A Study on Target Positioning Error and Its Impact on Dose Variation in Image-Guided Stereotactic Body Radiotherapy for the Spine

Purpose: To investigate the amount of target positioning error and evaluate its dosimetric impact during image-guided stereotactic body radiotherapy for single-fraction spine treatment. Methods and Materials: A prescription dose of 15 Gy and five to nine coplanar intensity-modulated beams were used. The patient was immobilized with a custom-fit vacuum mold, and the target was localized with a volumetric cone-beam CT image. A robotic couch with six degrees of freedom was used for target adjustment. For evaluation a cone-beam CT image was obtained at the end of treatment. Both target positioning error and its dosimetric impact were investigated for the first 9 cases. Results: For cases studied, translational errors were 0.9 ± 0.5 mm (lateral), 1.2 ± 0.9 mm (longitudinal), 0.7 ± 0.6 mm (vertical), and 1.8 ± 1.0 mm (vector), and rotational errors were 1.6 deg. ± 1.3 deg. (pitch), 0.8 deg. ± 0.9 deg. (roll), and 0.8 deg. ± 0.4o (yaw). For the clinical target volume, D95 (dose to 95% of target volume), D90, Dmax, and Dmean were evaluated. Only 1 case showed significant dose variations, reaching up to 18% in D95. The spinal cord dose was evaluated by observing D0.1 (dose to 0.1 cm3), D0.5, D1.0, and Dmax. Although 1 case showed a dose change reaching up to 30% in Dmax, cord dose was within the planning tolerance limit in all but 2 cases (3% higher in one and 0.4% higher in the other). Conclusion: The implemented image-guided stereotactic body radiotherapy provides precise target localization. However, despite reasonably precise spatial precision, dosimetric perturbation can be significant because of both extremely steep dose gradients and close distances between the target and the spinal cord.