Design of a QA method to characterize submillimeter-sized PBS beam properties using a 2D ionization chamber array

Area of Science:

• Medical Physics
• Radiation Oncology
• Quality Assurance

Background:

• Pencil beam scanning (PBS) requires precise quality assurance (QA) to ensure patient safety and treatment efficacy.
• Accurate measurement of beam width and position is critical for PBS delivery.
• Existing QA methods may be complex or require specialized equipment for sub-millimeter beam characterization.

Purpose of the Study:

• To investigate the feasibility of using a 2D ionization chamber array, specifically the MatriXX detector, for characterizing sub-millimeter PBS beam properties.
• To develop a method that overcomes limitations of standard detectors, such as coarse pixel spacing and volume averaging, for accurate QA.
• To simplify the QA workflow in proton therapy by utilizing commonly available equipment.

Main Methods:

• A novel method was designed involving a cross-like irradiation pattern to increase sampling points.
• A modified Gaussian fitting technique was employed to correct for volume averaging effects of the detector pixels.
• Detector signals were simulated with added random noise and setup errors to mimic real-world measurements.

Main Results:

• The developed method accurately characterized sub-millimeter pencil beams (σ = 0.7 mm) with uncertainties better than 3% relative to σ.
• Even with limited coverage (60% of the detector), sub-millimeter beams (σ = 0.9 mm) were characterized with <3% uncertainty.
• Without specialized patterns or corrections, the method could determine the properties of larger beams (σ = 3.6 mm to 5.0 mm) with <3% uncertainty.

Conclusions:

• The MatriXX detector, with the proposed method, can effectively characterize sub-millimeter PBS beam properties, simplifying QA procedures.
• This approach enables the use of standard ionization chamber arrays for precise measurement of narrow pencil beams in proton therapy.
• The developed technique enhances the efficiency and accuracy of periodic QA in proton therapy.