Investigating the effect of a magnetic field on dose distributions at phantom-air interfaces using PRESAGE® 3D dosimeter and Monte Carlo simulations

Filipa Costa,Simon J Doran,Simon Duane,Simeon Nill,Ilias Billas,Ian M Hanson,David Shipley,John Adamovics,Uwe Oelfke

doi:10.1088/1361-6560/aaaca2

Abstract

Dosimetric quality assurance (QA) of the new Elekta Unity (MR-linac) will differ from the QA performed of a conventional linac due to the constant magnetic field, which creates an electron return effect (ERE). In this work we aim to validate PRESAGE® dosimetry in a transverse magnetic field, and assess its use to validate the research version of the Monaco TPS of the MR-linac. Cylindrical samples of PRESAGE® 3D dosimeter separated by an air gap were irradiated with a cobalt-60 unit, while placed between the poles of an electromagnet at 0.5 T and 1.5 T. This set-up was simulated in EGSnrc/Cavity Monte Carlo (MC) code and relative dose distributions were compared with measurements using 1D and 2D gamma criteria of 3% and 1.5 mm. The irradiation conditions were adapted for the MR-linac and compared with Monaco TPS simulations. Measured and EGSnrc/Cavity simulated profiles showed good agreement with a gamma passing rate of 99.9% for 0.5 T and 99.8% for 1.5 T. Measurements on the MR-linac also compared well with Monaco TPS simulations, with a gamma passing rate of 98.4% at 1.5 T. Results demonstrated that PRESAGE® can accurately measure dose and detect the ERE, encouraging its use as a QA tool to validate the Monaco TPS of the MR-linac for clinically relevant dose distributions at tissue-air boundaries.

Highlights

The integration of magnetic resonance imaging (MRI) into a radiotherapy treatment platform provides images with high soft-tissue contrast that can be used to perform accurate image-guidance during treatment delivery, giving confidence to apply radiation with tight planning target volumes margins, and potencially reducing the toxicity risks and leading to higher tumour control
PRESAGE R undergoes an optical density (OD) change as the leuco-dye is oxidised from leuco malachite green to malachite green with a peak absorption maximum around 633 nm (Guo et al 2006)
PRESAGE R cuvettes irradiation For the reproducibility experiment, the cobalt source samples gave a coefficient of variance with a mean over all times and doses of 1.8% and range of [0.9%, 4.1%], whilst the equivalent result for samples irradiated on the linac was 2.9% [0.4%, 5.0%]

Summary

Introduction

The integration of magnetic resonance imaging (MRI) into a radiotherapy treatment platform provides images with high soft-tissue contrast that can be used to perform accurate image-guidance during treatment delivery, giving confidence to apply radiation with tight planning target volumes margins, and potencially reducing the toxicity risks and leading to higher tumour control. There are currently five separate designs for integrating an MRI scanner and a radiotherapy treatment device (Fallone et al 2009, Raaymakers et al 2009, Keall et al 2014, Mutic and Dempsey 2014, Mutic et al 2016). These designs differ in the method of radiation delivery (linear accelerator (linac) or cobalt teletherapy), the orientation of the main (B0) magnetic field (parallel or perpendicular to the radiation beam) and the magnetic field strength (0.35 T to 1.5 T). The impact of the Lorentz force on the secondary electrons creates a reduced build-up distance and a strong dose increase at the proximal side of an air cavity and a reduction at its distal side (Raaijmakers et al 2004, 2005, Raaijmakers et al 2007a, 2007b)

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