Analysis of interaction between number of implant catheters and dose-volume histograms in prostate high- dose-rate brachytherapy using a computer model.

PURPOSE: In prostate high-dose-rate brachytherapy, to determine before implant, using the standard geometric optimization algorithm, whether there is an optimal number of catheters.
MATERIALS AND METHODS: Transrectal ultrasound images of the prostate from 24 patients were transferred into the brachytherapy planning system. Urethra and prostate contours were digitized onto each axial slice of a CT scan, as well as hypothetical locations of the catheters (2/3 of the catheters along the prostate contour, 1/3 around the urethra). Each prostate was implanted with 9, 12, 15, 18, and 21 catheters. Dosimetry was optimized using a geometric optimization algorithm; prescription isodose was chosen so that 95% of planning target volume was covered by the 100% isodose.
RESULTS: A significant increase in mean volume of prostate receiving 150% of the dose (V150) when the number of catheters decreased (p < 0.0001). The 9-catheter group significantly differed from each of the other groups; no difference was seen in V150 among the 21-, 18-, and 15-catheter groups. Parallel results were observed for urethra V150 and homogeneity index; there was no difference in conformity index by catheter group.
CONCLUSION: V150 increased when fewer catheters were used. There was no significant difference among the 21-, 18-, and 15-catheter groups: the geometric optimization routine probably compensated for the larger distance between dwell positions. Based on the technique described in our study, we conclude that 15 to 21 catheters seem to cover the prostate adequately without creating excess hot spots.