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Characterization of the Limitations of the Mobius3D Dosimetric Leaf Gap Offset Correction

A Shepard*, J Hansen, M Belanger, S Frigo, University of Wisconsin-Madison, Madison, WI

Presentations

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

Room: AAPM ePoster Library

Purpose: To assess the applicability and limitations of the dosimetric leaf gap offset correction (DLGOC) in the Mobius3D secondary dose-check software.

Methods: The procedure recommended by Mobius3D for DLGOC optimization was followed for models of a TrueBeam and TrueBeamSTx. Mobius3D calculations with varying DLGOC values were compared to measurements of seven IMRT test plans (target volume: 16-1634 cm³). Plans were delivered to a Tomotherapy “cheese” phantom with A1SL ion chamber inserts along the central plane of the phantom. The optimal DLGOC was determined from a linear fit of the dose difference between measurements and the corresponding Mobius3D calculations for a range of DLGOC values. This process was performed collectively across all seven plans for each photon energy on both machines, as well as for each plan individually. The DLGOC resulting in an average dose difference of 0% across all plans was taken as the collective optimal value.

Results: The resulting DLGOC across all energies and machines ranged from -0.71mm to 1.85mm. With the optimal DLGOC in place, the variability in the calculated dose difference for the seven plans relative to the measured dose ranged from -2.6% to 6.1%. When optimized on a per-plan basis, the DLGOC ranged from -5.38mm to 4.59mm. In general, plans requiring a more positive DLGOC corresponded to a smaller average leaf position from the central axis.

Conclusion: The determination of the optimal DLGOC is impacted by the suite of test plans chosen for analysis, making it essential that representative plans of the expected clinical cases are chosen during commissioning. Additionally, the variability in dose difference across plans for a single DLGOC, and the variation in the optimal DLGOC per-plan, indicates that the full spectrum of plans cannot be completely represented by a single DLGOC. The functionality of multiple DLGOC values may be necessary for increased accuracy.

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Keywords

Modeling, MLC

Taxonomy

TH- External Beam- Photons: General (most aspects)

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