SU‐E‐T‐417: Registration with and Without the Cranium When Using the ExacTrac System for Frameless Intracranial SRS

Abstract

Purpose: To determine if the inclusion or exclusion of the cranium during image registration affects the delivery accuracy when using Brainlab Novalis ExacTrac (ExacTrac) for frameless intracranial SRS. Methods: Tests performed on a Varian TrueBeam™ STX linac with the 6D robotic couch ExacTrac system (0.6‐mm 3D mechanical isocenter accuracy established at acceptance). A head phantom with a 7‐mm dia. steel ball insert was the experimental subject. The hidden target was localized with the Brainlab H&N localizer. 1.5‐cm square fields were planned to treat the hidden target using the Brainlab planning system. The target was initially positioned at isocenter using the ExacTrac IR system. Then, stereoscopic kV images were acquired. The images were registered, using the bony anatomy, to the DRRs in two ways: with and without the cranium. Fifty‐one auto‐registration actions were performed to determine the residual registration uncertainty. Translational and rotational couch shifts were applied. Radiographic film captured the treatment field images at gantry angles of 0°, 90°, 180°, and 270°, and couch rotations of 0°, 45°, and 90°. The experiment was repeated with the cranium digitally removed from the registration using the ROI tool of the ExacTrac software. The treatment field hidden target images were analyzed using RIT113 software. Results: The residual uncertainties of translational and rotational shifts were ±0.13 mm and ±0.13°, respectively. The center of the hidden target was within 1.0 mm of radiation isocenter for all gantry angles and couch rotations (range 0.15 to 1.0 mm) whether the cranium was included or excluded from the registration. However, registration without the cranium produced small but significant improvements in accuracy of treatment delivery based on two‐tailed student t‐test analysis (p = 0.05). Conclusion: Removing the cranium with the ROI tool may help improve the treatment delivery accuracy when using the ExacTrac system for intracranial lesions.