J G M Kok

Integrating a 1.5 T MRI scanner with a 6 MV accelerator: proof of concept

At the UMC Utrecht, The Netherlands, we have constructed a prototype MRI accelerator. The prototype is a modified 6 MV Elekta (Crawley, UK) accelerator next to a modified 1.5 T Philips Achieva (Best, The Netherlands) MRI system. From the initial design onwards, modifications to both systems were aimed to yield simultaneous and unhampered operation of the MRI and the accelerator. Indeed, the simultaneous operation is shown by performing diagnostic quality 1.5 T MRI with the radiation beam on. No degradation of the performance of either system was found. The integrated 1.5 T MRI system and radiotherapy accelerator allow simultaneous irradiation and MR imaging. The full diagnostic imaging capacities of the MRI can be used; dedicated sequences for MRI-guided radiotherapy treatments will be developed. This proof of concept opens the door towards a clinical prototype to start testing MRI-guided radiation therapy (MRIgRT) in the clinic.

Towards MRI-guided linear accelerator control: gating on an MRI accelerator

To boost the possibilities of image guidance in radiotherapy by providing images with superior soft-tissue contrast during treatment, we pursue diagnostic quality MRI functionality integrated with a linear accelerator. Large respiration-induced semi-periodic target excursions hamper treatment of cancer of the abdominal organs. Methods to compensate in real time for such motion are gating and tracking. These strategies are most effective in cases where anatomic motion can be visualized directly, which supports the use of an integrated MRI accelerator. We establish here an infrastructure needed to realize gated radiation delivery based on MR feedback and demonstrate its potential as a first step towards more advanced image guidance techniques. The position of a phantom subjected to one-dimensional periodic translation is tracked with the MR scanner. Real-time communication with the MR scanner and control of the radiation beam are established. Based on the time-resolved position of the phantom, gated radiation delivery to the phantom is realized. Dose distributions for dynamic delivery conditions with varying gating windows are recorded on gafchromic film. The similarity between dynamically and statically obtained dose profiles gradually increases as the gating window is decreased. With gating windows of 5 mm, we obtain sharp dose profiles. We validate our gating implementation by comparing measured dose profiles to theoretical profiles calculated using the knowledge of the imposed motion pattern. Excellent correspondence is observed. At the same time, we show that real-time on-line reconstruction of the accumulated dose can be performed using time-resolved target position information. This facilitates plan adaptation not only on a fraction-to-fraction scale but also during one fraction, which is especially valuable in highly accelerated treatment strategies. With the currently established framework and upcoming improvements to our prototype-integrated MRI accelerator, we will realize more intricate MRI-guided linear accelerator control in the near future.

First patients treated with a 1.5 T MRI-Linac : Clinical proof of concept of a high-precision, high-field MRI guided radiotherapy treatment

The integration of 1.5 T MRI functionality with a radiotherapy linear accelerator (linac) has been pursued since 1999 by the UMC Utrecht in close collaboration with Elekta and Philips. The idea behind this integrated device is to offer unrivalled, online and real-time, soft-tissue visualization of the tumour and the surroundings for more precise radiation delivery. The proof of concept of this device was given in 2009 by demonstrating simultaneous irradiation and MR imaging on phantoms, since then the device has been further developed and commercialized by Elekta. The aim of this work is to demonstrate the clinical feasibility of online, high-precision, high-field MRI guidance of radiotherapy using the first clinical prototype MRI-Linac. Four patients with lumbar spine bone metastases were treated with a 3 or 5 beam step-and-shoot IMRT plan. The IMRT plan was created while the patient was on the treatment table and based on the online 1.5 T MR images; pre-treatment CT was deformably registered to the online MRI to obtain Hounsfield values. Bone metastases were chosen as the first site as these tumors can be clearly visualized on MRI and the surrounding spine bone can be detected on the integrated portal imager. This way the portal images served as an independent verification of the MRI based guidance to quantify the geometric precision of radiation delivery. Dosimetric accuracy was assessed post-treatment from phantom measurements with an ionization chamber and film. Absolute doses were found to be highly accurate, with deviations ranging from 0.0% to 1.7% in the isocenter. The geometrical, MRI based targeting as confirmed using portal images was better than 0.5 mm, ranging from 0.2 mm to 0.4 mm. In conclusion, high precision, high-field, 1.5 T MRI guided radiotherapy is clinically feasible.

Consequences of air around an ionization chamber : Are existing solid phantoms suitable for reference dosimetry on an MR-linac?

