A sponge slowly watered from a watering can gets wet all the way through. But if you dump the same bucket of water on it all at once, the middle gets soaked while the edges stay almost dry. That's roughly how a material behaves when it absorbs energy nonlinearly: the more intense the beam, the more it gets stuck at the point of impact.

In the new study, this image is turned into a computer model. The medium is divided into 250×250 cells, like a field divided into 10-centimeter squares. At the center is an energy source that spreads and is absorbed. Two modes are compared: continuous (like slow dripping) and pulsed (like a sudden splash)—both with the same total dose. Absorption is defined in four ways: constant, linear, saturating, and threshold.

For nonlinear laws, the pulsed mode reduces energy in the ring around the center: in the very core, absorption increases slightly, but in the surroundings it drops. This explains the sparing effect of FLASH therapy: the tumor in the center gets its dose, while the healthy tissue around gets less. The effect depends on the absorption threshold: at a low threshold, an overly intense pulse can cause harm; at a high threshold, the benefit plateaus. A sponge that absorbs differently suggests how to tune pulse intensity for specific tissues.