Automated Film-Based Quality Assurance Framework for Elekta Unity Mlc Position Verification Using Stakitt Test
Abstract
Purpose
To develop and evaluate a film-based picket fence (PF) test and automated analysis for determining multi-leaf collimator (MLC) position accuracy on the Elekta Unity MR-Linac.
Methods
An elongated film strip was created by joining three Gafchromic RTQA2 sheets end-to-end, providing coverage for 73 of the 80 Unity MLCs. The film was placed at isocenter (143.5cm SAD) between two 2-mm sheets of copper to address electron return effects and minimize Lorentz Force impact on secondary electrons. A modified PF test, named stakitt, was performed on the film using 5 central pickets of 0.5cm width spaced 2cm apart alongside two lateral comb patterns. Individual MLCs were identified using the comb patterns and the central MLCs were marked by elongated teeth. In-house python scripts registered and analyzed films to determine MLC positions using a single scan of the full strip. Results were compared to a routine PF test of the central 30 MLCs using an onboard electronic portal imaging device (EPID). The detection algorithm's sensitivity was validated by digitally introducing a controlled 1-mm error.
Results
The mean absolute error (MAE) and maximum leaf error detected using a 250 MU stakitt delivery and automated analysis were 0.09±0.07mm and 0.33mm respectively, both within an institutional tolerance of 0.5mm of expected values. These values agree with a PF test of the central 30 MLCs obtained via EPID (MAE = 0.05±0.03mm, maximum = 0.19mm). The deliberate 1mm shift in MLC positions was detected within 0.03mm of its expected position and appropriately flagged.
Conclusion
The film-based stakitt procedure addresses PF difficulties inherent to the Unity system by evaluating MLCs outside of the EPID’s field of view and using the test’s signature comb pattern to aid in leaf identification. Altogether, the procedure allows for the routine and efficient quality assurance of MLC positional accuracy on the Unity MR-Linac.