Purpose: To measure off-central axis dosimetric leaf gap (DLG) and investigate its impact on measured vs. calculated dose agreement for multi-met stereotactic radiosurgery (SRS) plans.
Methods: DLG values were measured on an HDMLC-Varian Edge linear accelerator for 6MV FFF photon beam. Ionization chamber measurements were performed parallel and perpendicular to leaf motion along lines that include the central axis (CAX). VMAT plans were generated for a range of target-to-isocenter distances from 0cm to 8cm in steps of 2cm. All VMAT plans were re-calculated by varying DLG values from 0.1mm to 1mm and measured with Sun Nuclear's ArcCHECK. Measured vs. calculated dose agreement was assessed with three different gamma criteria. For each target-to-isocenter distance, an optimal DLG value that maximized gamma-pass rate was identified.
Results: Measured DLG values decreased away from CAX, with perpendicular direction showing a more substantial drop-off compared to parallel direction. The outer (5mm) MLC pairs showed on average a DLG value of 0.09 mm compared with 0.16 mm for the inner (2.5 mm) MLC pairs and 0.18 mm at the CAX. Parallel to the leaf motion at 4-6 cm off-axis, the average DLG value was 0.15 mm. This drop is less pronounced at 1.5-4 cm off-axis, where the average DLG was 0.16 mm. Only a strict gamma-passing criterion like 2%/1mm resulted in ArcCHECK detecting differences in optimal DLG for varying target-to-isocenter distances. Over the range of measured distances, only a DLG value of 0.2 mm was identified as providing at least 90% pass-rates. Use of other DLG values led to higher pass-rates at small target-to-isocenter distances, yet for larger distances, pass-rates below 90% were observed.
Conclusion: With the current treatment planning system limitation of a single DLG value, the importance of optimizing DLG over a range of target-to-isocenter distances relevant for multi-met SRS has been shown.
Stereotactic Radiosurgery, Dosimetric Leaf Gap