PURPOSE A protocol for reference dosimetry for the MR-linac is under development. The 1.5 T magnetic field changes the mean path length of electrons in an air-filled ionization chamber but has little effect on the electron trajectories in a surrounding phantom. It is therefore necessary to correct the response of an ionization chamber for the influence of the magnetic field. Solid phantoms are used for dosimetry measurements on the MR-linac, but air is present between the chamber wall and phantom insert. This study aimed to determine if this air influences the ion chamber measurements on the MR-linac. The absolute response of the chamber and reproducibility of dosimetry measurements were assessed on an MR-linac in solid and water phantoms. The sensitivity of the chamber response to the distribution of air around the chamber was also investigated. METHODS Measurements were performed on an MR-linac and replicated on a conventional linac for five chambers. The response of three waterproof chambers was measured with air and with water between the chamber and the insert to measure the influence of the air volume on absolute chamber response. The distribution of air around the chamber was varied indirectly by rotating each chamber about the longitudinal chamber axis in a solid phantom and a water phantom (waterproof chambers only) and measuring the angular dependence of the chamber response, and varied directly by displacing the chamber in the phantom insert using a paper shim positioned at different orientations between the chamber casing and the insert. RESULTS The responses of the three waterproof chambers measured on the MR-linac were 0.7%-1.2% higher with water than air in the chamber insert. The responses of the chambers on the conventional linac changed by less than 0.3% when air in the insert was replaced with water. The angular dependence of the chambers ranged from 0.6% to 1.9% in the solid phantom on the MR-linac but was less than 0.5% in water on the MR-linac and less than 0.3% in the solid phantom on the conventional linac. Inserting a shim around the chamber induced changes of the chamber response in a magnetic field of up to 2.2%, but the change in chamber response on the conventional linac was less than 0.3%. CONCLUSIONS The interaction between the magnetic field and secondary electrons in the air around the chamber reduces the charge collected from 0.7% to 1.2%. The large angular dependence of ion chambers measured in the plastic phantom in a magnetic field appears to arise from a change of air distribution as the chamber is moved within the insert, rather than an intrinsic isotropy of the chamber sensitivity to radiation. It is recommended that reference dosimetry measurements on the MR-linac can be performed only in water, rather than in existing plastic phantoms.

Integrated megavoltage portal imaging with a 1.5 T MRI linac.

In this note, the feasibility of complementing our hybrid 1.5 T MRI linac (MRL) with a megavoltage (MV) portal imager is investigated. A standard aSi MV detector panel is added to the system and both qualitative and quantitative performances are determined. Simultaneous MR imaging and transmission imaging can be performed without mutual interference. The MV image quality is compromised by beam transmission and longer isocentre distance; still, the field edges and bony anatomy can be detected at very low dose levels of 0.4 cGy. MV imaging integrated with the MRL provides an independent and well-established position verification tool, a field edge check and a calibration for alignment of the coordinate systems of the MRI and the accelerator. The portal imager can also be a valuable means for benchmarking MRI-guided position verification protocols on a patient-specific basis in the introductory phase.

Relative dosimetry in a 1.5 T magnetic field: an MR-linac compatible prototype scanning water phantom

The MR-linac is a hybrid MRI radiotherapy system allowing dose delivery in a 1.5 T magnetic field. This paper presents the design and performance of a prototype MR-linac compatible scanning water phantom. Since a scanning water phantom requires dose detectors, the performance air-filled ionization chambers in the magnetic field was characterized. We have found that the linearity and reproducibility of an ionization chamber are unaffected by the magnetic field. Also, moving the ionization chambers in a magnetic field during irradiation does not affect the dose response. When scanning in-plane profiles, the change in irradiation orientation can influence the ionization chamber dose response by up to 0.4%. However this effect can be eliminated by rotating the ionization chamber by 90° before measuring the in-plane profile. The performance of the total scanning water phantom was validated at a clinical setup in a 0 T magnetic field. There was no significant difference between the dose profiles measured with a standard clinical scanning water phantom and the profiles measured with the MR-linac compatible scanning water phantom. The performance of the MR-linac scanning water phantom in the MR-linac was validated using Gafchromic EBT2 film. There was no significant difference in dose profiles between the MR-linac scanning water phantom and the radiochromic film. These results indicate that automated scanning water phantom measurements using ionization chamber detectors are possible in the MR-linac.

Installation of the 1.5 T MRI accelerator next to clinical accelerators: impact of the fringe field.

