Effect of 0.35 T and 1.5 T magnetic fields on superficial dose in MR-guided radiotherapy of laryngeal cancer

Mireille Conrad; Riccardo Dal Bello; Janita E van Timmeren; Nicolaus Andratschke; Lotte Wilke; Matthias Guckenberger; Stephanie Tanadini-Lang; Panagiotis Balermpas

doi:10.1016/j.ctro.2023.100624

Effect of 0.35 T and 1.5 T magnetic fields on superficial dose in MR-guided radiotherapy of laryngeal cancer

Clin Transl Radiat Oncol. 2023 Apr 1:40:100624. doi: 10.1016/j.ctro.2023.100624. eCollection 2023 May.

Authors

Mireille Conrad^{1

2}, Riccardo Dal Bello², Janita E van Timmeren³, Nicolaus Andratschke², Lotte Wilke², Matthias Guckenberger², Stephanie Tanadini-Lang², Panagiotis Balermpas²

Affiliations

¹ Institute of Radiation Physics, Lausanne University Hospital, Rue du Grand-Pré 1., 1007 Lausanne, Switzerland.
² Department of Radiation Oncology, University of Zurich, Rämistrasse 100, 8091 Zurich, Switzerland.
³ Department of Radiation Oncology, Radboud University Medical Centre, Geert Grooteplein Zuid 32, 6525GA Nijmegen, The Netherlands.

Abstract

Background: Treatment of head and neck cancer on linear accelerators with on-board magnetic resonance imaging (MR-linac) might be beneficial to reduce side effects and increase accuracy. For many head and neck cancer patients, dose coverage of the often superficially located planning target volumes (PTVs) is required. This study examines the impact of the electron return effect (ERE) on the surface dose in MR-guided radiotherapy (MRgRT) compared to conventional radiotherapy.

Materials and methods: For this bicentric dosimetric study, 14 cases of laryngeal carcinomas with PTVs reaching up to the skin surface were included. For each patient, five different plans were compared, two VMAT plans (with and without a 5 mm bolus) and three IMRT MRgRT plans (0.35 T, 1.5 T and 0 T, each without bolus). Dose distributions were also validated with film measurements.

Results: A similar coverage on the most superficial 3-5 mm of the PTV was achieved in the VMAT plans with bolus and the MRgRT plans for both 0.35 T and 1.5 T. However, coverage on this region was usually not achieved for VMAT without bolus and the 0 T plans. The film measurements on phantoms confirmed the results with the relative error never exceeding the calculated differences between the plans.

Conclusion: The present study could demonstrate that the ERE for both commercially available MR-linac variants provides sufficient coverage of the superficial tissue layers in MRgRT-plans for laryngeal carcinoma.

Keywords: Electron return effect; Head and neck; MR-Linac; MR-guided radiotherapy; Radiotherapy.