Author: Martha J. Malin, Matthew Schelin 👨🔬
Affiliation: NYU Langone Health 🌍
Purpose: To evaluate the agreement between ExacTrac Dynamic (ETD) and CBCT-based positioning for spine SRS/SBRT treatments during initial patient setup and repositioning after intrafraction motion.
Methods: Data was analyzed from 154 spine SRS/SBRT fractions delivered in 2023 on a Varian Edge linac equipped with a 6 degree-of-freedom couch and ETD. Initial patient positioning was CBCT-based using auto-registration to bone with a region of interest focused on the target volume. After applying CBCT shifts, initial ETD images were taken and registered to the planned position (“ETD-offset”). If ETD-offset was within tolerance, treatment began with ETD tracking motion relative to the CBCT-based initial position (“ETD-motion”). CBCT was used to reposition patients if either ETD-offset or ETD-motion was out of tolerance. Treatment fractions with out-of-tolerance initial ETD-offsets, but no subsequent out of tolerance ETD-motion (n=19) were analyzed to evaluate agreement in initial positioning. The magnitude of the difference between the initial ETD-offset and the shifts from the subsequent CBCT were evaluated (“ETD-offset*”). Treatment fractions with at most two out of tolerance ETD-motions (n=30) were used to quantify the agreement between ETD and CBCT-based intrafraction re-positioning. The magnitude of the difference between the out-of-tolerance ETD-motion and the shifts applied from the subsequent CBCT were evaluated (“ETD-motion*”). This analysis was completed both with and without the initial ETD-offset accounted for in the calculation of ETD-motion*.
Results: The ETD-offset* and ETD-motion* average (Stdev) magnitudes were 0.65 mm (0.54 mm) and 1.04 mm (0.55 mm). The percentage of the ETD-offset* and ETD-motion* sample size that had a magnitude > 1 mm was 10.5% and 50.0%. Accounting for ETD-offset reduced ETD-motion* 36.7% of the time. When ETD-motion was out of tolerance, the subsequent CBCT indicated motion had been within tolerance 20.0% of the time.
Conclusion: ETD-CBCT agreement is better at initial setup than during intrafraction motion tracking.