Optimizing TrueBeam jaw calibrations to reduce positional errors at customizable gantry and collimator angles

Clinical medical physicists in a radiotherapy setting are aware of the importance of maintaining linac collimators with high positional accuracy, especially when used to form junctions; however, there is a lack of quality control options available. This is especially true when trying to improve jaw matching in junctions or when trying to maintain accurate jaws across gantry angles other than 0 degrees.

In this work, we address this quality assurance niche by developing a jaw calibration optimization program, which defines jaw calibration positions to minimize jaw positional errors and junction matching errors over customizable gantry and collimator angles.

A jaw calibration optimization program was developed and evaluated on two Varian TrueBeam linacs. Half beam blocked MV EPID images were acquired to characterize jaw positions and clinically relevant breast junction errors across a set of gantry and collimator angles. Collimator encoder values were correlated with measured jaw positions, allowing the jaws to be set precisely during calibration. The Jaw calibration program analyzed DICOM images, modelled encoder-position relationships, and optimized the calibration origins for all four jaws. The jaw calibration was performed and then verified by redoing the analysis with the new jaw settings, and standard QA of graph paper jaw position, light field radiation coincidence, and jaw matching.

The jaw calibration was found to reduce clinically relevant junction errors on both units, from a maximum of 1.5 mm to 0.3 mm on Unit A and 1.2 mm to 0.7 mm on Unit B. The sum of cold junctions was reduced from 9.5 mm to 0.8 mm and from 4.9 to 0.6 mm, respectively. Mean jaw positional errors decreased significantly for some jaws and remained constant for others. Predicted jaw positions following calibration agreed well with measurements.

Our results demonstrate the viability of this jaw calibration optimization program as a quality assurance tool. This program has been made open source and provides a unique QA solution for a common issue in centers operating linear accelerators for radiotherapy.