A powerful laser might one day allow doctors to zap brain tumors without harming healthy tissue, an improvement on today’s X-ray therapies. First, though, the German team developing it will have to get the laser working, demonstrate that they can generate cancer-killing particles exactly where they want them, and make the whole thing affordable.
The plan is to build a petawatt laser and use the quadrillion watts of power it produces to vaporize a target, creating protons that will then penetrate a tumor cell and kill it without affecting the surrounding tissue. Proton beam therapy is already used in a number of hospitals around the globe, but those systems rely on large particle accelerators to generate the protons, limiting where they’re available. Thomas Cowan, director of the Institute of Radiation Physics at Helmholtz-Zentrum Dresden-Rossendorf (HZDR), in Germany, hopes the laser can overcome the size constraints.
“In principle, you can have an accelerator on a table-top—a very large table,” he says. Cowan spoke to a group of journalists during a March press tour of eastern German research institutions sponsored by the German Academic Exchange Service, a publicly funded organization promoting higher education in Germany.
A majority of brain tumors are treated with x-rays, says Wolfgang Enghardt, head of medical radiation physics at HZDR. But some of the X-rays can wind up in tissue beyond the edges of the tumor, and some parts of the brain, such as the brain stem and the optic nerves, can be particularly sensitive to the radiation. Protons, on the other hand, tend not to scatter in tissue; how far they penetrate depends on their kinetic energy, which promises to make targeting more precise. “With protons, if you do it correctly, the dose drops to nearly zero behind the tumor,” Enghardt says.
The team is working with a titanium:sapphire laser that produces pulses of 800-nm light that last about 15 femtoseconds. The beam strikes a metal target—researchers have been working with gold, but think titantium may work better—with so much energy that it turns the metal into a plasma. That in turn strips protons off hydrocarbon atoms on the surface of the metal and fires them into a target.
So far researchers have been using a laser that produced 150 terawatts of power. They’ve recently upgraded it to 500 TW—half of their eventual 1-PW goal—and they are planning experiments at that level. The proton beams they’ve produced provide 85 megaelectron-volts. To treat a brain tumor, they’ll need to reach about 230 MeV.
Cowan says it will take another five to 10 years to complete the laser, and then researchers will have to prove the therapy’s effectiveness. Enghardt says that, while proton therapy has proved to be effective for brain tumors, there’s no evidence that it’s superior to conventional treatments outside of the central nervous system, for instance in liver or prostate cancer or carcinoma. There’s also a question of whether it will be economically feasible. While proton therapy is twice as effective as x-rays in destroying cancer cells, Enghardt says, the laser system is currently 10 times as expensive.