Evaluation and improvement of angular response for a commercial 2D detector array for patient-specific QA

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Quality Assurance

Background:

• MatriXX ionization chamber arrays are crucial for IMRT/VMAT dose verification.
• Beam profile offsets in MatriXX measurements can lead to inaccurate quality assurance (QA) fails, especially with oblique beam incidence at various gantry angles.

Purpose of the Study:

• To investigate the beam profile offset of the MatriXX array at different setup conditions.
• To identify methods to minimize or eliminate this offset for improved accuracy in IMRT/VMAT plan verification at original gantry angles.

Main Methods:

• Measured beam profiles using MatriXX at various depths (0.5 mm increments) and gantry angles (0°–360°).
• Determined the optimal depth for QA measurements by identifying the depth with the minimum measured profile offset.

Main Results:

• Beam profile offset varied with gantry angle and depth, exceeding 3 cm at the vendor-recommended depth for lateral beams.
• A minimum offset (near 0) was achieved at approximately 2.5 mm below the vendor-suggested depth.
• Using this optimal depth improved QA results (3%/2 mm Gamma analysis) with a 99.4% passing rate for IMRT/VMAT plans, compared to 94.1% at the vendor-recommended depth.

Conclusions:

• Performing QA measurements at the determined optimal depth significantly enhances accuracy.
• This optimization reduces unnecessary repeated QA and plan modifications caused by system errors, improving clinical workflow efficiency.