SU‐GG‐I‐119: MRI‐Guided Transrectal Robotic Prostate Biopsy Validation

PURPOSE: We report a quantitative evaluation of the clinical accuracy of an MRI‐guided robotic transrectal prostate biopsy system that has been in use for over five years at the U.S. National Cancer Institute. Through this retrospective study we also expect to gain insight to the nature and interplay of error factors in MRI‐guided transrectal robotic prostate biopsy. METHODS: Pre‐ and post‐insertion MR images show that while the whole group of relevant organs moves deformably, the prostate, rectum, and pubic bone shows little deformation according to Karnik et al. (SPIE Medical Imaging, vol. 7625, 2010.) Hence our validation framework entails two‐stage rigid volume registration using mutual information between the pre‐and post insertion images, using the Insight Toolkit. Global registration over the whole scene capture coupled gross prostate motion. Next, to account for residual decoupled prostate motion we re‐register the global image with a region of interest including the prostate only. RESULTS: The two‐stage registrations scheme was validated by 3D overlays of the segmented prostates and it was accurate up to 2 mm. A total of 82 biopsies from 21 patients were evaluated. The mean target displacement, needle placement error, and biopsy error was 5.9 mm, 2.3 mm, and 4 mm, respectively. This suggests that targets dislocate during the procedure, leading to often clinically significant biopsy errors, even in cases where needle placement in scanner coordinates may seem clinically acceptable. CONCLUSION: The mean anatomical biopsy error was 4 mm, which considering the minimal size of significant prostate cancer (about 4mm) is on the border of being clinically acceptable. Even considering imperfections in the registration framework (slice thickness, segmentation errors and assuming local rigidity) these findings indicate that compensation of target motion is necessary before inserting a biopsy needle.