Approximately half of all cancer patients receive radiation as part of their treatment. The aim is to kill cancer cells using high-energy radiation. However, the radiation also affects the surrounding tissue. This is especially true for X-rays, which—unlike protons—pass through the body. As a result, they can also damage healthy tissue behind the tumor (see below). "Roughly speaking, proton therapy reduces the burden on healthy tissue by a factor of two to three. That's why it's considered a superior form of radiation therapy," says Jan Unkelbach, research group leader for medical physics at the Department of Radiation Oncology at the University Hospital Zurich.
Facility without a massive steel structure
However, proton therapy is technically challenging and very expensive. Currently, the only place in Switzerland offering proton therapy is the Paul Scherrer Institute (PSI) in Villigen. "Globally, there are just over 100 proton therapy facilities—and more than 10,000 devices for X-ray radiation therapy," Unkelbach explains. "That's why only about one percent of radiation treatments today are done with protons." Together with his team and cooperation partners at the PSI, Unkelbach has explored various ways to make proton therapy accessible to more patients in a research project funded by the Swiss Cancer Research foundation.
A key idea is to combine proton therapy with X-rays. For instance, Unkelbach's team has investigated the treatment quality that could be achieved with a simplified proton therapy system, which would do away with the more than 100-ton steel structure (or, in technical jargon: the gantry). In proton therapy centers, this structure is used to direct the proton beam in all possible directions so that the patient lying on the treatment table can be irradiated from the optimal angle.
Compensating for limited irradiation angles
A proton accelerator without this massive structure would only produce a beam with a fixed direction, but the device "would be quite compact and could be installed in an existing hospital at significantly lower costs," says Unkelbach. In the researchers' hypothetical scenario, the fixed proton beam is aimed at a patient bed in a conventional X-ray therapy room. If a robotic arm rotates the bed slightly to the left or right, the patient is irradiated from different angles. "However, only from the side, and not from the front or back, for example," Unkelbach explains.
He and his team have used model calculations to show that this limitation in the irradiation angle can be compensated by simultaneously administering X-rays. The X-rays kill the tumor tissue that the protons cannot reach. "We have shown that combining a simplified proton therapy system with traditional X-ray therapy can achieve very good treatment quality," says Unkelbach. "And that cancer patients with the most commonly irradiated tumor types—such as tumors in the prostate, breast, lung, and head and neck—would benefit from combined treatments thanks to reduced radiation exposure."
Development has so far gone in the opposite direction
Unfortunately, there is a catch to these promising results: such a simplified proton therapy system does not (yet) exist. "In recent decades, development has gone in the opposite direction: the goal has always been to build the most perfect device possible," says Unkelbach. Although his team's results have generated significant interest at professional conferences, discussions with representatives of leading companies have only yielded cautious responses. "While our research can demonstrate the potential of combined radiation therapy," says Unkelbach, "whether the industry is willing to undertake the costly development of a simplified proton therapy system is up to them."