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An Alternative Approach to Verify 6FFF Beam Dosimetry for Ethos and MR Linac Without Using a 3D Water Tank

Y Hao1*, B Cai1, O Green1, N Knutson1, S Yaddanapudi2, T Zhao1, V Rodriguez1, M Schmidt1, S Mutic1, B Sun1, (1) Washington University School of Medicine, St. Louis, MO, (2) University of Iowa, Iowa City, IA


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

Room: AAPM ePoster Library

Purpose: The current approach for linac beam dosimetry verification are typically performed utilizing a 3D water tank system. The entire 3D beam scanning process is cumbersome, labor intensive, error prone and costly. This is especially challenging for the new Ethos system and MR linacs. This work proposes an alternative approach to verify 6FFF beam dosimetry for Ethos (Varian Medical Systems, Palo Alto, CA), MRIdian ViewRay (ViewRay, Cleveland, OH) and other Linacs with 6FFF beam with 2D ion chamber arrays.

Methods: The PDD and profiles of an Ethos system, an MR linac and several linacs with 6FFF beams were measured at the nominal beam current with a TG-51 compliant 1D tank and 2D ion chamber arrays, respectively. The beam energy was detuned by changing the bending magnet current on one of the TrueBeam linac. The PDD and profiles were measured for the detuned beam energy. Several profiles features were calculated. Correlations between profiles features and PDDs were evaluated for the sensitivity of beam energy to beam profile changes.

Results: Strong correlations were found between profiles and PDDs for all the Linacs with 6FFF beam we investigated. Both the profile slope and unflatness are proportional to PDD at 10 cm depth of the beam. The correlations with PDD are 0.97 and 0.86 for slope and unflatness, respectively. In our experiment, less than 0.5% of PDD change can be observed on the profile variations.

Conclusion: Our work shows the feasibility of verifying 6FFF beam quality of Ethos, MR Linac and other Linacs by using 2D ionization chambers array devices. This new approach provides a simplified method for routine beam quality check without using 3D water tank system while maximizing cost effectiveness and efficiency.

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Dosimetry, Quality Assurance


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

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