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. (C) 2002 Elsevier Science Ireland Ltd. All rights reserved.