Reducing Variability In Truebeam Jaw Calibration through a QA Track+ Integrated Encoder Workflow
Abstract
Purpose
To implement a standardized, encoder‑driven TrueBeam jaw calibration workflow within QATrack+ that improves jaw‑junction dose uniformity and calibration precision. The workflow integrates EPID‑based jaw analysis, encoder‑target prediction, and longitudinal trending to reduce operator variability. This work was motivated by the limitations of a 2 mm action level per jaw, which can permit field‑edge mismatch in abutting fields and lead to unacceptable junction‑dose peaks or valleys.
Methods
Two QATrack+ testlists were developed: a monthly jaw‑trending test and an ad hoc calibration‑support test. Monthly trending uses EPID images at nominal jaw settings, with automated analysis determining radiation‑defined jaw positions, quantifying deviations from nominal, and predicting expected junction positions at isocentre. After machine service, an optical calibration provides the baseline needed for refinement. The EPID‑based jaw analysis test is then used to quantify residual offsets and guide targeted adjustments during precision calibration. Precision calibration refines the previous jaw calibration by using the current encoder readings and the measured accuracy of the jaws. QATrack+ determines the jaw‑position–encoder relationship for that session and calculates the optimal encoder values for the true calibration points. The user manually drives the jaw to these targets using the pendant, while QATrack+ reports the deviation between current and optimal encoder values to indicate when the position is acceptable. All final encoder values and measurements are stored as records.
Results
The workflow reduced inter‑user variability, improved jaw‑position accuracy across the calibration curve, and enhanced junction‑dose consistency. Monthly trending maintained predicted junction‑dose offsets within 5%.
Conclusion
The improved reliability enabled tightening jaw tolerances from 2 mm to 1 mm, providing a robust and consistent method for maintaining junction‑dose uniformity.