Improving IMRT quality control efficiency using an amorphous silicon electronic portal imager

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Technology

Background:

• Quality control (QC) is crucial for ensuring the accuracy and safety of intensity modulated radiation therapy (IMRT).
• Electronic portal imaging devices (EPIDs) offer potential for efficient IMRT QC checks.
• Amorphous silicon EPIDs are increasingly used in modern linear accelerators.

Purpose of the Study:

• To evaluate the usefulness and efficiency of amorphous silicon EPIDs for IMRT quality control.
• To assess EPID performance in measuring specific dosimetric parameters relevant to IMRT.
• To compare EPID measurements with ion chamber results for validation.

Main Methods:

• Measured dose linearity, segment reproducibility, and fractional monitor unit accuracy using EPID.
• Assessed low dose beam flatness and symmetry by comparing beam profiles.
• Developed and tested an MLC leaf calibration check using image subtraction.
• Compared EPID results with ion chamber measurements.

Main Results:

• EPID is a sensitive relative dosemeter, resolving dose differences of 0.01 cGy.
• Observed up to 7% dosimetric nonlinearity at low doses due to image lag/ghosting.
• Symmetry/flatness tests were efficient for low dose profiles.
• MLC test demonstrated high sensitivity to calibration changes and effectiveness with a composite reference image.

Conclusions:

• Amorphous silicon EPIDs are suitable for IMRT QC, particularly for segment reproducibility and MLC calibration checks.
• EPIDs are less suitable than ion chambers for absolute dosimetric checks due to nonlinearity.
• Automated EPID procedures offer significant time savings compared to traditional methods.