Towards MR-guided electron therapy: Measurement and simulation of clinical electron beams in magnetic fields. (October 2020)

• Record Type: Journal Article
• Authors: Kueng, R.; Oborn, B.M.; Roberts, N.F.; Causer, T.; Stampanoni, M.F.M.; Manser, P.; Keall, P.J.; Fix, M.K.
• Is Part Of: Physica medica. Volume 78(2020)
• Page Start: 83
• Page End: 92
• Publication Date: 2020-10
• Subjects: Electron therapy -- Magnetic fields -- MRI-linac -- Monte Carlo; Medical physics -- Periodicals; Biophysics -- Periodicals; Biophysics -- Periodicals; Imagerie médicale -- Périodiques; Radiothérapie -- Périodiques; Rayons X -- Sécurité -- Mesures -- Périodiques; Physique -- Périodiques; Médecine -- Périodiques; 610.153
• DOI: 10.1016/j.ejmp.2020.09.001
• Languages: English
• ISSNs: 1120-1797

Abstract

Highlights: Therapeutic small electron beams are investigated in MRI-linac style magnetic fields. A comprehensive Monte Carlo framework is established to support the experiments. Focusing of electron beams & reduced penumbra is observed in inline magnetic fields. The results motivate investigation of an electron mode in future inline MRI-linacs. Abstract: Purpose: In the current era of MRI-linac radiotherapy, dose optimization with arbitrary dose distributions is a reality. For the first time, we present new and targeted experiments and modeling to aid in evaluating the potential dose improvements offered with an electron beam mode during MRI-linac radiotherapy. Methods: Small collimated (1 cm diameter and 1.5 × 1.5 cm 2 square) electron beams (6, 12 and 20 MeV) from a clinical linear accelerator (Varian Clinac 2100C) are incident perpendicular and parallel to the strong and localized magnetic fields (0–0.7 T) generated by a permanent magnet device. Gafchromic EBT3 film is placed inside a slab phantom to measure two-dimensional dose distributions. A benchmarked and comprehensive Monte Carlo model (Geant4) is established to directly compare with experiments. Results: With perpendicular fields a 5% narrowing of the beam FWHM and a 10 mm reduction in the 15% isodose penetration is seen for the 20 MeV beam. In the inline setup the penumbral width is reduced by up to 20%, and a local central dose enhancement of 100% is observed. Monte Carlo simulations are in agreement with the measured dose distributions (2% or 2 mm). Conclusion: A new range of experiments have been performed to offer insight into how an electron beam mode could offer additional choices in MRI-linac radiotherapy. The work extends on historic studies to bring a successful unified experimental and Monte Carlo modeling approach for studying small field electron beam dosimetry inside magnetic fields. The results suggest further work, particularly on the inline magnetic field scenario.