In the UMC Utrecht a prototype MRI accelerator has been installed to investigate the feasibility of real-time, MRI guided radiotherapy. The system consists of a 6 MV Elekta (Crawley, UK) accelerator and a 1.5 T Philips (Best, The Netherlands) MRI system. The system is installed in a standard radiotherapy bunker. The bunker is at the corner of a block of six bunkers, so there are three neighbouring clinical Elekta accelerators. During ramping of the magnet, the magnetic fringe field in the two nearest bunkers was measured as a function of the magnetic field strength of the MRI magnet. At 8 m, a maximum increase of 1.5 G was measured, at 12 m, 0.6 G. This is up to three times the earths magnetic field. The clinical accelerators are needed to be re-calibrated in order to operate in such an external magnetic field. The resulting radiation field flatness of the clinical accelerators was measured and was similar to the situation before ramping the magnet.

Performance of a cylindrical diode array for use in a 1.5 T MR-linac.

At the UMC Utrecht, a linear accelerator with integrated magnetic resonance imaging (MRI) has been developed, the MR-linac. Patient-specific quality assurance (QA) of treatment plans for MRI-based image guided radiotherapy requires QA equipment compatible with this 1.5 T magnetic field. The purpose of this study was to examine the performance characteristics of the ArcCHECK-MR in a transverse 1.5 T magnetic field. To this end, the short-term reproducibility, dose linearity, dose rate dependence, field size dependence, dose per pulse dependence and inter-diode dose response variation of the ArcCHECK-MR diode array were evaluated on a conventional linac and on the MR-linac. The ArcCHECK-MR diode array performed well for all tests on both linacs, no significant differences in performance characteristics were observed. Differences in the maximum dose deviations between both linacs were less than 1.5%. Therefore, we conclude that the ArcCHECK-MR can be used in a transverse 1.5 T magnetic field.

Performance of a multi-axis ionization chamber array in a 1.5 T magnetic field

At the UMC Utrecht a prototype MR-linac has been installed. The system consists of an 8 MV Elekta linear accelerator and a 1.5 T Philips MRI system. This paper investigates the performance of the IC PROFILER™, a multi-axis ionization chamber array, in a 1.5 T magnetic field. The influence of the magnetic field on the IC PROFILER™ reproducibility, dose response linearity, pulse rate frequency dependence, power to electronics, panel orientation and ionization chamber shape were investigated. The linearity, reproducibility, pulse rate frequency dependence, panel orientation and ionization chamber shape are unaffected by the magnetic field. When the measurements results are normalized to the centre reference chamber, the measurements can commence unaltered. Orientation of the ionization chambers in the magnetic field is of importance, therefore caution must be taken when comparing or normalizing results from several different axes. IC PROFILER™ dose profiles were compared with film dose profiles obtained simultaneously in the MR-linac. Deviation between the film and the IC PROFILER™ data was caused by the noise in the film, indicating correct performance of the IC PROFILER™ in the transverse 1.5 T magnetic field.

TH‐E‐BRB‐09: Reference Dosimetry for an MRI‐Linac: The Magnetic Field Correction Factor

Purpose: To investigate if a standard dosimetry protocol (e.g. AAPM TG51) can be adapted to perform absolute dose calibration for a 1.5 T MRI‐linac system. Methods:Measurements were performed in an 1.5 T MRI‐linac system, a combination of a 1.5 T Philips Achieva MRscanner and a 6 MV Elekta linear accelerator. To determine the effect of a lateral 1.5 T magnetic field on the reading of a NE2571 Farmer type ionization chamber,measurements were performed with and without a 1.5 T magnetic field. The primary chamber was placed at the isocenter in a stationary water phantom (PTW, Freiburg, Germany). A second reference chamber was placed 168 cm behind the primary chamber on the central beam axis, where the magnetic field is 0 T.To ascertain correct functioning of the chamber inside a magnetic field,measurement reproducibility was investigated, as well as the influence of chamber orientation with respect to the central beam axis and the magnetic field (discussed in the supplement). Results: The magnetic field increases the ionization chamber reading by 4.8% resulting in a correction factor of 0.954 for this setup. The reproducibility of the ionization chamber did not appear to be affected by the magnetic field. The ionization chamber orientation in the MRI‐linac affects the reading with a maximum of 9.2%. However, for the standard dosimetry setup a relatively large orientation change of 10°, will influence the reading by less than 0.2%. Conclusions: These preliminary results show that a standard dosimetry protocols can be used to perform dosimetry in an MRI‐linac system, if a correction factor is applied for the magnetic field influence on the ionization chamber reading. Current research focuses on testing more NE2571 chambers and investigating the magnetic field influences on reading correction factors such as polarity and ion recombination. Results are expected shortly.