Purpose: To evaluate the positioning accuracies of two image-guided localization systems, ExacTrac and On-Board Imager (OBI), in a stereotactic treatment unit.
Methods and materials: An anthropomorphic pelvis phantom with eight internal metal markers (BBs) was used. The center of one BB was set as plan isocenter. The phantom was set up on a treatment table with various initial setup errors. Then, the errors were corrected using each of the investigated systems. The residual errors were measured with respect to the radiation isocenter using orthogonal portal images with field size 3 × 3 cm(2). The angular localization discrepancies of the two systems and the correction accuracy of the robotic couch were also studied. A pair of pre- and post-cone beam computed tomography (CBCT) images was acquired for each angular correction. Then, the correction errors were estimated by using the internal BBs through fiducial marker-based registrations.
Results: The isocenter localization errors (μ ±σ) in the left/right, posterior/anterior, and superior/inferior directions were, respectively, -0.2 ± 0.2 mm, -0.8 ± 0.2 mm, and -0.8 ± 0.4 mm for ExacTrac, and 0.5 ± 0.7 mm, 0.6 ± 0.5 mm, and 0.0 ± 0.5 mm for OBI CBCT. The registration angular discrepancy was 0.1 ± 0.2° between the two systems, and the maximum angle correction error of the robotic couch was 0.2° about all axes.
Conclusion: Both the ExacTrac and the OBI CBCT systems showed approximately 1 mm isocenter localization accuracies. The angular discrepancy of two systems was minimal, and the robotic couch angle correction was accurate. These positioning uncertainties should be taken as a lower bound because the results were based on a rigid dosimetry phantom.
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