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07:31
Characterization of Recombination Effects in a Liquid Ionization Chamber Used for the Dosimetry of a Radiosurgical Accelerator
Published on: May 9, 2014
The response of prototype plane-parallel ionization chambers in small megavoltage x-ray fields
Andrea L McNiven1, Matt Mulligan, Tomas Kron
1Department of Medical Biophysics, The University of Western Ontario, London, Ontario, Canada.
Medical Physics
|December 13, 2006
Summary
Accurate small-field dosimetry is crucial for modern radiotherapy. Prototype plane-parallel ionization chambers showed a significant under-response in small fields due to air cavity perturbation, requiring Monte Carlo-derived correction factors.
Area of Science:
- Medical Physics
- Radiation Oncology
- Dosimetry
Background:
- Modern radiotherapy techniques like intensity-modulated radiation therapy (IMRT) and stereotactic radiosurgery utilize small radiation fields.
- Accurate dose measurement in these small fields is essential for effective and safe patient treatment.
Purpose of the Study:
- To evaluate the dosimetric performance of prototype plane-parallel ionization chambers (2-20 mm diameter) in small stereotactic radiotherapy x-ray fields.
- To identify the causes of any observed discrepancies and determine the chambers' suitability for accurate small-field dose measurements.
Main Methods:
- Experimental measurements of relative ionization response for chambers in small circular fields (0.5-4 cm diameter) compared to a reference field.
- Comparison with other dosimeters: radiochromic film, micro-metal-oxide-semiconductor field-effect transistor (MOSFET), and diode.
- Monte Carlo (MC) BEAMnrc simulations to model the experimental setup and investigate the source of under-response.
Main Results:
- A significant under-response (approx. 40%) was observed for the smallest plane-parallel chamber compared to other dosimeters.
- MC simulations revealed that the air cavity of the custom chambers significantly perturbs secondary electron fluence in small fields.
- This perturbation leads to decreased fluence within the active volume, causing the chamber under-response.
Conclusions:
- The investigated prototype plane-parallel ionization chambers exhibit limitations for accurate small-field dosimetry due to air cavity effects.
- Significant corrections, particularly to the p(fl) factor, are necessary for accurate dose measurements in small fields.
- Monte Carlo simulations are a viable method for deriving these essential correction factors.
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