SU‐E‐T‐347: Effect of MLC Leaf Position Inaccuracy On Dose Distribution for Spinal SBRT with Different Energies and Dose Rates

Abstract

Purpose:To evaluate dosimetric impact of spinal SBRT when MLC leaf positions deviate from planning positions for different energies and doserates.Methods and Materials:18 localized spinal metastases patients were selected for SBRT using IMRT planning with 9 posterior beams delivered at gantry angles ranging between 100°–260°. A modern linear accelerator(Varian Turebeam STX with HDMLC 2.5 mm thick leaf at isocenter) IMRT plans were generated using both 6X and 6X_FFF(Flattening filter free) beams with a nominal prescription dose of 6 Gy/fraction to PTV. Doserates ranging from 200–600 MU/min for 6X and 400–1400 MU/min for 6X_FFF, with 200 increments were examined. A fixed amount(0.3, 0.5, 1, and 2 mm) of MLC‐leaf position deviation was simulated to each plan under following conditions: 1)only along X1 collimator; 2)with increments at both X1 and X2 collimator directions;3)with reductions at both X1 and X2 collimator directions. Dose was recalculated for each modified plans. Both original and modified plans were delivered using Turebeam STX machine and measured using both portal dosimetry and a 3D dosimeter(Delta4 of ScandiDos). Each field's Result were compared using following three parameters: the 95% iso‐dose level Conformal Index(95%CI), the spinal cord maximum dose(SCDmax), and the planned target volume(PTV) mean dose.Results:Dosimetric impacts on the 95%CI, SCDmax and the PTV mean dose are: 1)negligible if MLC‐leaf position deviation only along a single collimator direction ≥1.0 mm,2)substantial if MLC‐leaf position increment along both collimator directions ≥0.3 mm(95% CI decreases while SCDmax and PTV mean‐dose increase), 3)substantial if MLC‐leaf position reduction along both collimator directions ≥0.3 mm(95% CI first increases and then decreases while SCDmax and PTV mean‐dose decrease). Different energies and doserates demonstrated comparable dosimetric impacts.Conclusion:Substantial dose deviations could happen for spinal SBRT using IMRT plan with HD‐MLC if leaf position deviation ≥0.3 mm. The effects of different energy and doserate are negligible.