Monte Carlo simulations of out-of-field surface doses due to the electron streaming effect in orthogonal magnetic fields

Abstract

The out-of-field surface dose contribution due to backscattered or ejected electrons, focused by the magnetic field, is evaluated in this work. This electron streaming effect (ESE) can contribute to out-of-field skin doses in orthogonal magnetic resonance guided radiation therapy machines. Using the EGSnrc Monte Carlo package, a phantom is set-up along the central axis of an incident 10 10 cm2 7 MV FFF photon beam. The phantom exit or entry surface is inclined with respect to the magnetic field, and an out-of-field water panel is positioned 10 cm away from, and centered on, the isocenter. The doses from streaming backscattered or ejected electrons, for either a 0.35 T or 1.5 T magnetic field, are evaluated in the out-of-field water panel for surface inclines of 10, 30, and 45°. The magnetic field focuses electrons emitted from the inclined phantom. Dose distributions at the surface of the out-of-field water panel are sharper in the 1.5 T magnetic field as compared to 0.35 T. The maximum doses for the 0.35 T simulations are 23.2%, 37.8%, and 39.0% for the respective 10, 30, and 45° simulations. For 1.5 T, for the same angles, the maximum values are 17.1%, 29.8%, and 35.8%. Dose values drop to below 2% within the first 1 cm of the out-of-field water phantom. The phantom thickness is an important variable in the magnitude of the ESE dose. The ESE can produce large out-of-field skin doses and must be a consideration in treatment planning in the MRgRT work-flow. Treatments often include multiple beams which will serve to spread out the effect, and many beams, such as anterior–posterior, will reduce the skin dose due to the ESE. A 1 cm thick shielding of either a bolus placed on the patient or mounted on the present RF coils would greatly reduce the ESE dose contributions. Further exploration of the capabilities of treatment planning systems to screen for this effect is required.

In-Air Electron Streaming Effect for Esophageal Cancer Radiotherapy With a 1.5 T Perpendicular Magnetic Field: A Treatment Planning Study

“…This finding was subsequently confirmed by Malkov et al using the EGSnrc Monte Carlo simulations (17). Furthermore, Malkov et al implemented Monte Carlo simulations to study the ESE induced out-of-field surface dose enhancement with oblique phantom under various magnetic field strength, and they found that ESE can produce doses as high as 39.0 ± 0.2% of the maximum deliverable dose by the photon beam (18). An et al calculated and measured the air-electron stream doses outside the treatment field with ViewRay system by utilizing a custom-made acrylic phantom (19).…”

“…All the dose calculations were performed with "dose-to-medium" mode by using Monaco's GPU based Monte Carlo platform GPUMCD (15), with a statistical uncertainty of 1% per control point and a dose grid resolution of 0.1 cm. According to previous studies, ESE mainly affects the out-offield in-air dose distribution in CC direction (18)(19)(20)(21). For esophageal cancer case, due to the proximity to target, out-offield neck and chin skin are most likely to be influenced by ESE.…”

Optimizing MR-Guided Radiotherapy for Breast Cancer Patients

“…This was first observed and evaluated by Park et al (64), who, in the context of accelerated PBI delivered on the 0.35 T 60 Co ViewRay system, observed an electron stream in air extending toward the head and ipsilateral arm. This ESE is caused by electrons generated inside the body that, instead of scattering in random directions when leaving the body, start spiraling along the magnetic field (65). If unobstructed, this electron stream would reach the chin and arm, causing unwanted irradiation of the skin in these areas.…”

“…Depending on the location of the high-dose region in the breast, this electron stream can also be directed toward the feet (Figure 4). Studies on phantoms and early clinical experiences suggest that the treatment planning system is able to fully describe the ESE and that the use of bolus material to shield the body parts located in the electron stream showed effective reduction of the dose in these regions (64)(65)(66).…”

Online MR-guided radiotherapy – A new era in radiotherapy

“…The union of a linear accelerator and an MRI in a single device to facilitate online adaptive MR guided radiotherapy was complex and required almost 20 years from an idea to a clinically available system [1]. The reasons for this were manifold: Unlike with cone-beam CT guided radiotherapy; the magnetic field of an MR-linac is permanent and impacts the dose distribution [2]. In addition, the moving linear accelator influences the magnetic field.…”