Evaluating the dosimetric impact of intrafraction shifts on critical OAR doses for spine SBRT using an In-House simulator

This study evaluated the dosimetric impact of intrafraction motion during spine SBRT by developing a novel dose deviation simulator capable of analyzing translations and rotations in all six degrees of freedom. We retrospectively assessed the treatment plans from 18 spine SBRT patients treated at our institution with 24 Gy in two fractions by simulating shifts of 0.5-2 mm (translations) and 0.5-2° (rotations), and calculating changes in D0.03 cc to spinal cord, cord PRV, and thecal sac. Translational shifts, particularly in the vertical direction, produced the greatest observed deviations, with a 2 mm vertical shift yielding median increases, reported as percent of prescription dose, of 20.6% (4.93 Gy) to thecal sac D0.03 cc and 6.5% (1.55 Gy) to spinal cord D0.03 cc. Our simulator's accuracy was validated against treatment planning system-based forward calculations, showing mean differences of -1.3% ± 3.5% between the two systems. This study provides practical reference values to assist clinicians in rapidly evaluating the clinical significance of observed intrafraction motion. These findings may streamline decision-making during treatment and highlight the importance of precise positioning and motion monitoring in spine SBRT.