Impact of selection of post-implant technique on dosimetry parameters for permanent prostate implants.

PURPOSE: To investigate the variability of prostate implant quality indices between three different methods of calculating the post-implant dose distribution. METHODS AND MATERIALS: In a study of 9 permanent prostate implant patients, post-implant dosimetry was carried out using three methods of identifying seed positions within the prostate volume: (1) prostate volumes defined by transrectal ultrasound (TRUS) immediately following implant were registered with shift-film defined seed positions, (2) seeds were identified directly from the post-implant TRUS images, and (3) CT was used to define seed positions and prostate volumes from images acquired at 41-65 days post-implant. For each method, the volume of prostate receiving 90%, 100%, and 150% of the prescribed dose (V90, V100, V150) and the dose delivered to 90% of the prostate volume (D90) were calculated. RESULTS: Post-implant TRUS volumes were within 15% of the preimplant TRUS volumes in 8 of the 9 patients investigated. The post-implant CT volume was within 15% of the preimplant (TRUS) volume in only 3 of the 9 cases. The value of the dosimetry parameters was dependent on the method used and varied by 5-25% for V90, 5-30% for V100, 42-134% for V150, and 9-60% for D90. No simple relationship was found between change in volume and the resultant change in dosimetry parameter. Differences in dosimetry parameters due to source localization uncertainties was found to be small (< or = 10% for V100) when comparing methods (1) and (2). CONCLUSIONS: There are many uncertainties in the calculation of parameters that are commonly used to describe the quality of a permanent prostate implant. Differences in the parameters calculated were most likely a result of a combination of factors including uncertainties in delineating the prostate with different imaging modalities, differences in source identification techniques, and intraobserver variability.

Impact of prostate volume evaluation by different observers on CT-based post-implant dosimetry.

PURPOSE: An analysis of computed tomography (CT)-based dosimetry was performed to evaluate the variability of different observers' judgements in marking the prostate gland on CT films, and its effect on the parameters that characterise the prostate implantation quality. Accuracy of data entry by the first author in the process of dosimetry procedure has also been evaluated. MATERIALS AND METHODS: Four observers were asked to evaluate the prostate volume on CT films for six different patients. Each observer repeated the evaluation six times. The sample of patients has a prostate volume in the range of 21.4-42.0 cc derived from transrectal ultrasound volume study. After an average period of 6 weeks of the I-125 implantation, all patients had CT scans. CT-based post-implant dosimetry was performed and the dose volume histograms DVHs were calculated to report the re-constructed prostate volume, Vp100, Vp150, Vp90 and D90. Comparison between the four observers' output was performed. RESULTS: Comparison between the four observers shows that each observer has a different way of estimating the prostate on CT films. Observers' precision also varies according to the prostate volume and the image quality. This can cause a variation in the resulting D90 value by up to 50%. Analysis of data entry shows a high degree of accuracy. The error of digitizing the prostate is +/-0.19 cc. This is correlated to an error of +/-0.78 Gy of the D90. CONCLUSION: The evaluation of prostate gland volume on CT films varies between different observers. This has an effect on the dosimetric indices that characterise the implant quality in particular the D90.