Angular dependence correction of MatriXX and its application to composite dose verification

Area of Science:

• Medical Physics
• Radiation Oncology
• Dosimetry

Background:

• Accurate dose verification is critical for Intensity-Modulated Radiation Therapy (IMRT) and Volumetric Modulated Arc Therapy (VMAT).
• The MatriXX 2D ionization chamber array is a common tool for composite dose verification.
• Angular dependence of detectors can introduce inaccuracies in dose measurements.

Purpose of the Study:

• To measure and characterize the angular dependence of central and off-axis detectors in the MatriXX array.
• To develop and apply correction factors (CFs) to improve the accuracy of MatriXX dose verification.
• To validate the corrected MatriXX measurements against established dosimetry methods.

Main Methods:

• Measured MatriXX doses at various gantry (θ) and lateral angles using 6 MV and 10 MV photons in a phantom.
• Calculated doses using the Monte Carlo (MC) algorithm under identical conditions.
• Derived CFs from the ratio of measured to MC-calculated doses, normalized at θ = 0°.
• Validated corrected MatriXX doses with different field sizes, simple plans, and clinical IMRT/VMAT plans.

Main Results:

• Significant over- and under-responses of the MatriXX were observed at different gantry angles (e.g., up to 6% over-response at 90°, up to 15% under-response at 92° for 6 MV).
• Off-axis detectors showed larger CFs than central detectors at 92° (up to 7% and 6% difference for 6 MV and 10 MV).
• Corrected MatriXX doses agreed within 2% with ionization chamber measurements for simple plans.
• Gamma evaluation (3%/3 mm) showed good agreement between corrected MatriXX and treatment planning system (TPS) calculations for clinical plans.

Conclusions:

• The angular dependence of the MatriXX array significantly impacts dose measurement accuracy.
• Applying derived angular correction factors effectively improves the accuracy of MatriXX measurements.
• Corrected MatriXX measurements enhance the reliability of composite dose verification for IMRT and VMAT.