High-Contrast Imaging for Cancer Therapy with Protons

Medical physicist Dr. Aswin Hoffmann and his team from the Institute of Radiooncology — OncoRay at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) — are the first researchers worldwide to combine magnetic resonance imaging (MRI) with a proton beam, thus demonstrating that in principle, this commonly used imaging method can work with particle beam cancer treatments. This opens up new opportunities for targeted, healthy tissue-sparing cancer therapy.

Radiation therapy has long been part of the standard oncological treatment practice. A specific amount of energy, called dose, is deposited into the tumor tissue, where it will damage the cancer cells' genetic material, preventing them from dividing and, ideally, destroying them. The most commonly used form of radiation therapy today is called photon therapy, which uses high-energy x-ray beams. Here, a substantial portion of the beam penetrates the patient's body, while depositing a harmful dose in healthy tissue surrounding the tumor.

An alternative is radiation therapy with charged atomic nuclei, such as protons. The penetration depth of these particles depends on their initial energy. They release their maximum dose at the end of their trajectory. No dose will be deposited beyond this so-called "Bragg peak." The challenge for physicians administering this kind of therapy is to control the proton beam to exactly match the shape of the tumor tissue and thus spare as much of the surrounding normal tissue as possible. Before the treatment, they conduct an x-ray-based computed tomography (CT) scan to select their target volume.

"This has various disadvantages," Hoffmann says. "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." On top of this, proton therapy is more susceptible to organ motion and anatomical changes than radiation therapy with x-rays, which impairs the targeting precision when treating mobile tumors. At present, there is no direct way of visualizing tumor motion during irradiation. That is the biggest obstacle when it comes to using proton therapy. Physicians 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."

Hoffmann and his team want to change that. In cooperation with the Belgian proton therapy equipment manufacturer IBA, his group's objective is to integrate proton therapy and real-time MR imaging. Unlike x-ray or CT imaging, MRI delivers excellent soft-tissue contrast and enables continuous imaging during irradiation. "There are already two such hybrid devices for clinical use in MR-guided photon therapy; but none exists for particle therapy." This is mainly due to electromagnetic interactions between the MRI scanner and the proton therapy equipment. On the one hand, MRI scanners need highly homogeneous magnetic fields in order to generate geometrically accurate images. The proton beam, on the other hand, is generated in a cyclotron, a circular accelerator in which electromagnetic fields force charged particles onto a circular trajectory and accelerate them. The proton beam is also steered and shaped by magnets, whose magnetic fields can interfere with the MRI scanner's homogeneous magnetic field.

"When we launched the project three and a half years ago, many international colleagues were skeptical," Hoffmann explains. "Yet we were able to show in our experiments that an MRI scanner can indeed operate in a proton beam.”

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