Magnetic-Field-Induced Secondary-Electron Effects in MR-Guided Radiotherapy: From Dosimetric Mechanisms to Workflow-Based Clinical Risk Management.

Abstract

MR-guided radiotherapy (MRgRT) combines high-contrast MRI, online adaptation, and radiation delivery in a single treatment workflow. The static magnetic field, however, changes the transport of secondary electrons generated by megavoltage photons and can produce dose perturbations that depend strongly on anatomy and beam geometry. This review examines the underlying physics, with emphasis on Lorentz-force deflection, the electron return effect (ERE) at density interfaces, and the electron streaming effect (ESE) in favorable geometries. It also considers where conventional DVH-based review may miss small, spatially confined hot or cold regions. We discuss magnetic-field-aware dose calculation, Monte Carlo and accelerated methods, independent verification, and delivered-dose reconstruction in adaptive workflows. Evidence is then considered by anatomical site, including the head and neck, thorax, and abdomen. Across these settings, clinical importance is determined mainly by the location and size of the perturbation and its relation to normal-tissue tolerance. Data from 0.35 T and 1.5 T systems show that relevant effects can persist after reoptimization, vary with motion, and respond differently to additional beam angles. In representative planning studies, a 1.5 T field increased skin D2% by about 1.4 Gy in simulated lung SBRT, whereas reoptimization in head-and-neck plans reduced the mean increase in skin Dmax from 5.43 to 1.68 Gy. We therefore treat magnetic-field-induced dose effects as geometry-specific risks that should be identified and controlled prospectively. The review concludes with practical guidance on where to inspect dose, when additional verification is justified, and which mitigation strategies are supported by current evidence.