The ED system incorporated in a Varian TrueBeam STX accelerator was
tested using the Brainlab cranial verification phantom with its mid-brain
radiopaque sphere.
A simulation scan (slice thickness
of 1,25mm) of the phantom was obtained in a General Electric LightSpeed CT,
using the Cranial 4Pi Immobilization system. A treatment plan with 0º,
90º, 180º and 270º gantry angles beams collimated by the 15 mm Brainlab
circular cone was created in iPlan 4.5 treatment planning system. The isocenter
of the plan was placed at the center of the radiopaque sphere.
Repeatability of the surface positioning system was
evaluated placing the phantom with the same immobilization as above, letting
the system to perform the movements to bring the couch to the treatment
position. The procedure was performed five times without moving the phantom
between them.
SL system accuracy was determined using a
hidden target (HT) test acquiring integrated images with the linac electronic
portal image system, once the phantom was positioned at the isocenter,
according to the SL system. The HT test was
performed three times. For each HT test the assembly of the phantom and its
immobilization system was repeated.
The displacements that stereoscopic
KV x-ray and CBCT positioning systems calculate after the surface SL prepositioning
of the phantom were registered to obtain the accuracy of the SL system
independently. After applying the displacements calculated by each of the
systems, a HT test was done to assess their suitability as reference systems.