SU-E-T-156: Robust Algorithms for Correction of Predicted Electronic Portal Imager Response

Area of Science:

• Medical Physics
• Radiation Oncology
• Image-guided radiation therapy

Background:

• Commercial treatment planning systems (TPS) often underestimate electronic portal imaging device (EPID) response for off-axis intensity-modulated radiation therapy (IMRT) fields.
• This discrepancy can lead to inaccuracies in dose prediction and verification.

Purpose of the Study:

• To develop and evaluate novel algorithms for correcting EPID prediction errors in IMRT.
• To assess the ability of these algorithms and an image-to-dose conversion algorithm to identify TPS dose calculation errors.

Main Methods:

• Two correction algorithms were developed: a profile correction based on radial symmetry and a 2D correction matrix for asymmetric errors.
• Over 50 IMRT fields were tested, comparing corrected EPID predictions against measurements from a diode array and TPS.
• Induced TPS dose and beam-profile errors were used to evaluate the systems' sensitivity.

Main Results:

• Profile correction significantly improved agreement between TPS predictions and EPID measurements, increasing gamma pass rates by over 30% for off-axis fields.
• The 2D matrix correction provided an additional 5% average improvement in pass rates for complex fields.
• The EPID prediction system failed to detect induced TPS errors, while the image-to-dose conversion system and diode array successfully identified them.

Conclusions:

• Profile correction offers an effective and efficient, albeit approximate, method for improving EPID predictions.
• Matrix correction provides a more comprehensive solution but requires advanced image processing.
• Clinicians must be aware of potential limitations in EPID prediction systems for detecting TPS calculation errors that occur after initial fluence calculation.