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Updated: Feb 28, 2026
07:31
Characterization of Recombination Effects in a Liquid Ionization Chamber Used for the Dosimetry of a Radiosurgical Accelerator
Published on: May 9, 2014
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Sensitive volume effects on Monte Carlo calculated ion chamber response in magnetic fields
Victor N Malkov1, D W O Rogers1
1Department of Physics, Carleton Laboratory for Radiotherapy Physics, Carleton University, Ottawa, ON, Canada.
Medical Physics
|June 22, 2017
Summary
Monte Carlo (MC) simulations show that the ion chamber collection volume significantly impacts dose response in magnetic resonance-guided radiation therapy (MRgRT). Accurately defining this volume is crucial for MC-based ion chamber corrections in MRgRT.
Area of Science:
- Medical Physics
- Radiation Oncology
- Computational Dosimetry
Background:
- Magnetic resonance-guided radiation therapy (MRgRT) requires precise dose calculations.
- Accurate Monte Carlo (MC) models of ion chambers are essential for MRgRT.
- Ion chamber corrections are needed to account for magnetic field effects.
Purpose of the Study:
- To evaluate the sensitivity of ion chamber dose response in a magnetic field to the collection volume used in MC simulations.
- To assess the impact of varying the simulated collection volume on MC-calculated ion chamber corrections for MRgRT.
Main Methods:
- Utilized the EGSnrc egs_chamber application with a validated magnetic field transport code.
- Simulated dose response for several ion chambers (PTW 30013, 31006, 31010, Exradin A12S, A1SL) at varying magnetic field strengths.
- Varied the simulated sensitive region by excluding 0, 0.5, or 1 mm from the stem to assess volume sensitivity.
Main Results:
- Variations in simulated volume led to percent-level deviations in dose response, up to 3.39% at 0.5 T for the A1SL chamber.
- Larger ion chambers showed smaller, yet still significant, variations.
- Similar effects were observed with a 7 MV photon source, relevant for MR-linac machines, with a maximum deviation of 1.97% for a 1 mm reduction.
Conclusions:
- Small changes in the simulated ion chamber collection volume can cause substantial variations in MC-calculated dose response within a magnetic field.
- Accurate MC modeling of the true collection volume is critical for reliable ion chamber corrections in MRgRT.
- This volume averaging effect may also be relevant in small-field dosimetry, even without magnetic fields.
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