Impact of transrectal ultrasound- and computed tomography-based seed localization on postimplant dosimetry in prostate brachytherapy.

PURPOSE: To study the impact of seed localization, as performed by different observers using linked (125)I seeds, on postimplant dosimetry in prostate brachytherapy and, to compare transrectal ultrasound (TRUS)-based with CT-based approach for the dosimetric outcomes. METHODS AND MATERIALS: Nineteen permanent prostate implants were conducted using linked (125)I seeds. Postimplant TRUS and CT images were acquired and prostate glands were, after implantation, delineated on all images by a single oncologist, who had performed all 19 seeding procedures. Six observers independently localized the seeds on both TRUS and CT images, from which the principle dosimetric parameters V(100) (volume of prostate that received the prescribed dose), V(150) (volume of prostate that received 150% of the prescribed dose), and D(90) (minimal dose delivered to 90% of the prostate) were directly calculated for each patient. A single-factor analysis of variance was first applied to determine interobserver variability in seed localization. A nonparametric comparison of the approach using TRUS and CT was then carried out by the Wilcoxon paired-sample test. RESULTS: Analysis from the analysis of variance for TRUS showed that the null hypothesis for equal means, could not be rejected for all six observers based on a significance level alpha=0.05. TRUS-based and CT-based approaches were then cross compared by the Wilcoxon paired-sample test, which suggested that the null hypothesis was insignificant for V(100) and D(90), but was significant for V(150). CONCLUSIONS: Both TRUS- and CT-imaging modalities provided indistinguishable postimplant dosimetry results as far as V(100) and D(90) were concerned. There was comparable observer independence between TRUS- and CT-based seed localization for linked-seed implant procedures. With other advantages that TRUS-imaging modality had over CT in the evaluation of postimplant dosimetry, TRUS would be a preferred choice in conjunction with linked seeds for intraoperative procedures in prostate brachytherapy.

The effect of interobserver variability on transrectal ultrasonography-based postimplant dosimetry.

PURPOSE: To investigate interobserver variability in contouring the prostate on postimplant transrectal ultrasonography (TRUS) images and its effect on dosimetric parameters that quantify implant quality. METHODS AND MATERIALS: Twenty preplanned peripherally loaded prostate implants were performed using 125I seeds and spacers linked together in linear arrays that maintain precise seed spacing and prevent seeds from rotating about their longitudinal axis. A set of two-dimensional transverse images spaced at 0.50-cm intervals was obtained with a high-resolution TRUS probe at the conclusion of the procedure with the patient still under anesthesia. A high percentage of the seeds (> 85%) were localized based on their visible echoes. The remaining seeds were identified based on the known locations of the "missing" seeds in the arrays. Two experienced ultrasonographers and a prostate brachytherapist independently contoured the prostate on the postimplant TRUS images. The prostate volumes defined by each observer were used to calculate the minimal dose received by 90% of the prostate volume (D90) and the percentage of the prostate volume receiving 100% of the prescribed minimal peripheral dose (V100). The observers also contoured the prostate on six preimplant TRUS studies to compare the variability in defining the prostate on pre- and postimplant TRUS images. RESULTS: The mean postimplant prostate volumes ranged from 20.8 to 66.9 cm3 (median: 45.7 cm3). The standard deviations (SDs), which reflect the variation in the volumes of the three observers, ranged from 1.4% to 26.1% of the mean (median: 11%). Multiple pairwise comparisons showed that the prostate volumes delineated by observer 3 differed significantly from those of observers 1 and 2 (p < 0.003). The volumes of observers 1 and 2 were not significantly different (p > 0.5). The mean values of D90 ranged from 124.2 to 171.1 Gy (median: 154.7 Gy) having SDs that ranged from 0.6% to 24.4% of the mean D90 (median: 7.8%). The mean values of V100 ranged from 82.3% to 95.1% (median: 92.8%) having SDs that ranged from 0.4% to 11.2% of the mean V100 (median: 4.0%). The values of both D90 and V100 calculated from the volumes of observer 3 were significantly (p < 0.003) different from those of observers 1 and 2, which did not differ significantly (p > 0.5). There was less interobserver variability in contouring the preimplant TRUS volumes. The mean volumes ranged from 20.3 to 54.3 cm3 having SDs that ranged from 1.9% to 14.1% (median: 8.6%). CONCLUSIONS: Significant interobserver differences in delineating the prostate volume on postimplant TRUS images were observed; however, these differences were less than generally reported for postimplant CT images. The interobserver differences in contouring the prostate in both TRUS and CT images produced significant differences in the dosimetric parameters, D90 and V100.

Localization of linked 125I seeds in postimplant TRUS images for prostate brachytherapy dosimetry.

PURPOSE: To demonstrate that (125)I seeds can be localized in transrectal ultrasound (TRUS) images obtained with a high-resolution probe when the implant is performed with linked seeds and spacers. Adequate seed localization is essential to the implementation of TRUS-based intraoperative dosimetry for prostate brachytherapy. METHODS AND MATERIALS: Thirteen preplanned peripherally loaded prostate implants were performed using (125)I seeds and spacers linked together in linear arrays that prevent seed migration and maintain precise seed spacing. A set of two-dimensional transverse images spaced at 0.50-cm intervals were obtained with a high-resolution TRUS probe at the conclusion of the procedure with the patient still under anesthesia. The image set extended from 1.0 cm superior to the base to 1.0 cm inferior to the apex. The visible echoes along each needle track were first localized and then compared with the known construction of the implanted array. The first step was to define the distal and proximal ends of each array. The visible echoes were then identified as seeds or spacers from the known sequence of the array. The locations of the seeds that did not produce a visible echo were interpolated from their known position in the array. A CT scan was obtained after implantation for comparison with the TRUS images. RESULTS: On average, 93% (range, 86-99%) of the seeds were visible in the TRUS images. However, it was possible to localize 100% of the seeds in each case, because the locations of the missing seeds could be determined from the known construction of the arrays. Two factors complicated the interpretation of the TRUS images. One was that the spacers also produced echoes. Although weak and diffuse, these echoes could be mistaken for seeds. The other was that the number of echoes along a needle track sometimes exceeded the number of seeds and spacers implanted. This was attributed to the overall length of the array, which was approximately 0.5 cm longer than the center-to-center distance between the first and last seed owing to the finite length of the seeds at the ends of the array. When this occurred, it was necessary to disregard either the most distal or most proximal echo, which produced a 0.5-cm uncertainty in the location of the array in the axial direction. For these reasons, simply localizing the visible echoes in the TRUS images did not guarantee the reliable identification of the seeds. CONCLUSION: Our results have demonstrated that a high percentage (>85%) of the implanted (125)I seeds can be directly visualized in postimplant TRUS images when the seeds and spacers are linked to preclude seed migration and rotation and when the images are obtained with a high-resolution TRUS probe. Moreover, it is possible to localize 100% of the seeds with the mechanism of linked seeds because the locations of the missing seeds can be determined from the known construction of the arrays.