The role of post-implant dosimetry in the quality assessment of prostate implants. The RAH experience

The quality of a prostate implant using radioactive I-125 seeds can be assessed by performing post-implant dosimetry, which therefore has to be a reliable evaluation tool. At the RAH, we have noticed large variations in post-implant dosimetric parameters compared to the pre-implant data. The purpose of this study was to investigate the cause of these differences. Post-implant dosimetry was performed 4 weeks after implant on 15 prostate cancer patients. The CT images of the pelvis were exported to the planning system, where contouring of the target (prostate) was executed by 6 clinicians, followed by the post-implant dosimetry performed by a physicist. Pre- and post-implant dosimetric parameters were analyzed and compared. The average target volume based on the ultrasound measurements was 35.8 cc. Post-implant CT volumes were determined and averaged over the 15 patients by each clinician, and their average values vary from 28.9 cc to 67.9 cc. Beside the under/overestimation of the target on the CT there was also a ‘shift’ in the target base on the ultrasound image. By comparing pre-implant and post-implant dosimetric parameters we have encountered a significant discrepancy between target volumes based on ultrasound image and CT image. It was concluded that the accuracy of target coverage was partially connected to the poorer quality of the CT image compared to the ultrasound scan, patient’s anatomy, but mostly to the poor implantation of the base.