Assessing Patient Motion In Spine SBRT: Implications for VMAT Planning and Margin Selection
Abstract
Purpose
To evaluate intrafraction motion during spine SBRT with different immobilization systems and determine whether less rigid systems can offer similar geometric accuracy to near-rigid systems such as the BodyFIX during FFF VMAT treatments.
Methods
Motion data was evaluated for 3 immobilization systems: BodyFix BlueBag only, U-frame mask, and BodyFix BlueBag with wrap. For the first two systems, patient shifts were extracted from CyberKnife log files during the first 9-10 minutes of treatment to approximate the typical mid-treatment CBCT interval. For the third system, VMAT motion was evaluated with mid-treatment CBCT shifts. The mean, start-to-end, and maximum displacement in the CyberKnife groups were compared with the mid-treatment shifts in the VMAT group. A TOST test with a 0.5 mm threshold was applied to assess the equivalence between the two groups. Treatment margins for both groups were calculated using the extended Van Herk formalism to account for hypofractionation.
Results
228, 114 and 163 fractions were analyzed for the BodyFix no wrap, mask, and BodyFix with wrap cohorts. Both the mask and no wrap shift data were statistically equivalent to wrap for all translational directions when considering start-to-end displacement. In the with wrap group, mean mid-treatment shifts of 0.1± 0.4 mm, 0.0 ± 0.5 mm and 0.0 ± 0.5 mm, were observed in the S/I, L/R and A/P directions. Based on these shifts, an isotropic margin of 1.8 mm was calculated. Margins between 1.7 and 2.3 mm were estimated for mask and no wrap groups using the average shifts over each interval.
Conclusion
Margins calculated for the less rigid immobilization systems were within 0.5 mm of those for the with wrap cohort, suggesting adequate immobilization when imaging and repositioning is performed at 10-minute intervals. Future work will quantify the dosimetric impact of the motion for these less rigid immobilization systems.