Portal dose image verification: the collapsed cone superposition method applied with different electronic portal...

Area of Science:

• Medical Physics
• Radiation Oncology
• Imaging Technology

Background:

• Electronic Portal Imaging Devices (EPIDs) are crucial for verifying radiation delivery in cancer treatment.
• Accurate absolute dose verification requires comparing measured and calculated portal dose images (PDIs).

Purpose of the Study:

• To evaluate two commercial EPIDs for absolute dose calibration.
• To compare measured PDIs with calculated PDIs using a validated model.
• To assess the capability of calibrated EPIDs in verifying delivered patient radiation doses.

Main Methods:

• Two EPID types (liquid ion chamber matrix and fluoroscopic CCD camera) were calibrated to measure absolute dose.
• Portal dose images (PDIs) were acquired and transformed using specific calibration procedures.
• Measured PDIs were compared against calculated PDIs generated by a collapsed cone superposition model.
• Agreement was assessed using dose comparisons and gamma index analysis across various field shapes and phantom thicknesses.

Main Results:

• Both EPID systems demonstrated good agreement with calculated PDIs, with small average areas failing criteria ( <1% for matrix ion chamber, 7% and 1% for camera-based EPID at 6 and 15 MV).
• Central dose agreement was excellent: 0.1 ± 1.6% (1 SD) for the camera-based EPID and -0.1 ± 1.6% (1 SD) for the matrix ion chamber.
• The study confirmed the validity of the calculation method for different MV x-ray energies and phantom configurations.

Conclusions:

• Absolute dose-calibrated EPIDs can effectively validate delivered radiation doses.
• Comparing calculated and measured PDIs is a reliable method for quality assurance in radiotherapy.
• This technology enhances patient safety by ensuring accurate dose delivery.