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A large-area ionization chamber for portal image calibration.
M Partridge1, J R Symonds-Tayler, P M Evans
1Joint Department of Physics, Institute of Cancer Research/Royal Marsden NHS Trust, Sutton, UK.
Physics in Medicine and Biology
|March 11, 1999
Summary
Accurate electronic portal imaging requires correcting for linear accelerator (linac) output fluctuations. Using a specialized ionization chamber significantly improves calibration accuracy and reduces thickness errors in portal images.
Area of Science:
- Medical Physics
- Radiotherapy Imaging
- Radiation Oncology
Background:
- Linear accelerators (linacs) used in radiotherapy exhibit output fluctuations.
- These fluctuations can introduce significant errors in electronic portal images (EPIs).
- Accurate EPIs are crucial for treatment verification and quality assurance.
Purpose of the Study:
- To describe and compare methods for removing linac output fluctuation effects from EPIs.
- To evaluate the impact of incorporating ionization chamber measurements on calibration accuracy.
- To assess the long-term stability of calibration corrections.
Main Methods:
- Measuring linac output during imaging using a large-area ionization chamber.
- Recording linac output data concurrently with EPI acquisition.
- Testing quadratic thickness calibration schemes incorporating ionization chamber data.
- Comparing calibration accuracy and flatness with and without ionization chamber correction.
Main Results:
- Incorporating ionization chamber data significantly improved absolute calibration accuracy and image flatness.
- Immediately after calibration, mean systematic thickness error was ≤0.6 mm, with intra-image variation ≤±0.8 mm.
- Even with an unstable linac beam, corrections reduced mean thickness errors from 8.8 mm to <1.6 mm and intra-image variations from ±4.9 mm to ±1.4 mm over one month.
Conclusions:
- A large-area ionization chamber effectively measures linac output fluctuations during EPI acquisition.
- Using ionization chamber data in calibration schemes substantially enhances the accuracy and reliability of EPIs.
- This method provides robust, long-term correction for linac instability in radiotherapy imaging.