[Magnetic-resonance-guided radiotherapy : The beginning of a new era in radiation oncology?] / Magnetresonanzgeführte Strahlentherapie : Beginn einer neuen Ära in der Radioonkologie?

CLINICAL ISSUE: Image-guided radiotherapy (IGRT) using X­rays and cone-beam computed tomography (CT) has fostered precision radiotherapy. However, inter- and intrafractional variations of target volume position and organs at risk still limit target volume dose and sparing of radiosensitive organs at risk. METHODOLOGICAL INNOVATIONS: Hybrid machines directly combining linear accelerators and magnetic resonance (MR) imaging allow for live imaging during radiotherapy. PERFORMANCE: Besides highly improved soft tissue contrast, MR-linacs enable online, on-table adaptive radiotherapy. Thus, adaptation of the treatment plan to the anatomy of the day, dose escalation and superior sparing of organs at risk become possible. ACHIEVEMENTS: This article summarizes the underlying intention for the development of MR-guided radiotherapy, technical innovations and challenges as well as the current state-of-the-art. Potential clinical benefits and future developments are discussed. PRACTICAL RECOMMENDATIONS: Increasing availability of MR imaging at linear accelerators calls for the ability to review and interpret MR images. Therefore, close collaborations of diagnostic radiologists and radiation oncologists are mandatory to foster this fascinating technique.

Direct determination of [Formula: see text] for cylindrical ionization chambers in a 6 MV 0.35 T MR-linac.

To ensure accurate reference dosimetry with ionization chambers in magnetic resonance linear accelerators (MR-linacs), the influence of the magnetic field on the response of the ionization chambers must be considered. The most direct method considering the influence of magnetic fields in dosimetry is to apply an appropriate absorbed-dose-to-water primary standard. At PTB, a new water calorimeter has been designed which is capable to determine Dw,Q in an MR-linac. The new device allows the direct calibration of ionization chambers in terms of absorbed dose to water for MR-linac irradiation conditions. Hence, the correction factors [Formula: see text] can be determined which replace the current radiation-quality dependent correction factors [Formula: see text] for dosimetry in the presence of magnetic fields. In cooperation with Heidelberg University Hospital,[Formula: see text] factors were measured at the 6 MV 0.35 T Viewray MR-linac for different cylindrical ionization chambers with sensitive volumes ranging from 0.015 cm3 to 0.65 cm3. The chambers were placed both perpendicular and parallel in respect to the magnetic field. Standard uncertainties of about 0.5% were achieved.

On the feasibility of absolute 3D dosimetry using LiF thermoluminescence detectors and polymer gels on a 0.35T MR-LINAC.

BACKGROUND AND PURPOSE: As shown in our previous study, highly accurate absolute dosimetry in 3D is feasible by combining polymer gels (PG) with thermoluminescence dosimetry (TLD). In this setup, the thermoluminescence (TL)-based point dose information is used to renormalize the PG. This new PG-TLD reference system is now extended to measurements in magnetic fields. MATERIALS AND METHODS: Experiments were carried out on a conventional 6 MV linear accelerator (LINAC) and a 6 MV 0.35 T magnetic resonance (MR)-LINAC. Signal stability of TLD600 and TLD700 was examined without and with magnetic field. Afterwards, the combination of PAGAT PG and TL detectors was employed within a cylindrical phantom in presence of the magnetic field. Two scenarios were tested: (I) an air-filled phantom and (II) a water-filled phantom. For each scenario, two plans were irradiated: (a) opposed beams with a field size of 10 × 10 cm2 and (b) a 3D conformal plan assuring homogeneous target coverage using three equally distributed coplanar beams. RESULTS: Mean relative uncertainty of TL calibration reproducibility for TLD600/TLD700 was 0.49%/0.85% at the MR-LINAC and 0.48%/0.83% for the conventional LINAC. Individual TL calibration coefficients of TLD600 and TLD700 behaved differently in the presence of the magnetic field. An average difference of (3.29 ± 0.89)% occurred for all TLD600, whereas the result for TLD700 is not quite as clear with (1.09 ± 0.89)% after excluding some outliers. Using the TL dose information for PG renormalization, high 3D gamma passing rates were achieved using the 3%/2 mm criteria: 91.0% (Ia), 92.6% (Ib), 94.3% (IIa), 97.4% (IIb). CONCLUSION: This study shows that TL signal reproducibility is not affected by a low magnetic field. Nevertheless, absolute calibration coefficients of the individual detectors indicate a dependency on the magnetic field. Hence, a calibration at the appropriate LINAC type is recommended. Furthermore, the previously established renormalization method for PG was applied to measurements at a MR-LINAC and was verified as suitable for evaluations of homogeneous dose distribution in the target volume.

