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

• Published in: Physics in medicine & biology Vol. 63; no. 5; p. 05NT01
• Main Authors: Costa, Filipa, Doran, Simon J, Hanson, Ian M, Nill, Simeon, Billas, Ilias, Shipley, David, Duane, Simon, Adamovics, John, Oelfke, Uwe
• Format: Journal Article
• Language: English
• Published: England 26-02-2018
• Subjects: Gamma Rays; Humans; Magnetic Fields; Monte Carlo Method; Particle Accelerators; Phantoms, Imaging; Radiation Dosimeters - standards; Radiotherapy Dosage; Radiotherapy Planning, Computer-Assisted - methods
• ISSN: 1361-6560
• DOI: 10.1088/1361-6560/aaaca2

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.