An iterative EPID calibration procedure for dosimetric verification that considers the EPID scattering factor

Area of Science:

• Medical Physics
• Radiotherapy Physics
• Radiation Dosimetry

Background:

• Electronic Portal Imaging Devices (EPIDs) are increasingly used for dosimetric verification in intensity modulated radiotherapy.
• Current methods often assume the EPID's phantom scatter factor (Spe) equals that of a full phantom (Sp) or slab phantom (Sps), potentially causing absolute dosimetry errors.

Purpose of the Study:

• To present and validate an iterative calibration procedure for EPIDs that accurately determines Spe and the pixel value to dose rate calibration curve.
• To assess the impact of this new calibration on absolute dosimetry accuracy compared to traditional assumptions.

Main Methods:

• An iterative calibration procedure was developed to simultaneously update Spe and the EPID calibration curve.
• The EPID was irradiated with known field sizes and intensities, with initial dose rates estimated using ionization chamber measurements.
• Spe was derived from EPID measurements and used to refine the calibration curve iteratively until convergence.

Main Results:

• Hypothesis testing confirmed that assuming Spe = Sp or Spe = Sps introduces systematic errors, as large as 2% for large fields.
• The proposed calibration method, using the determined Spe, yielded dose calculations more consistent with ionization chamber measurements.
• EPID-measured relative dose profiles at dmax agreed well with film measurements, demonstrating the method's validity across various field types and sizes.

Conclusions:

• The iterative calibration procedure accurately determines the EPID phantom scatter factor (Spe), significantly improving absolute dosimetry accuracy in radiotherapy.
• This method minimizes systematic errors associated with conventional Spe assumptions, leading to more reliable dosimetric verification.
• The calibrated EPID provides accurate relative dose profiles, essential for quality assurance in intensity modulated radiotherapy.