Background and purpose

This study investigates the influence of an MRI linac fringe field on neighbouring radiotherapy accelerators and whether restoring their original steering calibration can be used to shorten downtime after changes of the magnetic field of the MRI linac.

Methods and materials

A 1.5 T MRI linac was installed in a treatment room, neighboured by three accelerators. A detector array was used to acquire gantry angle dependent beam flatness and symmetry parameters. Measurements were performed before and after ramping up the magnet, and again after adjusting the beam steering for deviations in flatness and symmetry.

Results

Changes in beam symmetry as a function of gantry angle of up to 4% (Linac A) and 1% (Linac B) were observed, causing beam parameters to be out of clinical tolerance. After steering adjustment, the parameters were within tolerance for all gantry angles. Linac C did not show any significant change.

Conclusion

The MRI linac magnetic fringe field can influence the beam steering of the accelerators in adjacent bunkers. The steering of two accelerators that were situated the closest to the MRI linac needed to be adjusted in order to bring beam parameters within tolerances. Adjusting the steering completely compensated the influence of the magnetic fringe field of the MRI linac.