Evaluation of Off-Axis Spatial Precision for Varian and Elekta Linear Accelerators
C Ramsey1,2*, J Bowling1,2, A Usynin2, I Pfeiffer2, S West2, D Chase2, (1) University of Tennessee, Knoxville, TN (2) Thompson Cancer Survival Center, Knoxville, TN
Presentations
(Saturday, 3/26/2022) [Central Time (GMT-5)]
Purpose: The coincidence of radiation and mechanical isocenter is typically measured using a Winston-Lutz test with a phantom placed at isocenter for fields shaped with the primary collimators. However, the treatment of multiple metastases with volume modulated arc therapy (VMAT) often involves target located away from isocenter. The purpose of this study is to evaluate the coincidence of the radiation and mechanical isocenter for asymmetric beams on a wide range of Varian and Elekta linacs.
Methods: The coincidence of the radiation and mechanical isocenter was measured (1) at isocenter and (2) 3-cm off axis for a Varian 21EX, two Varian TrueBeams, one Elekta Synergy, one Elekta Infinity, and one Elekta VersaHD. Ball bearing phantoms were placed on the treatment table and aligned with the center of on-axis and off-axis fields shaped with the multileaf collimator. A total of sixteen fields were used to evaluate the coincidence of the radiation and mechanical isocenter. These fields included combinations of four gantry angles, five couch angles, and five collimator angles. Images were obtained using the linacs’s electronic portal imaging device (EPID) and analyzed using the RIT TG-142 software.
Results: Measurements were performed at isocenter and at an off-axis position that was located 3-cm horizontally away from the gantry. The difference in spatial precision between the measurements on axis and off-axis for the Varian TrueBeam linear accelerators was -0.01 ± 0.06-mm, -0.17 ± 0.03-mm, and -0.12 ± 0.22-mm in the dX, dY, dZ directions respectively. For the Elekta linear accelerators, the difference in precision between the measurements on axis and off-axis was -0.42 ± 0.52-mm, -0.11 ± 0.11-mm, and -0.46 ± 0.43-mm in the dX, dY, dZ directions respectively.
Conclusion: For distances within 3-cm of isocenter, the coincidence of the radiation and mechanical isocenter was less than 1-mm for both Varian and Elekta linacs.
Keywords
Stereotactic Radiosurgery, Quality Assurance, Performance Tests
Taxonomy
TH- External Beam- Photons: Quality Assurance - Linear accelerator
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