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Dosimetric Leaf Gap of Varian HD120 for Accurate Dose Calculation

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J Kim

J Kim*, J Han , A Hsia , Z Xu , S Ryu , Stony Brook University, Stony Brook, NY


SU-I-GPD-T-467 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Dosimetric leaf gap (DLG) represents the round-leaf-end effect of multi-leaf collimators (MLC) in dose calculation that is important for treatment planning and MLC sequence optimization. We carried out a series of experiments to determine the DLG values for HD120 MLC equipped on Varian Edge machine for 6X, 6XFFF, and 10XFFF energy modes.

Methods: Three sets of experiments were conducted: 1) physical DLG measurements using sweeping-gap dynamic MLC plans, 2) DLG adjustment based on VMAT spine radiosurgery plan measurements, and 3) verification via film dosimetry and ion-chamber measurements using AAPM TG119 IMRT plans. Gafchromic EBT3 films were exposed in solid water phantoms for film dosimetry. Analytical Anisotropic Algorithm (v.13.6) was used for all dose calculations.

Results: The physical DLGs were measured as 0.39 mm, 0.27 mm, and 0.42 mm for 6X, 6XFFF, and 10XFFF respectively. The calculated doses using the measured physical DLG values were lower by 4.2% (6X), 3.7% (6XFFF), and 6.8% (10XFFF) than the actual measurements. The optimal DLG values, that minimized the errors near zero, were 1.1 mm, 0.9 mm, and 1.5 mm. The plan measurement errors were linearly related to the DLG values (mean r2=0.999). At the optimal DLG values, the IC measurement errors were <1% and the film gamma pass rates (3%/3mm) were > 97% for the spine plans. The absolute dose errors were <2% at 95% confidence limit for the TG119 IMRT plans (median 4.4% in TG119 report).

Conclusion: The calculated doses were systematically lower than the measured doses if the physical DLG values were used in treatment plans. It was necessary to increase the DLG values by about 3 folds in order to minimize the dose calculation errors within clinically acceptable level. The optimal DLG values may be specific to individual MLCs and beams. Thus, careful evaluation and verification are warranted.

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