Comparison of Intrafractional Motion Monitoring During Spine Stereotactic Body Radiation Therapy Using Multiple Immobilization Systems
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PO-GePV-P-43 (Sunday, 7/25/2021) [Eastern Time (GMT-4)]
Purpose: Given the proximity to spinal cord or cauda equina, spinal SBRT requires highly conformal doses with steep dose gradients. As a result, precise positioning and immobilization of the patient must be achieved and maintained for the duration of simulation/treatment. Our goal was to evaluate differences in intrafractional monitoring via Exactrac during spinal SBRT resulting from a change in immobilization systems used in our clinic.
Methods: Eight patients with a total of 10 spinal metastatic lesions were prospectively enrolled in a trial designed to evaluate setup/motion error for various combinations of immobilization and image guidance. All patients underwent 3 fraction spinal SBRT and were simulated/treated with one of two SBRT systems – Elekta BlueBAG/BodyFix (n=7) or Klarity SBRT system (n=3). After initial alignment to external marks, residual 6D alignment corrections were applied using the ExacTrac positioning system and subsequently verified to be within 1mm/1°. The ExacTrac x-rays system was further utilized to assess intrafractional motion and corrections were applied as necessary such that an overall positioning accuracy of 1mm/1° was maintained.
Results: Records for 28 treatment fractions were evaluated. The intrafractional shifts necessary to maintain 1mm/1° positioning accuracy were applied in 3/19 (15.7%) and 1/9 (11.1%) fractions with the BlueBAG/BodyFix and Klarity SBRT systems, respectively. During one fraction with the Klarity SBRT system, intrafractional shifts were applied twice, otherwise all shifts were applied once mid-treatment. All applied shifts were < 2mm/2° in each direction. The averaged translational magnitude of applied intrafractional shifts was 1.4mm and 2.0mm for the BlueBAG/BodyFix and Klarity SBRT systems, respectively.
Conclusion: Both spinal SBRT immobilization solutions achieve comparable results across the limited number of treatment fractions evaluated. Future study with a larger cohort is warranted. Some intrafractional motion was observed with both systems and demonstrates the necessity of motion management during treatment delivery for spinal SBRT.
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