Evaluation of initial setup accuracy and intrafraction motion for spine stereotactic body radiation therapy using stereotactic body frames

Purpose: The purposes of this study were (1) to evaluate the initial setup accuracy and intrafraction motion for spine stereotactic body radiation therapy (SBRT) using stereotactic body frames (SBFs) and (2) to validate an in-house-developed SBF using a commercial SBF as a benchmark. Methods and materials: Thirty-two spine SBRT patients (34 sites, 118 fractions) were immobilized with the Elekta and in-house (BHS) SBFs. All patients were set up with the Brainlab ExacTrac system, which includes infrared and stereoscopic kilovoltage x-ray-based positioning. Patients were initially positioned in the frame with the use of skin tattoos and then shifted to the treatment isocenter based on infrared markers affixed to the frame with known geometry relative to the isocenter. ExacTrac kV imaging was acquired, and automatic 6D (6 degrees of freedom) bony fusion was performed. The resulting translations and rotations gave the initial setup accuracy. These translations and rotations were corrected for by use of a robotic couch, and verification imaging was acquired that yielded residual setup error. The imaging/fusion process was repeated multiple times during treatment to provide intrafraction motion data. Results: The BHS SBF had greater initial setup errors (mean ± SD): - 3.9 ± 5.5 mm (0.2 ± 0.9°), - 1.6 ± 6.0. mm (0.5 ± 1.4°), and 0.0 ± 5.3 mm (0.8 ± 1.0°), respectively, in the vertical (VRT), longitudinal (LNG), and lateral (LAT) directions. The corresponding values were 0.6 ± 2.7 mm (0.2 ± 0.6°), 0.9 ± 5.3 mm (- 0.2 ± 0.9°), and - 0.9 ± 3.0 mm (0.3 ± 0.9°) for the Elekta SBF. The residual setup errors were essentially the same for both frames and were - 0.1 ± 0.4. mm (0.1 ± 0.5°), - 0.2 ± 0.4. mm (0.0 ± 0.4°), and 0.0 ± 0.4 mm (0.0 ± 0.4°), respectively, in VRT, LNG, and LAT. The intrafraction shifts in VRT, LNG, and LAT were 0.0 ± 0.4. mm (0.0 ± 0.3°), 0.0 ± 0.5 mm (0.0 ± 0.4°), and 0.0 ± 0.4 mm (0.0 ± 0.3°), with no significant difference observed between the 2 frames. Conclusions: These results showed that the combination of the ExacTrac system with either SBF was highly effective in achieving both setup accuracy and intrafraction stability, which were on par with that of mask-based cranial radiosurgery.