Towards MRI In Ultrasound-Guided High-Dose-Rate Prostate Brachytherapy: Mental Fusion Vs Deformable Image Registration
Abstract
Purpose
While transrectal ultrasound (TRUS) is the primary modality for guidance and planning in high-dose-rate prostate brachytherapy (HDR-BT), magnetic resonance imaging (MRI) is required for identifying and targeting the dominant intraprostatic lesion (DIL). The purpose of this work is to investigate the role of deformable image registration (DIR) in comparison to mental fusion.
Methods
This study utilized imaging data for three prostate cancer patients who underwent salvage or monotherapy HDR-BT treatments. Pre-treatment MRI scans were used in the clinical treatment via mental fusion into the TRUS space. Retrospective MRI-to-TRUS DIR was performed using MIM Software tools. The specific workflow involved an initial manual rigid registration, followed by a surface-based prostate-to-prostate deformation. Mental fusion and DIR results were compared via volume assessment, Dice similarity coefficient (DSC), and dose volume histogram analysis.
Results
The DIL was an average of 45% smaller with DIR compared to mental fusion, reflecting the more generous target definition with the simpler approach. One lesion, measuring only 0.5 cm3 on MRI, did not overlap with the mentally-fused DIL when deformably registered, with a DSC of 0.002. The other two lesions had an average DSC of 0.40 ± 0.08. When it comes to the dose distribution, however, the results were similar regardless of fusion technique. For mental fusion compared to DIR, average V100% was 100% vs 98%, while D90% was 125% vs 119%.
Conclusion
Results of this study show that for whole-gland treatment while considering the DIL, mental fusion is sufficient in terms of the resulting dose distribution. For the purpose of DIL boost or targeted treatment, a deformable registration approach would be ideal to reduce the possibility of geographic miss. Future work will expand this retrospective dataset, consider inter/intra-observer variability, and integrate this registration approach prospectively in HDR-BT for salvage and monotherapy.