Evaluation of the Intra-fractional Motion in Stereotactic Body Radiation Therapy for Metastatic Spinal Tumors

The purpose of this study was to evaluate the intra-fractional motion (IFM) in stereotactic body radiation therapy (SBRT) for metastatic spinal tumors according to different irradiation site groups. We analyzed IFM during SBRT in 31 patients (32 lesions) treated at our institution from January 2022 to April 2023. IFM was evaluated in 3 groups according to the targeted spinal regions as follows: the cervical and upper thoracic, the lower thoracic, and the lumbar-sacral (L-S). IFM was defined as the displacement measured by acquired with the orthogonal kV-kV imaging system (ExacTrac; BrainLab, Munich, Germany) before and after irradiation. Correlations between IFM and treatment time were examined. The planning target volume (PTV) was determined by adding a 2 mm margin to the clinical target volume. The results showed that IFM for three groups was within 2 mm/2° for more than 95% of all fractions. There was no strong correlation between IFM and treatment time. The correlation coefficient between IFM and treatment time was up to 0.43 in the anterior-posterior direction of the L-S group. In conclusion, the IFM of SBRT for spinal metastases at our institution was within 2 mm for more than 95% of all fractions, which was encompassed by the PTV.