ESTRO 2025 - Abstract Book

S3703

Physics - Quality assurance and auditing

ESTRO 2025

4468

Digital Poster Use of CBCT as a Quality Assurance Tool for Frame-Based SRS Treatments

Carlos M Rivera, Mauricio Acosta, D Jay J Wieczorek, Yongsook C Lee, Matthew D Hall, Robert H Press, Vibha Chaswal, Tatiana Bejarano, Michael W McDermott, Minesh P Mehta, Rupesh Kotecha, Alonso N Gutierrez, Ranjini Tolakanahalli Radiation Oncology, Miami Cancer Institute, Miami, USA Purpose/Objective: Gamma Knife Icon™ and Esprit™ systems utilize cone-beam computed tomography (CBCT) to define a stereotactic coordinate space for patient localization of both frame-based and frameless treatments. While frameless stereotactic radiosurgery (SRS) offers workflow flexibility, a frame-based approach remains the gold standard for localization of small targets. This study evaluates the impact of uncorrected (residual) patient offsets or frame slippage on target parameter metrics in SRS treatments when CBCT-reported corrections are omitted. Material/Methods: Consecutive patients treated with single-fraction SRS between 2017 and 2024 using frame-based immobilization and stereotactic pre-treatment CBCT at a single, tertiary-care institution were analyzed. Translational and rotational setup errors were extracted by registering pre-treatment CBCT (CBCT Tx ) to stereotactic reference CBCT (CBCT Ref ) images. Examinations with large positional shifts and significant coverage drops resulting in replans were selected for dosimetric evaluation. Transformation matrices derived from CBCT registrations were applied to original shot coordinates to simulate uncorrected treatments. Target coverage (TC) and Paddick Conformity Index (PCI) were compared between the original corrected plans and the uncorrected simulated scenarios. Results: From the database, 364 patients with 913 examinations and 4,466 lesions with at least one CBCT Tx imaging were analyzed. Mean translational and rotational shifts for these patients were: X = 0.06 ± 0.12mm, Y = 0.07 ± 0.07mm, Z = 0.09 ± 0.11mm; Pitch = 0.05 ± 0.06 ∘ , Yaw = 0.06 ± 0.08 ∘ , Roll = 0.05 ± 0.06 ∘ . Eight patients (2.2%) required redefinition of the stereotactic reference due to frame slippages, with mean translational and rotational differences of: X = 0.09 ± 0.10mm, Y = 0.14 ± 0.11mm, Z = 0.32 ± 0.20mm; Pitch = 0.13 ± 0.07 ∘ , Yaw = 0.10 ± 0.11 ∘ , Roll = 0.04 ± 0.03 ∘ . TC decreased by an average of 5.77± 8.36% (Uncorrected (0.94 ± 0.08%) v. corrected (1.00 ± 0.00%; p < 1e-4) while a decrease of 0.06 ± 0.07 was noted for PCI (Uncorrected (0.56 ± 0.22%) v. corrected (0.61 ± 0.21%; p < 1e-4). Conclusion: CBCT image guidance for defining stereotactic reference in frame-based immobilization systems serves as a valuable quality assurance tool to quantify frame slippage and minimize geographic miss and associated reduction in plan quality.