Simplifying EPID dosimetry for IMRT treatment verification.

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Dosimetry

Background:

• Electronic portal imaging devices (EPIDs) are crucial for intensity-modulated radiation therapy (IMRT) dose verification.
• Current backprojection models require patient-specific transmission measurements, which are time-consuming.
• The need for additional 'open images' prolongs the verification procedure.

Purpose of the Study:

• To modify an existing backprojection model for EPID-based IMRT dose verification.
• To eliminate the requirement for patient-specific transmission measurements, thereby accelerating the process.
• To assess the accuracy and clinical applicability of the modified model.

Main Methods:

• A novel model was developed to calculate phantom/patient transmission, incorporating beam hardening and off-axis EPID response effects.
• Model parameters were empirically determined using polystyrene phantoms and various photon beam energies (6, 10, 18 MV).
• The model was validated against EPID dose verification measurements for IMRT treatments across multiple cancer sites.

Main Results:

• The new model demonstrated good agreement with doses determined by the traditional method using measured transmission.
• Average differences in in vivo isocenter dose were within 2.6% for 6 MV, 0.2% for 10 MV, and 2.2% for 18 MV beams.
• Clinical implementation of the model is projected to save approximately 450 hours per year by eliminating open image measurements.

Conclusions:

• Calculating transmission instead of measuring it results in minimal differences in isocenter dose (generally <2%).
• The modified model is suitable for clinical implementation in IMRT dose verification.
• The elimination of open image measurements significantly streamlines the verification process.