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Accuracy of Isocenter Location and Rotation Axis: Possible Consequences for Stereotactic Irradiations and Measurements at Elekta Linacs

L Wack*, O Sauer , University Hospital Wuerzburg, Wuerzburg, BY


(Sunday, 7/14/2019)  

Room: ePoster Forums

Purpose: To assess the impact of isocenter shifts due to gantry and table rotation during stereotactic radiosurgery on PTV coverage (TVC), Paddick’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 5cm³. The changes of CI and GI for the 80% isodose, and TVC respectively D₉₈, 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 0.45cm³ to 10.4cm³ and evaluated in Pinnacle.

Results: For a PTV of 1cm³, 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. D₉₈ 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 2cm³.

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.


Stereotactic Radiosurgery, Quality Assurance


TH- External beam- photons: intracranial stereotactic/SBRT

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