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Verification of MV Radiation Isocenter Using the CIRS ISO Phantom Model 23A

W Donahue1*, H Chen1, M Lazea2, Z Chen1, B Wang1, (1) Yale Univ. School of Medicine, New Haven, CT, (2) CIRS, Norfolk, VA


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: TG-142 recommends annual verification of the mechanical and radiation isocenters. Older machines with advanced radiation techniques may require more frequent monitoring. However, these tests can be time consuming. The purpose of this study is to evaluate the accuracy and efficiency of a new CIRS ISO Phantom for these tests.
Methods: Isocenter measurements were performed on an Elekta Synergy using traditional techniques and a CIRS isocenter phantom. For traditional techniques, the gantry, collimator, and couch isocenters were measured using starshots with films and the radiation isocenter was measured using a Winston-Lutz test with a ball bearing (BB) phantom. Both were analyzed with a commercial image analysis software (PipsPro v. 5.5) These measurements were repeated using the CIRS ISO Phantom Model 23A and analyzed by the CIRS ISO Analyze software. The time spent in collecting the data and performing the analysis were recorded, respectively. The results of the analyses were compared for the repeatability and accuracy.
Results: The ISO Phantom data collection and analysis was 3.4 times faster than the traditional techniques (19±1 minutes vs 64±3 minutes). The starshot data collected with the ISO Phantom was within 0.4 mm of the film based one. This discrepancy is due to the fact that the ISO Phantom results are directly impacted by the initial alignment with the lasers, while starshots are not. The Winston-Lutz test performed using both methods agreed within 0.07 mm.
Conclusion: The CIRS ISO Phantom accurately measured the radiation MV isocenter in a reduced amount of time, making more routine measurement of these quantities convenient. This is especially important for older machines where mechanical wear may cause increased variation in the isocenter position and require more frequent checks than annually.

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Funding Support, Disclosures, and Conflict of Interest: CIRS provided the phantom and software at no cost for user evaluation prior to commercial release. Yale University performed the evaluation of phantom and the data collected was used to generate this abstract.


Linear Accelerator, Quality Assurance


TH- External Beam- Photons: Quality Assurance - Linear accelerator

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