Standardizing the Mlc Gap and Characterizing Its Effect on the Dosimetric Leaf Gap
Abstract
Purpose
To Standardize multi-leaf collimator (MLC) leaf-end dosimetry by adjusting the physical MLC gap across multiple linacs within an enterprise.
Methods
A tunable parameter within the treatment planning system (TPS) to characterize the leaf-end dosimetry – the Dosimetric Leaf Gap (DLG) – was measured across 11 Truebeam linear accelerators with the intent to homogenize the gap by modifying the physical gap between the two carriages of MLC. DLG measurements and patient specific quality assurance measurements were taken before and after each gap modification. The patient QA includes portal images of the chair and pyramid patterns as well as four patient plans. The change in DLG and the change in gamma pass rates for 2%/2mm and 3%/3mm criteria were recorded to characterize the clinical impact of the gap offset.
Results
Prior to DLG standardization, DLG values ranged from [1.53, 2.01], [1.42, 1.8], [1.68, 2.18], and [1.59, 2.04] for 6X, 6X-FFF, 10X, and 10X-FFF energy modes. Attempts were made to adjust the DLG parameter to the lowest of these parameters. Gap adjustments were calculated from difference in the DLG with requested Gap modifications matching post-adjustment DLG values by an average of 0.03mm.
Conclusion
The change in physical leaf gap closely mirrored the change in DLG as expected. The difference in portal pass rates was clinically insignificant, allowing the clinic to phase in DLG standardization with minimal impact to plan quality.