Experimental characterization of magnetically focused electron contamination at the surface of a high-field inline MRI-linac

Natalia F Roberts; Elizabeth Patterson; Urszula Jelen; Trent Causer; Lois Holloway; Gary Liney; Michael Lerch; Anatoly B Rosenfeld; Dean Cutajar; Bradley M Oborn; Peter Metcalfe

doi:10.1002/mp.13847

Experimental characterization of magnetically focused electron contamination at the surface of a high-field inline MRI-linac

Med Phys. 2019 Dec;46(12):5780-5789. doi: 10.1002/mp.13847. Epub 2019 Oct 21.

Authors

Natalia F Roberts^{1

2

3}, Elizabeth Patterson¹, Urszula Jelen³, Trent Causer^{1

4}, Lois Holloway^{1

2

3

5

6

7}, Gary Liney³, Michael Lerch¹, Anatoly B Rosenfeld¹, Dean Cutajar¹, Bradley M Oborn^{1

4}, Peter Metcalfe^{1

3}

Affiliations

¹ Centre for Medical Radiation Physics, University of Wollongong, Wollongong, NSW, 2522, Australia.
² Centre for Oncology Education and Research Translation, Wollongong, NSW, 2522, Australia.
³ Ingham Institute for Applied Medical Research, Liverpool, NSW, 2170, Australia.
⁴ Illawarra Cancer Care Centre, Wollongong Hospital, Wollongong, NSW, 2500, Australia.
⁵ Department of Medical Physics, Liverpool and Macarthur Cancer Care Centres, Liverpool, NSW, 2170, Australia.
⁶ South Western Sydney Clinical School, University of New South Wales, Liverpool, NSW, Australia.
⁷ Institute of Medical Physics, University of Sydney, Camperdown, NSW, 2505, Australia.

PMID: 31633212
DOI: 10.1002/mp.13847

Abstract

Purpose: The fringe field of the Australian MRI-linac causes contaminant electrons to be focused along the central axis resulting in a high surface dose. This work aims to characterize this effect using Gafchromic film and high-resolution detectors, MOSkin^TM and microDiamond. The secondary aim is to investigate the influence of the inline magnetic field on the relative dose response of these detectors.

Methods: The Australian MRI-linac has the unique feature that the linac is mounted on rails allowing for measurements to be performed at different magnetic field strengths while maintaining a constant source-to-surface distance (SSD). Percentage depth doses (PDD) were collected at SSD 1.82 m in a solid water phantom positioned in a low magnetic field region and then at isocenter of the MRI where the magnetic field is 1 T. Measurements for a range of field sizes were taken with the MOSkin^TM , microDiamond, and Gafchromic^® EBT3 film. The detectors' relative responses at 1 T were compared to the near 0 T PDD beyond the region of electron contamination, that is, 20 mm depth. The near surface measurements inside the MRI bore were compared among the different detectors.

Results: Skin dose in the MRI, as measured with the MOSkin^TM , was 104.5% for 2.1 × 1.9 cm² , 185.6% for 6.1 × 5.8 cm² , 369.1% for 11.8 × 11.5 cm² , and 711.1% for 23.5 × 23 cm² . The detector measurements beyond the electron contamination region showed agreement between the relative response at 1 T and near 0 T. Film was in agreement with both detectors in this region further demonstrating their relative response is unaffected by the magnetic field.

Conclusions: Experimental characterization of the high electron contamination at the surface was performed for a range of field sizes. The relative response of MOSkin^TM and microDiamond detectors, beyond the electron contamination region, were confirmed to be unaffected by the 1-T inline magnetic field.

Keywords: dosimetry; electron contamination; inline MRI-linac; magnetic field; skin dose.

MeSH terms

Electrons*
Magnetic Phenomena*
Magnetic Resonance Imaging / instrumentation*
Particle Accelerators / instrumentation*
Surface Properties

Abstract

MeSH terms

Grants and funding