Development of an Electronic Portal Imaging Device Dosimetry Method

Area of Science:

• Medical Physics
• Radiation Oncology
• Imaging Technology

Background:

• Electronic Portal Imaging Devices (EPIDs) are crucial for radiotherapy quality assurance.
• Backscatter and off-axis effects in EPID images present significant challenges for accurate dose assessment.

Purpose of the Study:

• To develop and validate a simple and effective method using correction matrices to rectify EPID image responses.
• To enable accurate conversion of EPID pixel values to absolute dose for improved radiotherapy dosimetry.

Main Methods:

• Simultaneous measurement of square fields using an ionization chamber array (ICA) and EPID.
• Calculation of dose-to-pixel value ratio to create field-specific correction matrices.
• Generation of a universal correction matrix by adapting large-field data for small fields.
• Application of the correction matrix to EPID images and conversion to absolute dose.

Main Results:

• The developed method achieved high gamma pass rates: 99.6% ± 0.94% for 3%/3 mm and 95.48% ± 1.03% for 2%/2 mm (5% threshold).
• Average gamma values were low, indicating excellent agreement between calculated and measured doses (0.28 ± 0.04 and 0.42 ± 0.05).
• Experimental results confirmed accurate correction of EPID images and reliable conversion to absolute dose.

Conclusions:

• The proposed correction matrix method effectively addresses backscatter and off-axis effects in EPID imaging.
• This technique enhances the accuracy of EPID dosimetry, establishing it as an efficient tool for radiotherapy quality assurance.