WE-G-BRB-01: Per-Frame Analysis of MatriXX Data

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Quality Assurance

Background:

• The MatriXX is a widely used ionization chamber array for intensity-modulated radiation therapy (IMRT) quality assurance (QA).
• Discrepancies between planned and measured dose distributions are often observed despite minimal expected angular dependence of the ionization chambers.
• Accurate QA is crucial for ensuring effective and safe delivery of IMRT and VMAT treatments.

Purpose of the Study:

• To identify all factors affecting the measurement accuracy of the MatriXX device.
• To develop a per-frame post-processing strategy to mitigate these factors.
• To improve the passing rate of IMRT and VMAT plans assessed by the MatriXX.

Main Methods:

• Developed software to read MatriXX dose frame sequences and apply correction factors.
• Calculated angular correction factors based on measured vs. planned dose at the isocenter for various photon energies.
• Implemented background subtraction using a predictive model and machine output corrections; identified the effective measurement plane.

Main Results:

• The per-frame analysis software significantly improved clinical passing rates for IMRT QA.
• Average passing rate increased from 93.6% (no corrections) to 97.3% (with per-frame corrections) in a cohort of 800 patients.
• The study identified and addressed angular effects, background evolution, machine output variations, and the effective measurement plane.

Conclusions:

• All factors impacting MatriXX measurement accuracy, including angular effects, background evolution, machine output, and plane of measurement, were identified.
• A novel per-frame post-processing strategy effectively eliminates the impact of these factors.
• The proposed strategy enhances the reliability of IMRT and VMAT QA, leading to higher passing rates.