Comprehensive fluence model for absolute portal dose image prediction.

Area of Science:

• Medical Physics
• Radiation Oncology
• Image Analysis

Background:

• Amorphous silicon electronic portal imaging devices (a-Si EPIDs) are crucial for verifying radiation therapy treatments, especially intensity modulated radiation therapy (IMRT).
• Accurate portal dose image prediction is essential for comparing measured and predicted images to ensure treatment accuracy.
• Modeling the complex fluence modulation in IMRT requires a robust fluence determination method.

Purpose of the Study:

• To develop and commission a physics-based fluence model for predicting portal dose images.
• To enhance the accuracy of treatment verification in IMRT using a-Si EPIDs.
• To validate the model's performance on clinical IMRT cases.

Main Methods:

• A two-source physics-based fluence model (focal and extrafocal Gaussian sources) was developed and commissioned using measured a-Si EPID images.
• The model incorporated specific multileaf collimator (MLC) and secondary collimator characteristics, including transmission factors, rounded leaf tips, and leakage.
• Monte Carlo simulations informed unique aspects of the model, such as non-Gaussian extrafocal fluence, separate energy spectra, and off-axis energy softening.

Main Results:

• The model predicted open-field profiles within 2% and 2 mm.
• An average of 96.6% of pixels in IMRT fields met the 2%, 3 mm agreement criteria.
• The model demonstrated accuracy comparable to existing methods for IMRT pretreatment verification.

Conclusions:

• The developed physics-based fluence model provides accurate portal dose image prediction for IMRT.
• This model is suitable for pretreatment verification of complex clinical IMRT cases.
• The findings support the use of a-Si EPIDs for enhanced radiation therapy quality assurance.