The effect of needle number on the quality of high-dose-rate prostate brachytherapy implants

The aim of this study is to evaluate the effect of the number of needles on the quality of dose distributions in high-dose-rate (HDR) prostate implants regarding target coverage, dose homogeneity and dose to organs at risk. Treatment plans of 174 implants were evaluated using cumulative dose-volume histograms. The plans were divided into three groups according to the number of implanted needles: <15: LNG (low number group), 15-17: MNG (medium number group) and >17: HNG (high number group). Treatment planning was based on transrectal ultrasound imaging. Dose-volume parameters for target (V90, V100, V150, V200, D90, D(min)) and quality indices (DNR, DHI, CI, COIN) were calculated. Maximal dose in reference points and high dose volumes were determined for rectum and urethra. Nonparametric analysis of variance and correlation was used with regard to needle numbers. Between the groups differences were found only in the following parameters: V(p) was larger when more needles were used with the values of 22.8 cm(3), 28.0 cm(3) and 30.9 cm(3) for the three groups, and more needles were used when the central cross-section of the prostate was larger. V200 in MNG was lower than in LNG (12%, 14%). Dose to rectum was higher in MNG than in LNG (D(2): 51%, 47%). Doses to the urethra were higher in HNG than in MNG (D1: 142%, 137%, and D(0.1): 128%, 125%). There was no significant difference in other parameters. Different number of needles results significant differences in treatment plans. However, the optimal needle number depends on not only the size of the prostate, but also the individual anatomy of the patient. Based on our results, in most cases the use of 15-17 needles seems to provide a dosimetrically acceptable treatment plan in HDR prostate implants.