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Initial Performance Tests of a High-Field MR-Linac

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E Paulson

E Paulson*, X Chen, N Mickevicius, S Klawikowski, E Ahunbay , XA Li , Radiation Oncology, Medical College of Wisconsin, Milwaukee, WI


TU-FG-FS2-9 (Tuesday, August 1, 2017) 1:45 PM - 3:45 PM Room: Four Seasons 2

Purpose: MR-guided radiation therapy is an emerging high precision, adaptive treatment modality. We present here initial results of system level tests performed on a high-field MR-Linac system.

Methods: Measurements were performed on a 1.5T Elekta MR-Linac system over three consecutive days following magnet ramp up. MR system measurements included: magnetic field (B0) homogeneity, B0 stability, B0 versus gantry rotation, vendor-provided periodic image quality tests (PIQT), ACR MRI QA tests, 3D gradient linearity characterization, and motion accuracy assessment with dynamic motion phantom. Linac system tests included: cross-plane beam profile and gantry star-shots using Gafchromic film, onboard EPID MV image quality using the Las Vegas phantom, and delivery of static and IMRT fields onto an MR-compatible 3D diode array. When possible, tests were performed with and without irradiation, or with and without MR imaging.

Results: Mean B0 homogeneity over a 30 cm phantom with gantry at 0 degrees was -0.023 ppm. B0 instability was less than 0.003 ppm. With gantry rotation B0 ranged from -0.023 to 0.0366 ppm (0.064 – 0.27 ppm RMS). Vendor-provided PIQT passed successfully. ACR MRI QA tests passed successfully using 1.5T tolerances. Gradient linearity was within 1 mm for 20 cm radius from isocenter. Peak-to-peak dynamic target motion was estimated to be 4 cm with 4 second period, matching programmed values. No spurious signals were induced in phased-array RF coils by pulsatile irradiation. Gantry rotation isocenter fell within 0.65 mm radius. MV image quality tests passed successfully. A step and shoot IMRT plan was successfully delivered and measured. These initial results further demonstrate that the integration of a linac with a high field MRI does not compromise the performance of either system.

Conclusion: The integration of a linac and high field MRI does not compromise the performance of either system, offering a revolutionary technology for radiation therapy.

Funding Support, Disclosures, and Conflict of Interest: Elekta Instruments, AB

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