A proton beam aimed at a tumor cuts through soft tissue like a knife through butter, but its range depends on the subtle properties of water, the stuff we're mostly made of. Water is the body's main component, and how quickly protons slow down in it determines exactly where they deposit their energy.

Long-standing debates have hampered efforts to pin down the key number — the mean excitation energy (I): for liquid water, it's hard to measure directly because water evaporates. But amorphous ice (the same water, just frozen into a disordered structure) has a stopping power per unit density that matches liquid water at all proton energies. It's like comparing two recipes for the same dough: same flour, but in one case the batter is runny and in the other it's set, yet it bakes the same.

The authors used two computational methods: one accurate at high energies, the other at low energies, down to the peak of energy loss. Combining them, they obtained reliable stopping-power values and fed them into a program that simulates proton range in tissues. Comparison with precise depth-dose measurements showed that a value of I = 79.4 eV fits best.

Now doctors will have a firmer physical foundation for planning proton therapy. Water, which had always been a nuisance in measurements, has, through ice, shown us how to treat more precisely.