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Can a Commercial System for MR-IGRT Be Used to Treat Patients Without Acquiring a CT Scan?

H Wooten

H Wooten*, S Yaddanapudi , L Santanam , H Li , S Mutic , Washington University School of Medicine, St. Louis, MO


SU-E-J-204 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall

Patients treated using a magnetic-resonance image guided radiation therapy (MR-IGRT) system received both CT and MR simulations. During planning, the CT is used to determine relative electron density (RED) using a calibration table. This study aims to investigate the feasibility of MR-only treatments by comparing CT-computed dose distributions to those computed with combinations of water (1.0), lung (0.26) , tissue (1.02) , and bone (1.12) bulk RED overrides, and to identify the effects of the magnetic field on the RED-overridden doses.

Four patients who received treatment using a commercial MR-IGRT system were analyzed (1 lung, 2 abdomen, and 1 pelvis). The clinical plans were computed using the first fraction MRI as primary, and the simulation CT as secondary for REDs. Plans were re-optimized using default bulk RED overrides (water/lung and tissue/lung for the lung patient, water/bone, tissue/bone, water only, and tissue only for the abdomen and pelvis patients). Additionally, each plan was re-optimized to include the static magnetic field. All plans were normalized to the same PTV coverage as the clinical plan. Dose-difference volumes and DVHs were computed for bulk density override plans, and 3D gamma analyses between each plan and its accompanying magnetic field plan were performed using 3%/3 mm dose difference and distance-to-agreement criteria using the PTV and Skin as masking structures.

The average differences in PTV and organs-at-risk mean dose for all RED combinations tested were
-0.19 Gy (-0.62 – 0.06 Gy) and -0.34 Gy (-1.76 – 0.33 Gy), respectively. The average PTV and Skin gamma pass rates for all RED combinations tested were 99.88% (99.5% - 100%) and 98. 35% (96.3% - 99.6%). No systematic differences in DVHs or isodoses were observed.

It is likely that that a commercial MR-IGRT system may produce high quality treatment plans without the need for CT scans.

Funding Support, Disclosures, and Conflict of Interest: Authors of this abstract are members of the Washington University Radiation Oncology department, which has a research agreement with ViewRay, Inc.

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