The devices integrate a linac onto a biplanar rotating MR system, with the linac waveguide placed either between the MR's magnet planes or through the central opening of one of the planes (perpendicular and parallel configurations, respectively). The corresponding radiation field will therefore be either perpendicular or parallel to the main magnetic field, resulting in different physical or clinical properties. The entire system, including the magnet – rotates on a gantry to irradiate the target from any angle at 1 rpm.

The ability to interchange the system's configuration enables it to be adapted according to the clinical situation. "The parallel set-up avoids exit dose and dose perturbations at tissue-air/lung interfaces, and adds an insignificant entrance dose," explained Gino Fallone, head of the Cross Cancer Institute team. "The perpendicular set-up avoids entrance dose, but adds significant exit dose and hot/cold spots at tissue-air/lung interfaces."

Fallone and colleagues designed the system using a three-phase approach. Phase I involved the combination of a 6 MV linac with a head-sized permanent magnet, to demonstrate the feasibility of linac-MR integration. In 2008, this device was the first to deliver radiation during MR acquisition (see: MR-guided radiotherapy: one step closer).

The researchers then moved on to phase II, a whole-body system. Here, they interfaced a 6 MV linac with a 0.5 T superconducting whole-body open-bore magnet, with a 60 cm gap between the two poles to accommodate the patient. This prototype was used to demonstrate the structural and mechanical integrity of the system on a scaled-up rotating gantry.

Clinical installation

At the end of last year, the research team installed the phase II whole-body linac-MR system into an existing, clinical vault (5.9 x 6 m and 3.6 m in height). They note that this was achieved without having to remove the walls or ceiling. Instead, the system is delivered in parts that are small enough to fit through standard radiotherapy vault entrances, thus avoiding large construction costs.

Additionally, the MR system features a high-temperature superconducting magnet that uses a cryocooler instead of liquid helium, eliminating the need to construct a helium exhaust vent.

The team is now testing the imaging capabilities of the phase II linac-MR on humans and in July, the first high-quality MR images were acquired, says Fallone. "Conventional quality assurance measurements using the ACR [American College of Radiology] and other phantoms have demonstrated acquisition of good-quality, fast images appropriate for automatic tumour tracking and irradiation. These were recorded at the system's different angle of rotation," he told medicalphysicsweb. "We have also recorded good MR images of a human volunteer using simple conventional pulse sequences."

Design challenges

Writing in a recent research paper, Fallone described some of the physics and engineering challenges involved in developing the linac-MR system (Semin. Radiat. Oncol. 24 200). The key obstacle when creating a linac-MR is that the two devices are inherently incompatible.

For starters, RF noise from the linac and its associated components can reduce the quality of the MR image. The researchers quantified this noise and developed shielding techniques to eliminate it and restore the image signal-to-noise ratio (SNR). MR images can also be affected by currents induced in the MR coils by the megavoltage photon radiation. Again, this effect was quantified (and seen to reduce SNR by 15–18% at 250 MU) and methods developed to remove the effects of radiation-induced current in the MR images.

Conversely, the magnetic fields generated by the MR unit can impact the delivery of the therapeutic radiation beams. The researchers simulated the effects of MR fringe fields on the linac and revealed significant, negative effects on radiation output. They then developed active- and passive-shielding methods to eliminate these effects and recover full linac functionality.

Other important developments included optimization of the MR magnet design to improve image quality over a larger field-of-view and the design of tumour tracking techniques. The latter included generation of an automatic tumour-contouring algorithm to determine tumour position, a tumour-position prediction algorithm based on artificial neural networks and an MLC control system that moves the leaves in real time.

The team successfully demonstrated the use of the prototype to automatically contour, track and irradiate moving, lung-type targets with controlled MLC motion (see: Linac-MR achieves precise motion tracking).

Looking ahead

The next step in this project is the phase III system, an upgrade of the existing phase II device. This unit, which is currently under construction, will include an 85 cm bore system. Fallone envisages that the current phase II linac-MR system will begin use for patient treatments some time in 2015, once approval for clinical studies is granted by the federal regulatory agencies.

"Most tumours arise in soft-tissue. Because MRI provides the best soft-tissue visualization, the introduction of MRI into the planning and now the delivery process will significantly improve outcomes," Fallone explained. "In addition, this will also provide a potential treatment option to tumours in the mid-body region (such as the abdomen, cervix, kidney or pancreas), which are not conventionally treated by radiation because of organ motion or distortion. This may be the start of a whole new way of treatment."

• The Cross Cancer Institute team will be exhibiting at the ASTRO Annual Meeting in San Francisco, CA, with their research partner Paramed Medical Systems at booth #2341.

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