When a person's brain tumor is irradiated, some of the energy also hits healthy cells. A few months later, MRI scans of some patients show bright spots — places where tissue has changed due to radiation. Doctors call them "radiation-induced contrast enhancements." These spots aren't always dangerous, but they show where the brain took the hardest hit.
Scientists looked at data from 152 people treated with either photons (ordinary X-ray radiation) or protons. Protons are particles that hit the tumor more precisely and spare surrounding tissue more. But it turns out protons have their own quirk: their destructive power depends not only on the dose but also on how densely they deposit energy along their path. This property is called linear energy transfer. The denser the proton's energy deposition, the more damage to healthy cells.
The researchers built a mathematical model that predicts where spots will appear. For photons, the main factor was simply the radiation dose. For protons, it was the dose multiplied by the density of energy transfer. This means protons sometimes act more strongly than expected from older calculations. Now doctors can see dangerous zones in advance and plan radiation to spare the ventricles as much as possible.
This approach helps make treatment safer: preserve healthy tissue and reduce the risk of side effects. After all, the goal of radiation therapy is to destroy the tumor, not to harm the brain.
The brain's ventricles are cavities filled with cerebrospinal fluid. In an adult, their total volume is about 25–30 milliliters — about two tablespoons. It's precisely around these cavities that tissue turned out to be especially sensitive to radiation.
In the TV series Star Trek, doctors use proton beams for treatment, but there they always hit the target perfectly. In reality, physicists and doctors are still figuring out how protons behave in living tissue.