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Portal dosimetry using a liquid ion chamber matrix: dose response studies
1Department of Clinical Physics, Ontario Cancer Institute, Toronto, Canada.
Medical Physics
|July 1, 1995
Summary
This study shows that a liquid ion chamber electronic portal imaging device (EPID) can be accurately used for portal dosimetry. The EPID system demonstrated excellent stability and agreement with direct dose measurements.
Area of Science:
- Medical Physics
- Radiotherapy Physics
Background:
- Electronic portal imaging devices (EPIDs) are increasingly investigated for portal dosimetry applications.
- Liquid ion chamber matrix EPIDs offer a promising avenue for real-time dose monitoring in radiotherapy.
Purpose of the Study:
- To assess the performance of a commercial liquid ion chamber matrix EPID for portal dosimetry.
- To calibrate the EPID's pixel values against absolute dose and evaluate dose rate and field size dependencies.
Main Methods:
- Calibration of EPID pixel values using 6-MV X-ray beams with varying attenuators and source-to-detector distances.
- Determination of absolute dose using an ion chamber in a solid water phantom.
- Evaluation of pixel value dependence on dose rate and field size.
- Conversion of 2D pixel values to 2D dose maps with corrections for calibration field effects.
Main Results:
- EPID pixel values showed proportionality to the square root of the dose rate and were largely independent of field size.
- Converted 2D dose maps agreed well (typically <1% relative standard deviation) with direct ion chamber measurements in regions of shallow dose gradient.
- Both short- and long-term stability of the EPID system were within 1% relative standard deviation.
Conclusions:
- A commercial liquid ion chamber matrix EPID is suitable for portal dosimetry.
- The system demonstrates high accuracy and stability for potential on-line portal dosimetry applications.