Nov 30 2018
For the first time, researchers from the Institute of Radiooncology–OncoRay at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) have integrated magnetic resonance imaging, or MRI, with a proton beam and thus demonstrated that technically, this oft-used imaging technique can certainly work with particle beam cancer therapies.
Dr Aswin Hoffmann and his team installed an open MR scanner in the experimental room at the National Center for Radiation Research in Oncology–OncoRay. Conducting various experiments, the HZDR researchers were able to demonstrate that MRI can be combined with a proton beam. (Image credit: HZDR/R. Weisflog)
This discovery presents new avenues for targeted cancer therapy that spares healthy tissues. The results of the study, performed by medical physicist Dr Aswin Hoffmann and his research team, have been reported in the journal Physics in Medicine & Biology.
Traditionally, radiation therapy has been a part of the conventional oncological treatment practice. Within the tumor tissue, a particular amount of energy known as the dose is deposited, which causes damage to the genetic material of cancer cells, prevents them from dividing, and, preferably, destroys them.
Currently, photon therapy is the most frequently used form of radiation therapy, in which high-energy X-ray beams are used. In this therapy, a considerable part of the beam enters into the body of the patient and deposits harmful dose in healthy tissue enclosing the tumor.
Atomic nuclei as weapons against cancer
Radiation therapy is an alternative approach with charged atomic nuclei like protons. The particles’ penetration depth relies on their initial energy. In addition, these particles discharge their maximum dose toward the end of their trajectory. Beyond this so-called “Bragg peak,” no dose will be deposited. When administering this kind of therapy, physicians find it difficult to regulate the proton beam to accurately match the shape of the tumor tissue and therefore protect as much of the surrounding healthy tissue as possible in the process. Prior to the treatment, an X-ray-based computed tomography, or CT, scan is performed to choose their target volume.
“This has various disadvantages,” states Hoffmann. “First of all, the soft-tissue contrast in CT scans is poor, and secondly, dose is deposited into healthy tissue outside of the target volume.” In addition to this, proton therapy is more vulnerable to anatomical changes and organ motion when compared to radiation therapy with X-rays, which undermines the targeting precision during the treatment of mobile tumors.
Currently, no direct method is available for viewing the movement of tumor at the time of irradiation, and this is the most significant barrier when it comes to applying proton therapy.
We don’t know exactly whether the proton beam will hit the tumor as planned. Therefore, physicians today have to use large safety margins around the tumor. But that damages more of the healthy tissue than would be necessary if radiation were more targeted. That means we are not yet exploiting the full potential of proton therapy.
Dr Aswin Hoffmann, Institute of Radiooncology–OncoRay, Helmholtz-Zentrum Dresden-Rossendorf.
First prototype for MR-guided particle therapy
Hoffmann and his research team wished to change that. In collaboration with IBA (Ion Beam Applications SA)—the Belgian proton therapy equipment manufacturer—the aim of Hoffmann’s research group is to incorporate real-time MR imaging and proton therapy. Different from CT imaging and X-ray, MRI delivers exceptional soft-tissue contrast and allows continuous imaging at the time of irradiation. “There are already two such hybrid devices for clinical use in MR-guided photon therapy; but none exists for particle therapy.”
This is primarily because of electromagnetic interactions between the proton therapy equipment and the MRI scanner. In contrast, MRI scanners require highly homogeneous magnetic fields so as to produce images that are geometrically accurate. On the other hand, the proton beam is produced in a cyclotron—a circular accelerator, wherein charged particles are forced by electromagnetic fields onto a circular trajectory and are expedited.
Also, the proton beam is steered and shaped by magnets, the magnetic fields of which can obstruct the homogeneous magnetic field of the MRI scanner.
When we launched the project three and a half years ago, many international colleagues were skeptical. They thought it was impossible to operate an MRI scanner in a proton beam because of all the electromagnetic disturbances. Yet we were able to show in our experiments that an MRI scanner can indeed operate in a proton beam. High-contrast real-time images and precise proton beam steering are not mutually exclusive.
Dr Aswin Hoffmann, Institute of Radiooncology–OncoRay, Helmholtz-Zentrum Dresden-Rossendorf.
The majority of experts predicted another difficulty to transpire from proton beam behavior: when electrically charged particles move in the magnetic field of an MRI scanner, the beam is deflected by Lorentz forces from its straight trajectory. Yet, here too, the team was able to show that this deflection can be predicted and therefore can be corrected easily.
Competence center featuring a cyclotron and a large experimental room
In order to study these mutual interactions, Hoffmann and his group applied the experimental room at the National Center for Radiation Research in Oncology–OncoRay. Operated by TU Dresden, HZDR, and University Hospital Carl Gustav Carus, this joint research platform was established in 2005 as a new center of excellence. Since the University Proton Therapy Dresden (UPTD) was founded in 2014, patients have been getting proton therapy in the OncoRay facility, and currently, over 120 scientists at OncoRay perform studies on novel technologies and methods for radiation therapy.
“Our mission is to individualize proton therapy biologically and to optimize it technologically towards its physical limits,” stated Hoffmann, head of the research team on MR-guided radiation therapy at the HZDR.
With its own cyclotron, OncoRay delivers the proton beam into both the therapy room and the experimental room. The latter is used by Hoffmann and his colleagues for their research purposes. With the aid of IBA and the Paramed MRI Unit of ASG Superconductors SpA, the team installed an open MRI scanner in the proton beam path, achieving the first-ever prototype of MR-guided particle therapy. “We are lucky to have an experimental room that is large enough to accommodate an MRI scanner. That is one of OncoRay’s unique features.”
Knee phantom, mixed sausage, and predictable diversion
For the experiments on this first novel prototype, the researchers originally employed the so-called knee phantom—a tiny plastic cylinder filled with a range of differently shaped plastic pieces and an aqueous contrast liquid. Along with his team, Hoffmann used it to perform quantitative analyses of image quality. In another set of experiments, the team applied a piece of Dresden mixed sausage.
When the Dutch research group studied imaging for their MR-guided photon therapy device in 2009, they used pork chop. In 2016, Australian researchers demonstrated their MR-photon therapy device on a Kangaroo steak. Since we also wanted to go regional for our prototype in MR-guided particle therapy, we used Dresden mixed sausage.
Dr Aswin Hoffmann, Institute of Radiooncology–OncoRay, Helmholtz-Zentrum Dresden-Rossendorf.
Both sets of experiments with the sausage and with the knee phantom revealed that the image was not distorted by magnetic fields from proton therapy. They simply caused slight changes in the MR image, which can be easily rectified.
At present, the project is entering its next phase. The ultimate aim is to create the world’s first prototype for MR-guided particle therapy relevant for clinical applications.