (day 0) volumes obtained by U.S. and CT scanned images (student's t-test p=0.56). The mean half life of the edema was found to be (9.72 +/- 8.31) days (mean +/- 1 SD), ranging from 3.64 to 34.48 days. The mean values of V100 and FD90 from preimplant plan to postimplant plan at day 0 were decreased by 8.0% and 6.3%, respectively. On the other hand, the mean values of V100 and FD90 increased with increasing postimplant time and attained optimal values when postimplant volume reached the original volume of the prostate. The short half life 131Cs radioactive source delivered about 85% of the prescribed dose before the prostate reached its original volume. Therefore, improvement in V100 and FD90 due to edema decay does not improve the physical dose delivery to the prostate. It is important to note that at the time of 131Cs implant, the effect of edema must be accounted for when defining the seed positions. Implants performed based only on the guidance of a preimplant volume study would result in poor dosimetric results for 131Cs implants.