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  • Title: Off-iso Winston-Lutz test on seven linear accelerators.
    Author: Gao J, Anand D.
    Journal: J Appl Clin Med Phys; 2024 Oct; 25(10):e14470. PubMed ID: 39042435.
    Abstract:
    PURPOSE: The aim of this study is to find optimal gantry, collimator, and couch angles for performing single isocenter, multiple target stereotactic radiosurgery (SIMT-SRS). Nineteen angle sets were tested across seven linear accelerators for radiation-isocenter coincidence and off-isocenter coincidence. The off-isocenter Winston-Lutz test was performed to evaluate the accuracy of isocenter alignment for each angle set, and optimal angle sets as well as maximum off-isocenter distance to target for each angle set was determined. The influence of simulated patient weight on off-iso Winston-Lutz test accuracy was also inspected. METHOD: The SNC MultiMet-WL phantom and MultiMet-WL QA Software v2.1 were used for the direct measurement and analysis of the off-iso Winston-Lutz test (also referred to as Winston-Lutz-Gao test). A two-step method was developed to ensure precise initial placement of the target. Nineteen beams were delivered at 6X energy and 2 × 2 cm field size to each of six targets on the MultiMet Cube with couch kicks at five cardinal angles (90°, 45°, 0°, 315°, and 270°). To reduce imaging uncertainty, only EPID was used in target alignment and test image acquisition. A total of 200 Ibs (90.7 kg) of weight was also used to mimic patient weight. All tests were performed on both the free table and the weighted table. RESULTS: For two new TrueBeam machines, the maximum offset was within the 1 mm tolerance when the off-iso distance is less than 7 cm. Two older VitalBeam machines exhibited unfavorable gantry, couch, and collimator (GCC) angle sets: Linac No. 3 at (0,90,0), (0,270,0) and Linac No. 4 at (0,45,45) and (0,90,0). The C-Series Linacs failed in the majority of GCC angle sets, with Linac No. 5 exhibiting a maximum offset of 1.53 mm. Four of seven machines show a clear trend that offset increases with off-isocenter distance. Additionally, the IGRT table was less susceptible to the addition of simulated patient weight than the ExactCouch. CONCLUSION: Among the seven linear accelerators addressed, newer model machines such as the Varian TrueBeam were more precise than older models, especially in comparison to the C-Series Linacs. The newer machines are more suitable for delivering SIMT-SRS procedures in all GCC angle sets, and the results indicate that newer TrueBeams are capable of performing SIMT-SRS procedures at all angle sets for targets of off-iso distances up to 7 cm. The trend that offset between the target center and radiation field center increases with off-iso distance, however, does not always hold true across machines. This may be comprised by the EPID's severe off-axis horn effect. Lastly, the IGRT couch was less susceptible to patient weight compared to ExactCouch in the off-isocenter Winston-Lutz test.
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