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Annual Scientific Meeting of the Canadian Organization of Medical Physicists — July 9–12, 2014, The Banff Centre, Banff, Alberta, Canada

Magnetic Field Effects on Ion Chamber Response in MR-Linac Systems

Magnetic resonance image-guided linear accelerators are reshaping adaptive radiotherapy by combining real-time soft-tissue imaging with external beam delivery. Australian centres that have invested in hybrid systems such as the Elekta Unity or ViewRay MRIdian face a pressing clinical challenge: when an ionisation chamber operates inside the bore, the static magnetic field alters the trajectory of secondary electrons in the irradiated medium. The reading of a Farmer-type or micro-chamber therefore deviates from conventional calibration conditions. Understanding these deviations matters for physicists in Sydney, Melbourne, Perth and Brisbane who must keep treatments within accepted tolerance levels.

The 2014 Annual Scientific Meeting of the Canadian Organization of Medical Physicists in Banff brought together researchers tackling these questions. Sessions on hybrid commissioning covered cavity theory, correction factors and detector selection, and Australian delegates compared notes about TRS-398-style protocols extended for magnetic environments. The archived proceedings remain a useful resource, and anyone exploring the scientific program details will find relevant abstracts on chamber response in static fields.

How the Static Field Changes Chamber Behaviour

Inside the bore of an MR-Linac, secondary electrons liberated by photon interactions are pushed sideways by the Lorentz force. Rather than travelling in straight radial paths between the electrodes, the electrons spiral around field lines and may re-enter the active volume multiple times. When a chamber is orientated with its long axis parallel to the magnetic field, the response can differ from the perpendicular orientation by several percent. This orientation dependence has been documented by groups at the University of Sydney and the Peter MacCallum Cancer Centre.

The magnitude of the effect scales with chamber volume. Larger collecting volumes integrate current over more voxels, so stochastic perturbations from the field tend to average down. Smaller chambers such as micro-ionisation or pinpoint designs can show exaggerated shifts because individual electron tracks dominate the response. Quality assurance teams in Adelaide and at the Royal Brisbane and Women's Hospital have noted that even chambers with identical nominal volumes behave differently depending on wall material and electrode geometry. Generic correction factors are unreliable without site-specific validation.

Volume Averaging in Gradient and Small Fields

Volume averaging has long been a consideration for small-field dosimetry, but the magnetic field introduces a second layer of complexity. In a transverse field, the effective point of measurement of an ionisation chamber shifts slightly because the electron fluence no longer spreads symmetrically around the central axis. For stereotactic fields smaller than 3 cm by 3 cm, the chamber reading can underestimate or overestimate the dose at the reference point depending on orientation. Many Australian physicists performing Winston-Lutz-style checks now incorporate an additional uncertainty budget to capture this geometric effect.

Gradient fields complicate the picture further. The Elekta Unity uses a 1.5 T magnet whose uniformity varies slightly across the treatment volume, while the ViewRay MRIdian operates at 0.35 T with different gradient profiles. Reference dosimetry performed at the isocentre may not represent chamber response at off-axis positions, particularly for treatments involving the spinal cord or breast where lateral distances exceed 15 cm. Researchers at Liverpool Hospital in Sydney have documented response variations of up to 1.4 per cent between isocentric and offset measurements, reinforcing the case for chamber-specific Monte Carlo simulations.

Applying Correction Factors in the Clinic

The standard approach introduces a quality correction factor, k_B,Q, which adjusts a chamber's calibration coefficient from the conventional cobalt-60 reference field to the MR-linac environment. Published datasets provide values for popular chamber models, including the PTW 30013, IBA FC-65G and Standard Imaging A1SL. These factors are derived from Monte Carlo calculations benchmarked against experimental readings in a research linac coupled with a magnet. ARPANSA is exploring a national primary standard for MR-Linac reference dosimetry.

For centres that have not measured k_B,Q locally, vendor-supplied factors or those in the AAPM TG-308 report offer a reasonable starting point. The task group consolidates data from multiple institutions and ranks which chambers are least sensitive to magnetic field perturbations. Australian practice typically pairs a small-volume chamber for field-size-dependent measurements with a larger reference chamber for absolute output calibration, hedging against orientation-dependent bias. This pragmatic approach reflects the "she'll be right" engineering culture that values robustness over chasing the last fraction of a percent.