End-to-end test of an online adaptive treatment procedure in MR-guided radiotherapy using a phantom with anthropomorphic structures.

Online adaptive treatment procedures in magnetic resonance (MR)-guided radiotherapy (MRgRT) allow compensating for inter-fractional anatomical variations in the patient. Clinical implementation of these procedures, however, requires specific end-to-end tests to validate the treatment chain including imaging, treatment planning, positioning, treatment plan adaption and accurate dose delivery. For this purpose, a new phantom with reproducibly adjustable anthropomorphic structures has been developed. These structures can be filled either with contrast materials providing anthropomorphic image contrast in MR and CT or with polymer dosimetry gel (PG) allowing for 3D dose measurements. To test an adaptive workflow at a 0.35 T MR-Linac, the phantom was employed in two settings simulating inter-fractional anatomical variations within the patient. The settings included two PG-filled structures representing a tumour and an adjacent organ at risk (OAR) as well as five additional structures. After generating a treatment plan, three irradiation experiments were performed: (i) delivering the treatment plan to the phantom in reference setting, (ii) delivering the treatment plan after changing the phantom to a displaced setting without adaption, and (iii) adapting the treatment plan online to the new setting and delivering it to the phantom. PG measurements revealed a homogeneous tumour coverage and OAR sparing for experiment (i) and a significant under-dosage in the PTV (down to 45% of the prescribed dose) and over-dosage in the OAR (up to 180% relative to the planned dose) in experiment (ii). In experiment (iii), a uniform dose in the PTV and a significantly reduced dose in the OAR was obtained, well-comparable to that of experiment (i) where no adaption of the treatment plan was necessary. PG measurements were well comparable with the corresponding treatment plan in all irradiation experiments. The developed phantom can be used to perform end-to-end tests of online adaptive treatment procedures at MR-Linac devices before introducing them to patients.

Measurement of isocenter alignment accuracy and image distortion of an 0.35 T MR-Linac system.

For hybrid devices combining magnetic resonance (MR) imaging and a linac for radiation treatment, the isocenter accuracy as well as image distortions have to be checked. This study presents a new phantom to investigate MR-Linacs in a single measurement in terms of (i) isocentricity of the irradiation and (ii) alignment of the irradiation and imaging isocenter relative to each other using polymer dosimetry gel as well as (iii) 3-dimensional (3D) geometric MR image distortions. The evaluation of the irradiated gel was performed immediately after irradiation with the imaging component of the 0.35 T MR-Linac using a T2-weighted turbo spin-echo sequence. Eight plastic grid sheets within the phantom allow for measurement of geometric distortions in 3D by comparing the positions of the grid intersections (control points) within the MR-image with their nominal position obtained from a CT-scan. The distance of irradiation and imaging isocenter in 3D was found to be (0.8 ± 0.9) mm for measurements with 32 image acquisitions. The mean distortion over the whole phantom was (0.60 ± 0.28) mm and 99.8% of the evaluated control points had distortions below 1.5 mm. These geometrical uncertainties have to be considered by additional safety margins.

Radiation dosimetry in magnetic fields with Farmer-type ionization chambers: determination of magnetic field correction factors for different magnetic field strengths and field orientations.

The aim of this work was to determine magnetic field correction factors that are needed for dosimetry in hybrid devices for MR-guided radiotherapy for Farmer-type ionization chambers for different magnetic field strengths and field orientations. The response of six custom-built Farmer-type chambers irradiated at a 6 MV linac was measured in a water tank positioned in a magnet with magnetic field strengths between 0.0 T and 1.1 T. Chamber axis, beam and magnetic field were perpendicular to each other and both magnetic field directions were investigated. EGSnrc Monte Carlo simulations were compared to the measurements and simulations with different field orientations were performed. For all geometries, magnetic field correction factors, [Formula: see text], and perturbation factors were calculated. A maximum increase of 8.8% in chamber response was measured for the magnetic field perpendicular to chamber and beam axis. The measured chamber response could be reproduced by adjusting the dead volume layer near the chamber stem in the Monte Carlo simulations. For the magnetic field parallel to the chamber axis or parallel to the beam, the simulated response increased by 1.1% at maximum for field strengths up to 1.1 T. A complex dependence of the response was found on chamber radius, magnetic field strength and orientation of beam, chamber axis and magnetic field direction. Especially for magnetic fields perpendicular to beam and chamber axis, the exact sensitive volume has to be considered in the simulations. To minimize magnetic field correction factors and the influence of dead volumes on the response of Farmer chambers, a measurement set-up with the magnetic field parallel to the chamber axis or parallel to the beam is recommended for dosimetry.