Minimizing the magnetic field effect in MR-linac specific QA-tests: the use of electron dense materials

Abstract

To address the quality assurance (QA) of a MR-linac which is an MRI combined with a linear accelerator (linac), the traditional linac QA-tests need to be redesigned, since the presence of the static magnetic field in the MR-linac alters the electron trajectory. The latter causes the asymmetry in the dose kernel which is introduced by the magnetic field and hinders accurate geometrical QA-tests for the MR-linac. We introduced the use of electron dense materials (e.g. copper) to reduce the size of the dose kernel and thereby the magnetic field effect on the dose deposition. Two examples of QA-tests are presented in which the geometrical accuracy of the MR-linac was addressed; beam profile and star-shot measurements. The introduced setup was compared with a reference setup and both were tested on a conventional and the MR-linac. The results showed that the symmetry of the recorded beam profile was restored in presence of the copper material and that the isocenter size of the MR-linac can be determined accurately with the introduced star-shot setup. The use of electron dense materials is not limited to the presented QA-tests but has a broad application for beam-specific QA-tests in presence of a magnetic field.

Lung stereotactic body radiotherapy with an MR-linac – Quantifying the impact of the magnetic field and real-time tumor tracking

“…This model includes the treatment beam with a nominal beam energy of 7 MV, the MLC as well as the cryostat. Each of the 160 leaves of the MLC, which is fixed to 90° (coordinate system according to IEC-61217), has an isocenter-projected width of 7.15 mm and the maximum field size is 27.20 cm parallel to leaf direction and 57.15 cm perpendicular to leaf direction [21] . This large field of view allows treatment of peripheral tumors even though the treatment beam isocenter is fixed to the center of the bore.…”

Machine QA for the Elekta Unity system: A Report from the Elekta MR‐linac consortium

“…This will eliminate any systematic offsets relating from moveable collimator components. For a spoke film, the use of copper sheets 30,73 removes the dose shift caused by the magnetic field (see section 2.3). This results in the spokes effectively representing the photon beam trajectory.…”

“…This technique has been used to measure profiles and star shot films allowing for common QA techniques to be used from conventional radiotherapy systems with only minor modifications to the methods. 73…”

Multiplexed imaging for diagnosis and therapy

“…3c). Due to anatomical changes stemming from patient movement (such as breathing or digestion), imaging is ideally conducted in real time 229 ; in fact, the use of advanced real-time IGRT is rapidly growing through the development of MR-LINAC [230][231][232][233] and PET-LINAC 234 systems.…”