Radiological thickness measurement using a liquid ionization chamber electronic portal imaging device

Area of Science:

• Medical Physics
• Radiotherapy Technology
• Image Analysis

Background:

• Accurate compensator design is crucial for effective radiation dose delivery in cancer therapy.
• Electronic portal imaging devices (EPIDs) offer potential for real-time imaging during treatment.
• Standardizing EPID calibration for quantitative measurements like radiological thickness is an ongoing challenge.

Purpose of the Study:

• To develop and validate a calibration method for the Portal Vision EPID to generate radiological thickness maps.
• To assess the accuracy of the calibration method for compensator design in tangential breast irradiation.

Main Methods:

• Derived coefficients relating image intensity to radiological thickness using water-equivalent blocks.
• Investigated and corrected for system dose response, wedge effects, detector response variation with gantry angle, and field size variations.
• Quantified the intrinsic accuracy and residual errors associated with each parameter.

Main Results:

• The system dose response was modeled by a square-root function.
• Intrinsic system accuracy was found to be 1.9 mm, with orientation changes introducing up to 3.5 mm error.
• Correction for gantry angle variation yielded a 2.4 mm residual error, and field size correction resulted in 2.9 mm error.
• Overall calibration accuracy was approximately 4 mm (2% dose), deemed sufficient for compensator design.

Conclusions:

• The presented calibration method enables the Portal Vision EPID to produce accurate radiological thickness maps.
• This technique is suitable for compensator design in tangential breast irradiation, improving treatment planning precision.
• The study quantifies error sources, providing a reliable framework for quantitative EPID applications in radiotherapy.