Characterization of the on-board megavoltage imager in a magnetic resonance-guided radiotherapy machine for beam output checks.

We characterized the on-board megavoltage imager (MVI) of a magnetic resonance-guided radiotherapy machine for beam output checks. Linearity and repeatability of its dose response were investigated. Alignment relative to the beam under clinical circumstances was evaluated for a year using daily measurements. Linearity and short-term repeatability were excellent. Long-term repeatability drifted 0.8 % per year, which can be overcome by monthly cross calibrations. Long-term alignment was stable. Thus, the MVI has suitable characteristics for beam output checks.

Beam output checks of a commercial high-field magnetic resonance-guided radiotherapy machine with its on-board megavoltage imager.

Beam output checks of a commercial high-field magnetic resonance-guided radiotherapy machine can be performed with its on-board megavoltage imager (MVI). This is a fast and efficient method, but only recommended for daily checks. The aim of our study was to show its suitability for weekly checks by investigating its long-term agreement with the golden standard: ionization chamber measurements in a water tank. For one year, the output deviations obtained with both methods were compared. The difference was 0.1 ± 0.3 (1SD) percentage point. This indicated an excellent agreement, and translated into a tolerance level of ± 2 %.