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Commissioning of Magnetic Resonance Imaging-Based Tumor Tracking and Beam Control

O Green

O Green1*, L Rankine1 , B Cai1 , L Santanam1 , R Kashani1 , A Sharma2 , L Senadheera2 , C Mahaffey2 , S Mutic1 , (1) Washington University School of Medicine, St. Louis, MO, (2) ViewRay, Inc., Oakwood, OH


TH-AB-303-12 (Thursday, July 16, 2015) 7:30 AM - 9:30 AM Room: 303

Purpose: To develop a methodology for commissioning the real-time tumor tracking and beam control capabilities of a magnetic resonance imaging radiation therapy (MR-IGRT) system.
Methods: The MRIdian (ViewRay, Inc., Oakwood Village, OH) MR-IGRT machine consists of a 0.35-T magnet and a three-head Co-60 delivery system capable of real-time sagittal plane acquisition at 4 frames per second (fps). After obtaining a volumetric MR image, a "target" is chosen by contouring an area of interest with different signal intensity than the surrounding medium. A "boundary" is created by expanding the target by some margin or using a previous contour. After a sagittal plane is chosen, during delivery the system deforms the target contour on each acquired plane, and compares it to the boundary - if the target contour is observed outside the boundary, the beam is turned off. Since the system uses no external or internal surrogates, careful tests of the individual system components had to be created and executed to characterize the accuracy and potential dosimetric errors associated with real-time gating on real anatomy.
Results: Tests for spatial, temporal, and dosimetric accuracy were devised; these included latency, spatial integrity of the magnetic field, film, and ionization chamber measurements. A new set of phantoms was created, including a CIRS (Norfolk, VA) motion phantom that was modified to function in the magnetic field and have appropriate signal intensities. This phantom also included optical sensors that allowed for signal readout of its target position, and subsequent comparison to "beam off" signal on the heads. Boundary margins and other parameters were evaluated with film dosimetry by comparing the spatial distribution of the dose for static vs. gated deliveries. Ionization chambers were used to confirm an implemented shutter dose correction.
Conclusion: The first commercially-available MR-IGRT system's gating capabilities were characterized and successfully implemented in the clinic.

Funding Support, Disclosures, and Conflict of Interest: Drs. Green, Kashani, and Mutic have held non-compensated consultant roles, received honoraria, and had travel expenses paid by ViewRay, Inc.

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