Sci-Thurs PM: Delivery-06: 2-D lag and response nonlinearity corrections for dynamic IMRT verification using an EPID

Area of Science:

• Medical Physics
• Radiation Oncology
• Imaging Technology

Background:

• Electronic Portal Imaging Devices (EPIDs) are crucial for pre-treatment verification in intensity-modulated radiation therapy (IMRT).
• Existing EPID verification methods often overlook EPID lag and signal nonlinearities, which can introduce artifacts.
• These artifacts, including artificial asymmetries and amplitude changes, complicate the assessment of IMRT verifications.

Purpose of the Study:

• To develop and validate a 2-D pixel-by-pixel correction method for EPID lag and signal nonlinearities.
• To improve the accuracy of IMRT dose verification by mitigating image artifacts.
• To enhance the reliability of assessing clinical IMRT verifications.

Main Methods:

• Quantified Varian aS500 EPID lag and nonlinearity characteristics through experimental data.
• Developed a 2-D (pixel-by-pixel) correction algorithm based on these characteristics.
• Applied the correction to dynamic sliding-window IMRT delivery scenarios and clinical IMRT fields.

Main Results:

• The correction method successfully resolved discrepancies in symmetry for different sweeping directions.
• It reduced differences between measured and predicted amplitudes, especially for low monitor unit deliveries.
• Application to a clinical IMRT field reduced the percentage of pixels failing Gamma analysis (3%, 3mm) from 8.5% to 5.6%.

Conclusions:

• The developed 2-D correction technique effectively addresses EPID lag and signal nonlinearity artifacts.
• This method enhances the accuracy of EPID-based IMRT dose verification.
• It offers a valuable tool for resolving discrepancies in challenging clinical IMRT verifications.