On using the dosimetric leaf gap to model the rounded leaf ends in VMAT/RapidArc plans

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Physics

Background:

• Partial transmission through rounded leaf ends of Varian multileaf collimators (MLC) is modeled by the dosimetric leaf gap (DLG).
• Accurate DLG verification is crucial for dose delivery in volumetric modulated arc therapy (VMAT) plans, especially with gantry rotation.

Purpose of the Study:

• To compare measured doses (GAFCHROMIC film, ionization chamber) with treatment planning system (TPS) calculations.
• To determine optimal DLG values for clinical VMAT plans and investigate the limitations of a single-value DLG model.

Main Methods:

• Dose measurements using GAFCHROMIC film and ionization chamber in a phantom for whole brain with metastases VMAT plans.
• Comparison of measured doses with TPS calculations to identify optimal DLG values.
• Investigation of DLG as a function of distance in the beam's eye view (BEV) to model penumbra differences.

Main Results:

• The optimal DLG varied significantly across VMAT plans (0.93 ± 0.15 mm to 2.2 ± 0.2 mm), differing from sliding window plans (approx. 2.0 mm).
• A single DLG value failed to accurately reproduce dose profiles, overestimating in the proximal penumbra and underestimating in the distal penumbra.
• Modeling DLG as a function of BEV distance improved dose profile accuracy and explained the plan-specific nature of the conventional DLG.

Conclusions:

• A single, constant DLG is inadequate for accurate dose modeling in VMAT plans.
• The DLG should be considered a function of the distance in the BEV between the dose point and leaf ends to account for rounded MLC leaf geometry.
• This functional DLG approach offers a more accurate representation of dose delivery in VMAT and explains previous discrepancies.