Y Niu*, Y Yuan, A Rashid, D Pang, Medstar Georgetown University Hospital, Washington, DC

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  Y Niu

TU-D1030-IePD-F7-5 (Tuesday, 7/12/2022) 10:30 AM - 11:00 AM [Eastern Time (GMT-4)]

Exhibit Hall | Forum 7

Purpose: SRS MapCheck (SMC) is a high-resolution diode array developed for stereotactic radiosurgery dosimetry. We evaluated its performance in dosimetry of ZAP-X, which is a novel radiosurgery system characterized by 3MV flattening filter free (3MVFFF) radiation beams.

Methods: ZAP-X linac is mounted on a gyroscope-like gantry, which enables non-coplanar beams from ~2π steradians solid angles. Circular collimation varies from 4 to 25 mm in diameter. Patient table movement enables multi-iso treatment. An integrated kV imaging system provides head/phantom alignment. The SMC is a 77x77 mm2 diode array with 2.47mm inter-detector spacing, which is accommodated by a cylindrical PMMA phantom (i.e., StereoPHAN). In this study, absolute calibration was applied to the central diode with the calculated dose in a homogenous body from treatment planning system (TPS), which was verified using a miniature ion chamber. Conventional flattened 6MV field was used for relative array calibration, and the angular response function was scaled through measurements of parallel-opposed 3MVFFF fields. Measurements were performed to investigate the angular dependence of diode array. End-to-end tests were performed for selected patient plans, and gamma analysis results were compared between the tests of SMC and Gafchromic films.

Results: Measured result of ion chamber agrees with the TPS-calculated value within 2% at the absolute calibration point. The corrected angular response of central diode in transverse plane stayed within 4% deviation until the oblique beam direction approached the diode plane. In end-to-end tests, phantom alignment with onboard kV imaging system resulted in submillimeter dose-profile alignment, the average gamma analysis pass rates of patient plans using SMC/Gafchromic films are 97.6%/95.9% with 2%/1mm criteria.

Conclusion: The SMC diode array in the StereoPHAN phantom is a useful and convenient tool for ZAP-X dosimetry with sufficient accuracy. Further studies are needed to generate and validate the correction formalism tailored to 3MVFFF beams.

Keywords

Not Applicable / None Entered.

Taxonomy

TH- External Beam- Photons: Quality Assurance - IMRT/VMAT

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