We compare the accuracy of the off-axis Winston–Lutz (WL) test in two versions of ExacTracTM: version 6.0 (ETv6) and Dynamic (ETD) in the same linac (TrueBeam STx®).

An upgraded of the ExacTracTM system was done in our institution. It was designed as an off-axis WL test before the update for comparison purposes. A head 3D-printed phantom based on a patient’s computed tomography images was used. Nine metallic fiducials were inserted and distributed on the phantom. Each target (fiducial) was designed an off-axis WL test with eight different gantry/collimator/table combinations. The phantom was placed using two different ETv6 and ETD in the same linac, and cone-beam computed tomography and electronic portal imaging device (EPID) images were acquired. The 2D deviation between the centre of the fiducial and the radiation field was found and compared with the original digital reconstructed radiography (DRR) by the profiles.

The phantom allows the definition of a procedure to determine off-axis deviations in radiosurgery treatments. The displacements calculated from the WL test showed acceptable values for both versions taking into account 3D displacement tolerances of 1 mm. These values were reached with rigorous quality assurance (QA) linac tests performed routinely that include mechanical, MV/kV and image-guided radiotherapy (IGRT) tests. However, ETD indicated more accurate values for all the targets no matter the distance to the isocentre (3D displacements < 0·5 mm).

In terms of the IGRT correction without set-up displacements, ETD is up to twice as accurate as the ETv6, showing 3D displacements up to 0·5 mm in all targets.