Impact of postimplant edema on V 100 and D 90 in prostate brachytherapy: can implant quality be predicted on day 0?

Abstract

Purpose : To determine the effect of edema on the dosimetric parameters V 100 (percentage of prostate volume that received a dose equal to or greater than the prescribed dose) and D 90 (minimal dose delivered to 90% of prostate volume) in 125I prostate brachytherapy and to determine whether the edema can be used to predict implant quality on the day of the implant (Day 0). Methods and Materials : Fifty consecutive patients treated with 125I implants who had two postimplant CT scans were selected for this study. The mean interval between the studies was 46 ± 23 days. The implants were preplanned to deliver 150 Gy to the prostate plus a 3–5-mm symmetric dose margin using peripherally loaded 0.4–0.6-mCi (NIST-99) 125I seeds. A dose-volume histogram was compiled for each postimplant CT scan. The V 100 and D 90 from the first and second CT scans were compared to determine the effect of edema on these parameters. A multivariate regression analysis was performed to define the linear relationships for predicting the V 100 or D 90 at 30–60 days after implant from the magnitude of the edema and the values of V 100 and D 90 on Day 0. Results : V 100 and D 90 increased by 5% ± 6% and 15% ± 17%, respectively, during the interval between the first and second postimplant CT scans. The mean edema was 1.53 ± 0.20. The increases in V 100 and D 90 were found to be proportional to the edema and the values of V 100 and D 90 on Day 0. The increase in V 100 was also found to depend on the width of the preplan dose margin. Linear relationships were derived that predict the V 100 and D 90 at 30–60 days after implant with a standard error of ±4% and ±24 Gy, respectively. Conclusion : V 100 and D 90 increased by 5% ± 6% and 15% ± 17%, respectively, during the first 30–60 days after implant. The results of a multivariate linear regression analysis showed that the increases in V 100 and D 90 were proportional to both the magnitude of the edema and the values of these parameters on Day 0. The relationships derived by linear regression analysis predict V 100 and D 90 at 30–60 days after implant to within ±4% and ±24 Gy, respectively. However, predicting the 30–60-day V 100 and D 90 on Day 0 is a poor substitute for obtaining a 30–60-day CT scan, because the uncertainty in the predicted values is greater by a factor of ≥2. Nevertheless, on average, the predicted values should provide a more reliable estimate of the actual V 100 and D 90 than the Day 0 values that ignore the effect of edema altogether. The increase in V 100 was also found to depend on the width of the preplan dose margin; therefore, our results for V 100 are only valid for implants planned with a 3–5-mm margin.