[Evaluation of the Accuracy of Patent Fixation System Using a Bi-directional X-ray Image Matching System during Spine Stereotactic Body Radiation Therapy].

Purpose: To evaluate the accuracy of fixation devices during irradiation of our thoracic and lumbar vertebral stereotactic body radiation therapy (SBRT) and to suggest appropriate planning target volume margins.

Methods: Nine patients (45 sessions) with spinal metastases treated with spinal SBRT were studied. A fixation system with a torso shell was used as a fixture for thoracic and lumbar spinal lesions. In the patient setup, radiographic imaging was performed using Exactrac (Brainlab, Munich, Germany), followed by cone beam computed tomography (CBCT) confirmation to ensure that the images were matched within 0.3 mm just before irradiation. Irradiation was started after the collation, and additional X-ray imaging using Exactrac was performed immediately before, during, and immediately after treatment, respectively, to evaluate the fixation accuracy during irradiation.

Results: The mean ±1 standard deviation for each direction for movement during irradiation with the body shell was as follows: Lateral direction: 0.01±0.21 mm, head-tail direction: -0.01±0.18 mm, anteroposterior direction: 0.06±0.16 mm, roll: 0.02±0.19°, pitch: -0.02±0.29°, yaw: -0.03±0.23°.

Conclusion: The image guided radiotherapy system using X-ray imaging with Exactrac allowed the evaluation of positioning accuracy during irradiation. The body shell fixation system was able to ensure fixation accuracy within 1 mm during irradiation. In addition, the positioning during irradiation using Exactrac is very useful in the evaluation of positioning accuracy during irradiation in SBRT of the vertebral body, where very high irradiation accuracy is required.