Experimental verification of dose enhancement effects in a lung phantom from inline magnetic fields

Bradley M Oborn; Maegan A Gargett; Trent J Causer; Sarah J Alnaghy; Nicholas Hardcastle; Peter E Metcalfe; Paul J Keall

doi:10.1016/j.radonc.2017.09.012

Experimental verification of dose enhancement effects in a lung phantom from inline magnetic fields

Radiother Oncol. 2017 Dec;125(3):433-438. doi: 10.1016/j.radonc.2017.09.012. Epub 2017 Oct 3.

Authors

Bradley M Oborn¹, Maegan A Gargett², Trent J Causer³, Sarah J Alnaghy⁴, Nicholas Hardcastle⁵, Peter E Metcalfe⁴, Paul J Keall⁶

Affiliations

¹ Illawarra Cancer Care Centre, Wollongong, Australia; Centre for Medical Radiation Physics, University of Wollongong, Australia. Electronic address: brad.oborn@gmail.com.
² Northern Sydney Cancer Centre, St Leonards, Australia; Centre for Medical Radiation Physics, University of Wollongong, Australia.
³ Illawarra Cancer Care Centre, Wollongong, Australia; Centre for Medical Radiation Physics, University of Wollongong, Australia.
⁴ Centre for Medical Radiation Physics, University of Wollongong, Australia; Ingham Institute for Applied Medical Research, Liverpool, Australia.
⁵ Peter MacCallum Cancer Centre, Melbourne, Australia; Centre for Medical Radiation Physics, University of Wollongong, Australia.
⁶ Sydney Medical School, University of Sydney, Australia; Ingham Institute for Applied Medical Research, Liverpool, Australia.

PMID: 28985954
DOI: 10.1016/j.radonc.2017.09.012

Abstract

Background and purpose: To present experimental evidence of lung dose enhancement effects caused by strong inline magnetic fields.

Materials and methods: A permanent magnet device was utilised to generate 0.95T-1.2T magnetic fields that encompassed two small lung-equivalent phantoms of density 0.3g/cm³. Small 6MV and 10MV photon beams were incident parallel with the magnetic field direction and Gafchromic EBT3 film was placed inside the lung phantoms, perpendicular to the beam (experiment 1) and parallel to the beam (experiment 2). Monte Carlo simulations of experiment 1 were also performed.

Results: Experiment 1: The 1.2T inline magnetic field induced a 12% (6MV) and 14% (10MV) increase in the dose at the phantom centre. The Monte Carlo modelling matched well (±2%) to the experimentally observed results. Experiment 2: A 0.95T field peaked at the phantom centroid (but not at the phantom entry/exit regions) details a clear dose increase due to the magnetic field of up to 25%.

Conclusions: This experimental work has demonstrated how strong inline magnetic fields act to enhance the dose to lower density mediums such as lung tissue. Clinically, such scenarios will arise in inline MRI-linac systems for treatment of small lung tumours.

Keywords: Lung dose enhancement; MRI guided radiotherapy; Magnetic fields.

MeSH terms

Humans
Lung Neoplasms / radiotherapy*
Magnetic Fields*
Magnetic Resonance Imaging / methods*
Monte Carlo Method
Phantoms, Imaging
Radiation Dosage
Radiotherapy, Image-Guided / methods*