(Saturday, 4/7/2018)
Room: Foyer
Purpose: To perform three-dimensional (3D) isocentricity test of a medical linear accelerator (LINAC) using a 3D dosimeter (PRESAGE). The test includes the isocenter size and position measurement for gantry and couch rotations, whose precise knowledge is important for the accurate delivery of non-coplanar treatment plans.
Methods: A 3D dosimeter was set up on the treatment couch of a Varian TrueBeam LINAC and the coincidence with the setup lasers was marked using metal ball bearings. Irradiations were performed under gantry angles of 0°, 50°, 160°, and 270° with the couch fixed at 0° and subsequently, under couch angles of 10°, 330°, 300°, and 265° with the gantry fixed at 270°. The 1 cm2 (at 100 cm SAD) square fields were delivered at 6 MV with 800 MU/field. After irradiation the dosimeter was scanned using a single-beam optical scanner and images were reconstructed with sub-millimeter resolution using filtered back-projection. The stack of reconstructed images was rotated in 3D to extract stacks perpendicular to the gantry and couch rotation axes showing the star shots. Beam trajectories and the smallest circle enclosing these were drawn and extracted from both images. Information from the third orthogonal stack was used for 3D localization.
Results: The gantry and couch rotation isocenter diameters were measured to be 0.41 mm and 0.70 mm while the distances to the setup center in 2D/3D measured 1.30/1.73 mm and 0.16/0.72 mm, respectively. The isocenter sizes as well as the 2D couch isocenter distance agree with the film measured values. The gantry isocenter distance, however, was not reproducible and is likely due to a setup center registration error.
Conclusion: This study demonstrates that 3D dosimeters can be used to concurrently measure isocenter sizes and their position with appropriate procedure and provides more information than 2D measurements.
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