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Isocenter Calibration Concept and Feasibility for MR-Guided Radiation Therapy (MRgRT)

J Winter

J Winter1*, M Carlone2, M Westmore1, S Breen2, T Stanescu5, M Dahan1, D Jaffray2, (1) IMRIS, Winnipeg, Manitoba, (2) Princess Margaret Hospital, Toronto, ON

SU-C-103-6 Sunday 1:00PM - 1:55PM Room: 103

Purpose: To describe and assess feasibility of a novel MR-to-Linac isocenter calibration tool for magnetic resonance guided radiotherapy (MRgRT™).

Methods: The MRgRT system co-developed by IMRIS and Varian employs a movable MR system, which travels into a Linac vault. To support MR-based couch corrections based on MR-to-MR registration, we developed a software tool to calibrate the MR-to-Linac coordinate system transformation and apply it to MR images acquired for treatment guidance. To assess feasibility, we quantified repeatability of the movable MRI system at MR isocenter by securing an ACR phantom to a fixed IMRIS interventional table, and moving the MR system between the two adjacent rooms simulate normal function. We performed 18 cycles over two days in which we acquired 3D MRI datasets with 1 mm isotropic resolution. We assessed position variability by registering the MR volume for each cycle to the first volume, and tabulating transform parameters. Separately, we investigated if a calibrated position encoder on the magnet mover could minimize the z-direction shift variability.

Results: Greatest isocenter variability was observed in the z direction, with mean shift of 0.21 +/- 0.12 mm over both days. The greatest rotational variability was observed about the z-axis, with mean rotation of 0.8 +/- 0.8 mrad over both days. Maximum shift was 0.51 mm in the z direction, and maximum rotation was 1.7 mrad about the z-axis. In the separate repeatability experiment we found that the calibrated position encoder reduced the mean z-shift variability from 0.59 +/- 0.35 mm to 0.17 +/- 0.13 mm.

Conclusion: We showed that the movable MRI system provides a consistent within-day and between-day isocenter location, and z-shift variability can be minimized with a position encoder. This work demonstrates the feasibility of using an MR-to-Linac isocenter calibration tool for the MRgRT system.

Funding Support, Disclosures, and Conflict of Interest: Jeff Winter, Michael Westmore and Meir Dahan are all employees of IMRIS, the company co-developing the MRgRT system. Funding for the MRgRT project at Princess Margaret Hospital was provided by the Canadian Foundation for Innovation.

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