Purpose: To validate the 2D/3D registrations by comparing the registration accuracy, image quality, and dose with the 3D/3D registrations.
Methods: Cranial (CIRS STEEV) and body (BrainLab Pelvis) phantoms were used as an imaging target. Each phantom was placed near the treatment positions with deliberate small random translations and rotations. The 6D transformations were measured independently by seven techniques: 3D/3D match with CBCT, 2D/3D match with kV/kV on TrueBeam (kV/kV-T), kV/kV with ExacTrac (kV/kV-E), and kV/MV with four MV MU settings (2.5X-Hi, 2.5X-Lo, 6X-Hi, 6X-Lo). The 3D/3D registrations were used as the ground truth to evaluate the accuracy of the planar image group using 2D/3D registration. The difference between 3D/3D and 2D/3D was calculated and assessed with the Wilcoxon Rank-Sum Test (p-value<0.05). A Varian Las Vegas phantom was used to compare the image quality of different kV or MV to evaluate the impact of image quality at registration. 2 OSLD chips were placed at the surface, mid-plane, and exit of the phantoms to measure imaging doses.
Results: From 23 measurements (cranial-11, body-12), the mean residual errors of each 2D/3D method in each translational and rotational dimension were small (<±0.3mm/0.3°). kV/kV-E and kV-2.5X-Lo had the smallest standard deviations (SD) of 0.3mm/0.3° in all dimensions; kV/kV-T, kV-2.5X-Hi, and kV-6X-Hi showed SD of 0.5-0.7mm/0.3-0.5° at Lateral/Roll direction; kV-6X-Lo had the highest SD of 1mm/0.8° at Lateral/Roll direction. Compared to 3D/3D, 2D/3D registrations showed no significant difference (p-values: 0.13-1.00). 2.5X showed comparable contrast to kV. 6X contrast was worse than kV/2.5X. The maximum surface/mid-plane/exit doses (mGy) were 23.0/16.9/8.8, 24.8/28.9/19.7, 0.22/0.16/0.02, and 1.50/0.23/0.05 for 2.5X, 6X, 80/100kV-E, and 80/100kV-T, respectively.
Conclusion: All 2D/3D registration methods in this study showed no significant difference to 3D/3D. 2.5X has comparable image quality to kV and lower doses than 6X. The MV imaging doses were 10 times higher than kV.