Small Field Dosimetric Characterization of 7MV FFF Photons for Stereotactic MR-Guided Adaptive Radiotherapy Using the Unity MR-Linac
M Hwang*, X Chen, S Oh, D Lee, D Pavord, B Coopey, J Sohn, Radiation Oncology, Allegheny Health Network, Pittsburgh, PA Drexel University College of Medicine, Philadelphia, PA
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PO-GePV-T-376 (Sunday, 7/10/2022) [Eastern Time (GMT-4)]
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Purpose: Interaction between the magnetic fields and the radiation particles in the MR-Linac (MRL) affects the dosimetric properties in comparison to the conventional linear accelerator. Our goal was to analyze the dosimetric characteristics of small fields in the MRL, which are largely used for stereotactic MR-guided Adaptive Radiotherapy treatment with online plan adaptation to shape.
Methods: EBT3-based film measurements were performed using a cubic phantom with water-equivalent 1cm thick slabs. The phantom was oriented and affixed rigidly to the Unity couch at 133.5 cm SSD. Precisely-cut radiochromic films were irradiated between the compression slabs in two orientations, parallel and orthogonal to the beam central axis. The 2D dose distributions were measured at various field sizes and depths, and analyzed using the Film Pro software. Square fields ranged from 0.5 cm to 5 cm were investigated with a 0.5 cm or 1 cm interval. Single beam treatment plans were designed using the Monaco with the Monte Carlo calculation. Change of focal spots, percentile depth profiles, dose distributions, penumbrae, relative output factors, and symmetry/flatness were evaluated and compared with ones calculated by the Monaco and an in-house Monte Carlo Method.
Results: The radiation fields of the 7MV FFF photons of the Elekta Unity MR-Linac system showed change in dose profiles and PDDs depending on the field size, in particular in the transverse orientation. When it becomes smaller than a 1.0 x 1.0 cm2, location of the maximum dose swifts up to 2.5 mm and penumbra size increases up to 41.7 % only in the positive transverse orientation. The change decreases as the field size increases to the 5.0 x 5.0 cm².
Conclusion: Interaction between the magnetic fields and the radiation particles in the MR-Linac was much significant in the small field compared to large fields in terms of the location of the maximum dose and penumbra size. .
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