For MRI-based treatment, the contour-based method has been reported to match DRR to the portal images. However, it is time-consuming to contour all bony structures on MRI images and additional localization error may be introduced by contouring. In this study, we propose to use 3D MRI images directly to match the cone-beam CT (CBCT) images for localization. The localization accuracy with MRI-CBCT matching was analyzed by comparing the corresponding CT-CBCT matching.
Materials/Methods
Ten patients treated for intracranial lesions were analyzed in this retrospective study. In addition to conventional treatment planning CT, planning MRI images were acquired for each of these patients for better soft tissue delineation. The MRI images were fused to the planning CT during the clinical planning process. On-board CBCT images were acquired prior to treatment. All ten patients were treated based on planning CT-CBCT matching for position correction. For this study, both CT-CBCT matching and MRI-CBCT matching were performed to localize the treatment isocenter. The couch shifts for isocenter placement for each matching were recorded to identify the localization accuracy. The differences of couch shifts between the MRI-CBCT matching and CT-CBCT matching were analyzed to assess the accuracy of the MRI-CBCT matching relative to the CT-CBCT matching. The matching was performed manually using bony structures and ventricles as landmarks. The slice thickness for MRI, CT and CBCT images was 2.5 mm.
Results
Figure 1 shows the split view of the MRI-CBCT matching for a patient with three orthogonal views. The differences of the couch shifts between the MRI-CBCT matching and CT-CBCT matching for all ten patients were within 2 mm in both the lateral and vertical directions and within 3 mm in the longitudinal direction. The mean values of the shifts and standard deviations were 1.0 ± 0.8 mm, 0.8 ± 0.8 mm, and 1.6 ± 1.0 mm in the lateral, vertical and longitudinal directions, respectively.
Conclusions
Target localization based on matching between 3D planning MRI and on-board CBCT is feasible for the radiation treatment of intracranial lesions, such as CBCT-guided radiosurgery with MRI-based planning.
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