3078 - Defining the Geometric Accuracy of Surface Guidance and Cone-Beam CT Setup and Intrafraction Motion Detection for Open-Face Mask-Based Hyperarc Intracranial Stereotactic Radiotherapy

Purpose/Objective(s): The objective of this study is to define the geometric roles of surface-guided radiotherapy (SGRT) and cone-beam CT (CBCT) in patient setup and to quantify true intrafraction motion using post-treatment CBCT in open-face mask–based intracranial stereotactic radiotherapy (SRT).

Materials/Methods: A retrospective analysis was performed on 50 patients and 156 treatment fractions delivered using linac based open-face mask based intracranial radiotherapy. The median number of metastases was 4 (range, 1-82). Treatment was delivered in 1, 3, and 5 fractions in 17 (34%), 12 (24%), and 21 patients (42%), respectively. Prescribed doses were 20 Gy in 17 patients (34%), 30 Gy in 12 (24%), 27 Gy in 9 (18%), 25 Gy in 6 (12%), and 35 Gy in 3 patients (6%). Initial positioning was performed with SGRT, followed by pre-treatment CBCT verification and 6 dimensional (6D) couch correction. After CBCT correction, the SGRT reference surface was reset to the corrected position to eliminate systematic offsets; post-treatment CBCT was then used to quantify intrafraction motion relative to this reference. Translational errors were expressed in millimeters, rotational components were converted to equivalent displacement assuming an 80-mm cranial radius, and the resulting 6D displacement distribution was characterized by the median and 95th percentile. Agreement between SGRT and pre-treatment CBCT shifts was evaluated to assess setup accuracy.

Results: The statistical analysis was carried out on the entire fraction-level dataset obtained from the study cohort. While SGRT effectively reduced significant initial positioning errors, its agreement with pre-treatment CBCT-derived shifts was limited, emphasizing the importance of volumetric imaging for accurate stereotactic verification. 6D setup deviations were computed and statistically assessed at the patient level, applying an 80 mm rotational radius to translate angular variations into linear displacement equivalents. The surface guidance system (Identify) vs Pretreatment CBCT 6D error comparison demonstrated a mean of 2.05 ± 1.69 mm and a median of 1.53 mm. Following CBCT guided correction and reference image redefinition, residual 6D displacement decreased substantially to 0.45 ± 0.21 mm, with a median of 0.45 mm with respect to posttreatment CBCT. The 95th percentile of post-correction residual error was found to be 0.72 mm.

Conclusion: This study demonstrates that SGRT effectively guides initial setup in HyperArc SRT workflow while its limited agreement with CBCT underscores the necessity of IGRT for sub-millimeter SRT accuracy. Our findings validate that the integration of CBCT-based correction with SGRT ensures intrafraction stability, providing a robust framework for high-dose HyperArc deliveries.