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SU‐F‐BRE‐09: Linac Isocenter Quality Assurance: A Stereotactic Approach
Author(s) -
McCabe B,
Li J
Publication year - 2014
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.4889047
Subject(s) - truebeam , isocenter , collimator , linear particle accelerator , optics , physics , dosimetry , collimated light , nuclear medicine , beam (structure) , laser , imaging phantom , medicine
Purpose: A quantitative method was designed to independently determine the couch/collimator/gantry isocenters, as well as the overall LINAC mechanical isocenter in 3D. Methods: Performed on both a Varian TrueBeam™ STx and Trilogy with the gold standard front pointer positioned at 100 cm SAD, and a Radionics XKnife™ (RX) attached to the couch. At gantry and couch 0°, the RX laser alignment attachment (RXLAA) was centered to the front pointer using the micrometers (0.1‐mm precision) on the RX. The 3D coordinates of the micrometers were recorded. The collimator was rotated to 90° and 270°. At each collimator rotation, the RXLAA was re‐centered to the front pointer and the micrometer coordinates recorded. At collimator and gantry 0°, the process was repeated for couch angles 0°/90°/270°. Finally, at collimator and couch 0°, the steps were repeated for gantry rotations 0°/90°/180°/270°. The centers/radii of the smallest bounding spheres for the collimator, couch and gantry walkout were calculated (using MatLab™). The smallest bounding sphere containing the collimator, couch and gantry walkout spheres was then calculated. The center of this all‐encompassing sphere is the overall mechanical isocenter of the LINAC. This position was dialed in on the RX. LINAC mechanical and radiation isocenter coincidence was determined by performing Winston‐ Lutz test at four cardinal gantry angles. Results: TrueBeam and Trilogy mechanical isocenters had overall walkout radii of 0.8 mm and 1.5 mm, respectively. For the TrueBeam and Trilogy 6‐MV beams, the radii of radiation isocenter were 0.3 mm and 0.4 mm, respectively, with distances between LINAC mechanical and radiation isocenters of 0.5 mm and 0.9 mm, respectively. Conclusion: This efficient and simple method allows for an independent and reliable quantitative assessment of LINAC isocenter in 3D with equipment typically available in a radiation oncology clinic. It can easily be performed for LINAC commissioning and annual QA.