Deformable image registration and delineation propagation

The DIR of pCT to daily CBCT images were performed using an Adaptive Monitoring tool available in the commercial image registration software. Figure 1 shows the DIR and delineation propagation workflow. DIR creation utilized the ‘Adaptive Monitoring’ navigator, which is comprised of three steps: (1) manual alignment, (2) rigid registration, (3) deformable registration. During the manual alignment step, a manual rigid alignment between the CBCT image and the pCT image was made using bony anatomy and the implanted gold fiducials as a guide. These were done independently of clinical set-up shifts for consistency. In step 2 the region of interest was adjusted to include the prostate and, using the manual alignment created in step 1 as a starting point, a rigid registration was created by the software. Step 3 uses the rigid registrations of step 2 as a reference to create DIR’s. After the rigid registration, the DIR algorithm was utilized to deformably register the pCT to the daily CBCTs. The DIR algorithm uses an intensity-based B-spline algorithm based on the Mattes formulation; the details have been described elsewhere^14,25. For poor quality deformations, the deformable image registration workflow was repeated with smaller ROIs (67 fractions). Poor quality deformations were defined as a DSC < 0.75 and MDA > 3.5 mm, just beyond the TG-132 tolerance recommendations. For the three fractions that continued to produce poor quality deformations, a structure-guided deformation (hybrid DIR) was employed.

Deformable image registration and delineation propagation workflow. All steps included in the Adaptive Monitoring Navigator on Velocity are inside the dotted-line box. The re-evaluation of poor-quality fractions (DSC < 0.75, MDA > 3.5 mm) is shown in the light-red box. Exportation of data to MATLAB for data extraction and analysis is done in the final green box.