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Updated: Mar 16, 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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Reference dosimetry in magnetic fields: formalism and ionization chamber correction factors
D J O'Brien1, D A Roberts2, G S Ibbott1
1Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, Houston, Texas 77030.
Medical Physics
|August 5, 2016
Summary
Magnetic resonance imaging-guided radiotherapy (MRIgRT) requires new dosimetry methods. This study proposes a formalism using magnetic field correction factors for ionization chambers, ensuring accurate dose calibration in MRIgRT.
Area of Science:
- Medical Physics
- Radiotherapy Technology
- Dosimetry
Background:
- Magnetic resonance imaging-guided radiotherapy (MRIgRT) offers superior imaging for radiation therapy compared to standard methods.
- Integrated MRIgRT machines require specialized dosimetry protocols due to magnetic field interference with ionization chambers.
- Accurate dose calibration is crucial for effective and safe radiotherapy delivery.
Purpose of the Study:
- To develop a formalism for reference dosimetry in integrated MRIgRT systems.
- To evaluate the impact of magnetic fields on beam quality specifiers like TPR10(20) and %dd(10)x.
- To calculate magnetic field correction factors for ionization chambers used in MRIgRT.
Main Methods:
- Monte Carlo simulations of an MR-linac and various ionization chambers (Exradin A19, NE2571, PTW Farmer).
- Calculation of magnetic field correction factors and assessment of beam quality specifiers under magnetic fields.
- Validation of simulation results using Fano cavity tests and preclinical MR-linac measurements.
Main Results:
- The TPR10(20) specifier was unaffected by magnetic fields, while %dd(10)x showed a 2.4% change with a 1.5 T transverse field.
- Calculated magnetic field correction factors depend on detector orientation, with minimal effect when the chamber is parallel to the field.
- Simulation parameters passed Fano cavity tests with high accuracy (~0.1%).
Conclusions:
- Reference dosimetry for MRIgRT is feasible using magnetic field correction factors.
- Careful selection of beam quality specifiers and ionization chamber orientation is essential for accurate dosimetry.
- The proposed formalism aims for uncertainties comparable to conventional methods, requiring further quantification.
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