A beam of carbon ions strikes the tumor precisely, like a surgeon who can only see the skin. Inside the body, the beam stops, and exactly where used to be a mystery. Now, helium is mixed into the same beam: it passes through the body like an X-ray, and its trail shows where the carbon went.

At the accelerator in the German center GSI, two types of ions were accelerated simultaneously — carbon and helium. They have nearly the same charge-to-mass ratio, so they travel together like two runners in a tandem. The helium fraction is only 7%, but that is enough: its signal clearly separates from the background left by carbon fragments.

On a phantom resembling a lung with a tumor, the new method showed accuracy better than 0.5 mm. Previously, doctors relied on calculations; now they see everything in real time. It is like driving a car using mirrors rather than memory of a map.

Both beams, the treating one and the illuminating one, are born simultaneously from the same accelerator. This means the image does not lag behind the treatment by a fraction of a second. The tumor does not have time to shift from breathing, and the doctor has time to adjust the aim.