Evaluation of the Dosimetric Impact of Rotational Setup Errors on Target and Organ at Risk In Spine SBRT
Abstract
Purpose
Spine stereotactic body radiotherapy (SBRT) requires high precision due to the anatomical proximity of target to the spinal cord and steep dose gradients. Rotational setup errors (SE) can affect the dose delivery to targets and organs at risk (OAR). This study evaluated the dosimetric impact of rotational SE in spine SBRT.
Methods
Fifteen patients treated with spine SBRT were retrospectively analyzed (2 cervical, 5 thoracic and 8 lumbar). PTV and spinal cord PRV margins were defined as 2 mm. The prescribed dose was 24 Gy in 2 fractions with a dose constraint of D0.03cc < 17 Gy for the spinal cord PRV or thecal sac. Dual-arc VMAT plans were generated using Eclipse. To simulate rotational SE, the center of gravity of the PTV was set as the isocenter, and the CT images and structure set were rotated around the isocenter. Rotation were applied in the pitch, yaw and roll directions from 0.5° to 3.0° at 0.5° intervals, and the dose distributions were recalculated. Evaluated dose metrics included D95% for CTV/GTV, D0.03cc for spinal cord and thecal sac. SE threshold causing a ≥5% dose deviation was determined.
Results
The dose change in CTV D95% and GTV D95% were within 1.5% even at the maximum rotation of 3.0° across all directions. The SE thresholds causing a ≥5% change in spinal cord D0.03cc were 3.0°/2.0°/2.5° (pitch/yaw/roll). For thecal sac D0.03cc the thresholds were 2.5°/3.0°/2.0° (pitch/yaw/roll). All cases met the dose constraints for the spinal cord at 3.0° SE.
Conclusion
In spine SBRT, target coverage remained robust (<1.5% change) against rotational SE up to 3.0°. Conversely, OARs demonstrated significant sensitivity, with ≥5% dose deviations occurring at 2.0° in the spinal cord and 2.5° in the thecal sac. Although clinical constraints were maintained, high OAR sensitivity underscores the necessity of strict rotational setup correction.