Chamber Selection Criteria for MR-Linac Use

  • Match chamber volume to clinical field size requirements
  • Prefer models with documented k_B,Q values from peer-reviewed sources
  • Verify chamber wall materials are non-ferromagnetic
  • Establish baseline orientation dependence before first clinical use
  • Cross-check against fibre-optic dosimetry during commissioning

Practical Quality Assurance Workflows

A typical MR-Linac commissioning workflow in Australia begins with an end-to-end test using a water phantom equipped with a fibre-optic dosimeter or a plastic scintillator. These detectors are essentially magnetic-field-immune and provide reference readings against which ion chamber measurements can be cross-checked. Once the baseline is established, monthly output constancy checks use the ion chamber with a fixed correction factor, and annual re-calibrations involve mailed comparisons with the ARPANSA primary standard. Several centres participate in the IAEA/WHO TLD audit network.

Daily and weekly quality assurance covers a familiar suite of mechanical and imaging tests, but the magnetic environment adds laser alignment checks at the bore entrance and verification of the shimming coils. Physicists must also monitor the helium fill level and cryo-cooler performance, because any drift in magnet strength alters chamber response. Engineers at GenesisCare and Icon Group sites have developed local scripts that flag unusual chamber readings and prompt a magnet health check, lowering the risk of an under- or over-dosed fraction.

Daily QA Priorities for an MR-Linac Programme

  • Verify magnet cold head temperature and helium level at start of day
  • Check laser alignment at bore entrance against wall-mounted markers
  • Confirm ion chamber constancy using a fixed k_B,Q correction
  • Run a quick image distortion check on the planning phantom
  • Review the previous day's adaptive plan approvals for anomalies

The Australian Clinical Landscape

Australia's MR-Linac footprint is modest but growing, with installations concentrated in New South Wales and Victoria. The Chris O'Brien Lifehouse in Sydney and the Peter MacCallum Cancer Centre in Melbourne have published clinical experience using MR-guided adaptive radiotherapy for prostate, pancreas and oligometastatic disease. Staffing models draw on the ACPSEM training program, with registrars rotating through a base hospital before specialising. Many senior physicists complete short observerships at Canadian or European centres, then bring those workflows home, adapting them to local Medicare billing structures and state-level credentialing.

Australian terminology differs subtly from North American usage. An ion chamber might be called a "thimble chamber" in a Melbourne department, while a Sydney physicist may refer to the same device as a "Farmer". Conference abstracts submitted locally tend to spell out acronyms in full, reflecting the cautious drafting style favoured by the Australasian College. Australian physics forums also show practitioners swapping tips about cable routing inside the bore to minimise image artefacts, an issue that rarely troubles colleagues using a conventional linac.

Looking Beyond Conventional Chambers

Detectors under development may eventually remove the magnetic field uncertainty entirely. Diamond detectors, plastic scintillators and MOSFET probes offer near-water equivalence with minimal magnetic sensitivity. Several Australian research groups, including teams at the University of Wollongong and RMIT, are collaborating with detector manufacturers to characterise prototype arrays for online treatment verification. The aim is a detector stack that can sit on the patient couch during beam-on time, providing real-time dose readout without disrupting the MR image.

In parallel, treatment planning systems are beginning to model chamber response explicitly, allowing physicists to predict k_B,Q values from the patient's anatomy and the planned field geometry. This personalised approach could reduce reliance on generic correction tables. International collaborations such as the MR-Linac Consortium, which includes Australian members, are pooling data to validate these algorithms. The conversations started at meetings like the one in Banff remain relevant, and the Banff conference social events helped cement the professional relationships that now drive multi-centre trials.

Share your chamber data with the Australian MR-Linac Working Group, register for the next ACPSEM magnetic field dosimetry webinar, and revisit the COMP 2014 Banff proceedings to compare your commissioning results with those of the original presenters. By pooling experience across Sydney, Melbourne, Brisbane and Perth, Australian physicists can refine k_B,Q values, tighten their uncertainty budgets, and give patients the full benefit of MR-guided adaptive radiotherapy.