Standard proton therapy is like a precise but light projectile: it hits the tumor dead-on, but it doesn't always pack enough punch. The reason is that protons have a low "killing power" (biological effectiveness), and some tumors manage to repair the DNA damage they inflict.
Researchers have found a way to amplify the blow without changing the beam itself. The idea is to pre-saturate the tumor with nitrogen-15 atoms — a rare but stable isotope of nitrogen. When protons slam into these atoms, short-lived alpha particles and carbon nuclei are born. These particles act like battering rams, shredding DNA far more brutally. In cell experiments, enriching with nitrogen-15 by 30% boosted the number of particularly dangerous double-strand breaks (those the cell can hardly fix) by 175% for alpha particles and by 53% for carbon.
Nitrogen-15 itself, delivered as the amino acid glutamine, didn't harm cells even at high doses. And after irradiation with a dose of 2 Gy, the survival of treated cells dropped by an additional 15.4% compared to standard proton therapy. This means the entire gain in effectiveness comes from the heavier DNA damage, not from the toxicity of the additive.
Nitrogen-15 is a rare guest on Earth: it makes up only 0.37% of the air, the rest being the common nitrogen-14. But it's stable and non-radioactive, so it's safe for the body.
The idea of turning harmless atoms into weapons against cancer echoes the "gray goo" principle from science fiction, except here the nanobots are replaced by atomic nuclei, and the target isn't the whole planet but a specific tumor.