Researchers in the Netherlands have shown that a radiotherapy photon beam can be guided to a tumour using magnetic resonance imaging (MRI). In a step that the researchers claim will "open the door to start testing MRI–guided radiation therapy in the clinic", the team showed that MR imaging with the radiation beam switched on does not degrade the performance of either the accelerator used to create the beam or the MR scanner.

Incorporating real-time image guidance into radiotherapy should boost tumour targeting accuracy, reduce the irradiation of sensitive neighbouring tissues and reduce side effects. Such guidance will be of particular benefit if a non-ionizing imaging technique such as MRI is employed. As such, a research team at the University Medical Center Utrecht in the Netherlands is working to integrate a linear accelerator with an MR scanner. Now the Utrecht team has demonstrated proof-of-concept operation of their system.

The prototype device comprises a 6 MV linear accelerator (linac) positioned laterally to a 1.5 T MRI system. The researchers modified both the MRI and accelerator to enable their simultaneous and unhampered operation. The linac was customized by replacing various steel components with non-magnetic versions, as well as mounting it on a wooden frame (instead of its steel gantry). This was done to ensure that the linac was not adversely affected by the high magnetic fields created by the MRI magnet.

Active shielding

The MR scanner was equipped with a replacement magnet and gradient coil that were designed to reduce the magnetic field strength in the region of the accelerator. The magnet is actively shielded, with most of the external field generated by the inner coils cancelled by a field generated from a pair of shield coils.

The team also redesigned the treatment room set-up. Instead of using the standard RF shielding method — placing the MRI inside a Faraday cage — shielding was achieved via two RF cages situated at either side of the MRI bore. In this design, the inner wall of the MRI cryostat becomes an integral part of the RF cage and the sample volume is shielded from the rest of the room, including the accelerator.

"Together, the magnetic decoupling and the RF decoupling made it possible to perform MRI with the radiation beam on,", said Bas Raaymakers from UMC Utrecht's department of radiotherapy.

"The key significance of this work is the fact that we show that real simultaneous irradiation and diagnostic-quality MR imaging is feasible," said Raaymakers. "Before implementation in the radiotherapy clinic we obviously need to make a few more steps, but the proof is there that makes it worth investing in these steps."

Proof of concept

The researchers performed initial imaging tests on volunteers (with the accelerator switched off) using standard sequences for prostate, brain and kidney MRI. All images were of diagnostic quality. They then examined the simultaneous use of the MRI and accelerator systems, by performing 1.5 T MR imaging on a pork sample during irradiation. Images taken with the radiation beam on and off were identical, and no degradation of linac performance was seen.

Working towards a clinical prototype, the next step is to incorporate a multileaf collimator (MLC), which needs to be non-magnetic as it will be located outside of the system's low-magnetic-field region. Meanwhile, constructing a gantry for accelerator rotation will facilitate treatments using an arc-therapy approach. Other plans include the development of megavoltage transparent RF coils that won't cast a shadow in the radiation field, and determination of the optimal MR sequences for guiding radiotherapy.

The Utrecht team is currently working with equipment-makers Elekta and Philips on these tasks. "We hope to start the first clinical tests in a year's time. Whether this involves patients remains to be seen," Raaymakers said.

He continued: "We are currently discussing with our physicians what the best introduction scheme is. Should we start with palliative patients, start by using MRI as improved position verification or should we start with a novel strategy such as irradiation of liver metastases? This choice will determine the technical requirements such as, for instance, full gantry rotation, and the accompanying time schedule."