Room: ePoster Forums
Purpose: To assess the impact of isocenter shifts due to gantry and table rotation during stereotactic radiosurgery on PTV coverage (TVC), Paddickâ€™s conformity (CI) and gradient index (GI) for small cerebral lesions.
Methods: The deviations studied included gantry sag, the distance between collimator and table axis to the gantry axis, and the distance between CBCT and MV isocenter. An in-house script for Pinnacle 16.2 applied angle-specific isocentric shifts to each beam on plans with spherical PTVs of 1, 2.5 and 5cmÂ³. The changes of CI and GI for the 80% isodose, and TVC respectively Dâ‚‰â‚ˆ, were determined. Winston-Lutz checks were performed on two Elekta Synergy linacs using the software ISO-CBCT (QualiFormeD, France). Isocenter shifts and the distance between MV and CBCT isocenter were determined for 20 gantry, 20 collimator and 13 table angles. The linac-specific shifts were applied to 18 clinical plans with PTVs ranging from 0.45cmÂ³ to 10.4cmÂ³ and evaluated in Pinnacle.
Results: For a PTV of 1cmÂ³, CI increased from 1.2 to 1.4 when shifting the CBCT isocenter by 1mm. CI barely changed due to gantry sag and rotation axis distance. The GI increased with a 1mm distance of the collimator axis from 3.8 to 4.2, but was robust to other deviations. Dâ‚‰â‚ˆ and TVC dropped to 91% (CBCT), 96% (table axis), 97% (collimator axis) and 99% (gantry sag). Observations for the larger target volumes were less pronounced. Measured mechanical shifts for our linacs stayed well below 1mm, except for a gantry- table axis distance of 1.2mm on Linac 1. This caused a drop of up to 7% in TVC for plans with PTVs below 2cmÂ³.
Conclusion: Quality reduction was most severe for CBCT-MV isocenter mismatch, followed by axis misalignments, and gantry sag. Moreover, our tool allows us to both determine and counteract mechanical shifts during planning.