Electron Streaming Effect Associated With the Elekta Unity Anterior Imaging Coil

Abstract

The presence of the static field in magnetic resonance guided radiation therapy (MRgRT) systems, such as the Elekta Unity MR-Linac (MRL), influences charged particle motion due to the Lorentz force in and around patients. Consequently, for the Unity, changes to out-of-field dose (OFD) relative to conventional linacs occur due to the electron streaming effect (ESE) and spiraling contaminant electrons (SCE). This work investigates OFD associated with irradiation of the anterior MR imaging coil, which is in situ for all treatments on the Unity. Film measurements and Monaco simulations were performed to quantify the magnitude of OFD at the superior and inferior ends of the coil as a function of coil tilt relative to the beam direction. The dependence of OFD on field size and the relative electron density (RED) assigned to the coil and surrounding air are reported. The doses at both coil ends were clinically significant, with nearly 23.0% of the Dmax dose to water being recorded for the largest field (8.0 × 22.0 cm2) and 6.8% for the smallest field (3.0 × 3.0 cm2). Monaco simulations of OFD agreed with film within 5.0%, when appropriate calculation conditions were set. OFD decreases as coil tilt is reduced, and there is no evidence of ESE when the coil is horizontal. Clinically, the potential magnitude of cranio–caudal streaming dose from a tilted coil necessitates the use of appropriate shielding. A clinical case involving coil-induced ESE during treatment of a lesion in the right angle is presented. The planning-based investigation revealed that ESE doses associated with the coil and an immobilization vacuum bag can be clinically significant.

Magnetic field induced dose effects in radiation therapy using MR‐linacs

“…10,83 The TPSs were compared to the independent Monte Carlo modeling or experimental measurements, and good agreement was found except for dose near heterogeneity interfaces. 35,45,96 It is vital to improving the TPSs to accurately reflect and account for the magnetic field induced dose perturbation at interfaces. This review does not cover every different aspect of MR-linacs compared to conventional linacs, such as MR safety, compatibility of the implants or QA devices, and others.…”

“…95 The out-of -field ESE dose from the surface coil was mitigated when coil tilt was reduced. 96 The lower strength 0.35 T magnetic field produces a slightly larger maximum ESE dose value as compared to 1.5 T. 92 It is explained by the fact that electrons under the lower magnetic field have a larger gyration radius, resulting in fewer liberated secondary electrons returned to the body surface by ERE. More electrons are escaped and streamed towards the out-of -field protruding surface.…”

“…It was measured that the RF coil could generate an ESE dose of 23% of the Dmax for a large field (8 × 22 cm 2 ) and 6.8% for a small field (3 × 3 cm 2 ). 96 Treatment beam transversing thinner tissue will experience less photon attenuation, produce more exit electrons, and be streamed to air. 92 Treatment regions such as esophageal are expected to involve higher ESE doses.…”

“…It should be noted that the RF coil placed above a patient could also generate significant streaming electrons 95 . The out‐of‐field ESE dose from the surface coil was mitigated when coil tilt was reduced 96 …”

ACPSEM position paper: the safety of magnetic resonance imaging linear accelerators

“…The same work reported approximately 14 Gy (36% of the prescribed dose) to the surface of bolus protecting the ear over the course of a supraclavicular nodal treatment. Additionally, doses due to ESE from the anterior imaging coil and vacuum immobilisation bag can be clinically significant [ 45 ]. The magnitude of the effect at the entrance surface of the anterior coil was comparable to that at the exit surface when the coil was tilted, with the two streams travelling in opposite directions (Fig.…”