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Beam Output Changes After Magnet Ramp Down in a 1.5 T Pre-Clinical MR-Linac

H Lee1*, G Bosco2 , G Ibbott1 , (1) UT MD Anderson Cancer Center, Houston, TX, (2) Elekta, Atlanta, GA


(Sunday, 7/29/2018) 2:05 PM - 3:00 PM

Room: Karl Dean Ballroom A2

Purpose: To evaluate beam measurement changes in a pre-clinical 1.5 T MR-linac immediately prior to and after ramp down of the magnet.

Methods: A rigid QA platform was used to measure cross-plane profiles with no physical shifts (influenced the most due to the orientation of the magnetic field). The radiation isocenter location was determined using a 10 mm BB (MV geometry phantom, similar to Winston-Lutz test) and onboard EPID before and after magnet ramp down. “Point� output doses were measured using an Exradin A1SL-MR ion chamber (15x15x15 cm³ water cube, at isocenter, d�₀) and EPID (1/pixel factor value). Ratio of outputs were measured using the A1SL-MR and EPID at depths of 5, 10, and 15 cm. Profiles were acquired using an IC Profiler MR with additional solid water build-ups of 4 mm (10x10 cm² field) and 2 cm (5x5, 7x7, 10x10, and 20x20 cm² fields). Profiles were acquired using a Starcheck maxi MR with 1 cm (10x10 cm² field) and 2 cm (7x7, 10x10, 15x15, and 20x20 cm² fields) additional build-up and analysed using StarReader (NCI).

Results: The radiation isocenter location did not shift in any direction after magnet ramp down. The A1SL-MR and EPID doses changed by +1.26% and +0.57%. d�₀/d₅ and d�₅/d₅ changed by <+0.13%. The CAX doses at shallow depths of 1.3/1.7 cm for IC Profiler/Starcheck changed by +5.92%/+2.05%. The IC Profiler/Starcheck measurements at 2.9/2.7 cm depth had an average change in CAX dose of +8.2±0.1%/+3.0±0.1%, up to 2.9%/3.4% difference in flatness, up to 4.2%/1.7% in symmetry, and X-axis beam center shift of -0.13±0.03/-0.18±0.04 cm, respectively.

Conclusion: Although the radiation isocenter location was not shifted between 1.5 T and 0 T, the measured cross-plane profile centers were shifted. These results suggest care should be taken when recording the device-measured CAX doses versus isocenter doses for MR-IGRT systems.


Not Applicable / None Entered.


TH- Radiation dose measurement devices: General (most aspects)

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