Patient dosimetry for hybrid MRI‐radiotherapy systems

Abstract

A novel geometry has been proposed for a hybrid magnetic resonance imaging (MRI)-linac system in which a 6 MV linac is mounted on the open end of a biplanar, low field (0.2 T) MRI magnet on a single gantry that is free to rotate around the patient. This geometry creates a scenario in which the magnetic field vector remains fixed with respect to the incident photon beam, but moves with respect to the patient as the gantry rotates. Other proposed geometries are characterized by a radiation source rotating about a fixed cylindrical magnet where the magnetic field vector remains fixed with respect to the patient. In this investigation we simulate the inherent dose distribution patterns within the two MRI-radiation source geometries using PENELOPE and EGSnrc Monte Carlo radiation transport codes with algorithms implemented to account for the magnetic field deflection of charged particles. Simulations are performed in phantoms and for clinically realistic situations. The novel geometry results in a net Lorentz force that remains fixed with respect to the patient (in the cranial-caudal direction) and results in a cumulative influence on dose distribution for a multiple beam treatment scenario. For a case where patient anatomy is reasonably homogeneous (brain plan), differences in dose compared to a conventional (no magnetic field) case are minimal for the novel geometry. In the case of a lung plan where the inhomogeneous patient anatomy allows for the magnetic field to have significant influence on charged particle transport, larger differences occur in a predictable manner. For a system using a fixed cylindrical geometry and higher magnetic field (1.5 T), differences from the case without a magnetic field are significantly greater.

Monte Carlo simulations of patient dose perturbations in rotational‐type radiotherapy due to a transverse magnetic field: A tomotherapy investigation

“…In general, physical constraints limit magnetic field arrangements in MR-guided treatment modalities to two basic configurations, longitudinal and transverse, as defined by the direction of the magnetic field relative to that of the incident photon beam. 7 The dose perturbations due to both magnetic field configurations have been well documented in simple slab geometries and fixed-angle IMRT scenarios. [7][8][9][10][11][12] Generally, the transverse magnetic field configuration, one where the magnetic field is perpendicular to the direction of the radiation beam, results in the electron return effect (ERE) which at high-to-low density interfaces, causes a lateral shift of the dose penumbra relative to the beam penumbra at depth, an asymmetric over-and underdosing in cavities, and an increased buildup region at low-to-high density interfaces.…”

New concept on an integrated interior magnetic resonance imaging and medical linear accelerator system for radiation therapy

“…Similar behaviors have also been observed in research from other groups. 26,27 We also calculated the dose distribution in the liver cancer patient, and the results are presented in Fig. 7.…”

Radiation induced currents in MRI RF coils: application to linac/MRI integration

“…Preliminary results from both groups have been published in dosimetry simulation papers (Kirkby et al 2008, Raaijmakers et al 2008 and more recently both groups have published MRI images taken with prototype units while the linac is producing radiation , Raaymakers et al 2009. During the course of a typical treatment using one of these integrated units, the MRI radio frequency (RF) coils will be exposed to the pulsed radiation beam of the linac causing unwanted radiation induced effects.…”