2.B. MLC position test

The MPC MLC positioning test uses an image of a static MLC comb‐shaped pattern. The position of each leaf is determined in these images and compared to the intended position. The MLC position is defined as the distance of the MLC tip from the MLCs center line, where the center line is determined as the line through the center of rotation of the MLC, perpendicular to the MLC direction of movement.

Similar images are also used to check the collimator angle readout and to find the position of the radiation isocenter relative to the collimator rotation axis. The mean and maximum differences between intended and measured positions for all the leaves in each MLC bank are included in the MPC report.

To assess the accuracy of the MPC MLC positioning test for the Halcyon system, we intentionally miscalibrated one MLC position by 0.5, 1.0, 1.5, 2.0 mm, and 5.25 mm. This was achieved by changing the MLC offset — this is a setting not accesible to users, and required Varian engineering support. The MLC position values reported by MPC were compared with these known miscalibrated values. The accuracy of MPC MLC positioning for the TrueBeam has been well studied by Barnes et al., and was not repeated in this work.