An oscillating sweeping gap test for VMAT quality assurance

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Quality Assurance

Background:

• Volumetric Modulated Arc Therapy (VMAT) requires rigorous quality assurance (QA) to ensure accurate dose delivery.
• Existing QA methods may not fully capture uncertainties in VMAT delivery, including linac performance, dose calculation, and multileaf collimator (MLC) dynamics.
• Standard sweeping gap tests, effective for Intensity-Modulated Radiation Therapy (IMRT), need adaptation for VMAT's complex motion patterns.

Purpose of the Study:

• To develop and validate a novel oscillating sweeping gap test for VMAT QA.
• To simultaneously assess uncertainties in linear accelerator (linac) performance, dose calculation, and MLC parameters during VMAT.
• To provide a sensitive tool for commissioning treatment planning systems and performing routine linac QA for VMAT.

Main Methods:

• A new 'oscillating sweeping gap' test pattern was designed, extending the IMRT sweeping gap test for VMAT.
• This test involves a uniform MLC gap moving across the field during full gantry rotation, creating a quasi-uniform dose distribution.
• Delivered doses were measured using Matrixx, ionization chamber A12, and EDR2 films, and compared against Eclipse treatment planning system (TPS) calculations.

Main Results:

• The oscillating sweeping gap test effectively detected dose errors as deviations from the uniform background, sensitive to MLC gap size, gantry/MLC speeds, and monitor units.
• Dose calculation errors from insufficient control points manifested as streak artifacts, increasing with fewer control points and smaller MLC gaps.
• Gantry angle errors appeared as noise or hot/cold spots, and MLC leaf position discrepancies were identified, aligning with theoretical predictions.

Conclusions:

• The oscillating sweeping gap test is a valuable tool for VMAT QA, capable of identifying dose calculation and delivery errors.
• The test's sensitivity to various parameters makes it suitable for both VMAT system commissioning and routine linac QA.
• Uncertainties and dose errors determined by this VMAT test are comparable to those found with the standard sweeping gap test for IMRT.