Evaluation of Immobilization Device Performance In Reducing Intrafraction Motion for Linac-Based Upper Spine SBRT Using Mid- and Post-Treatment CBCT
Abstract
Purpose
Spine SBRT involves steep dose gradients near critical organs at risk, making treatments highly sensitive to intrafraction motion. This study evaluated the performance of immobilization devices used for LINAC-based upper spine (T2 and above) SBRT by using mid- and post-treatment CBCT imaging as a surrogate for intrafraction motion.
Methods
42 patients treated with upper spine SBRT between May 2023 and December 2025 at a tertiary academic center were retrospectively analyzed. The institutional SBRT workflow included a pre-treatment CBCT for initial alignment, a mid-treatment CBCT acquired between treatment arcs for repositioning as needed, and a post-treatment CBCT to assess patient motion during the latter portion of treatment. Over this period, three head-and-shoulder immobilization masks were used for upper spine SBRT: a Klarity White S-type, Klarity Green P-type, and Klarity Blue S-type mask. The treatment database was queried to identify eligible spine SBRT patients. Translational and rotational shifts from mid- and post-treatment CBCTs were extracted for each treatment fraction, and the immobilization device and treated vertebral level were obtained from treatment records. A total of 333 CBCT images were analyzed and stratified by vertebral level and immobilization device.
Results
Translational and rotational tolerances of ±2 mm and ±1° were applied based on institutional practice. Among immobilization devices, the Klarity White S-type mask demonstrated the poorest performance, with 28.6% and 26.7% of patients exceeding either translational or rotational tolerance on mid- and post-treatment CBCT, respectively. The Klarity Green P-type mask performed best, with 3.6% and 3% of patients exceeding tolerance on mid- and post-treatment CBCT respectively.
Conclusion
Immobilization device choice significantly impacts intrafraction motion for upper spine SBRT, and continuous longitudinal tracking of patient localization is an important QA tool for practice improvement for Spine SBRT programs.