Transit dosimetry in dynamic IMRT with an a-Si EPID

Area of Science:

• Medical Physics
• Radiation Oncology
• Diagnostic Imaging

Background:

• Amorphous silicon electronic portal imaging devices (a-Si EPID) are used in radiation therapy for dose verification.
• Accurate transit dosimetry with EPIDs requires careful calibration and characterization of their dosimetric response.
• Existing methods for EPID transit dosimetry can be complex and manufacturer-specific.

Purpose of the Study:

• To develop and validate a measurement-based model for calibrating a-Si EPID response for transit dosimetry.
• To characterize the effects of various parameters (field size, phantom thickness, off-axis distance) on EPID transit dosimetry.
• To investigate the impact of scattered radiation on EPID out-of-field response.

Main Methods:

• A measurement-based model was developed, comparing a-Si EPID response to a reference ionization chamber.
• Optimal ionization chamber reference depth and buildup thickness for EPID transit dosimetry were determined.
• An algorithm was created to automatically apply corrections for field size and phantom thickness to EPID images.

Main Results:

• The developed model demonstrated accurate dose calibration for a-Si EPIDs in transit dosimetry.
• Intensity-modulated radiation therapy (IMRT) fields tested showed high agreement (average 96.5% with Gamma index <1 at 10% threshold) with a 2D array dosimeter.
• Good agreement was also observed in out-of-field regions (average 94.8% with Gamma index <1 at 2% threshold).

Conclusions:

• The proposed measurement-based model provides a less complicated and effective method for a-Si EPID transit dosimetry.
• The method is applicable to all a-Si EPIDs, irrespective of the manufacturer.
• This approach enhances the accuracy and reliability of dose monitoring during radiation therapy delivery.