FLASH radiotherapy delivers the entire radiation dose in milliseconds instead of minutes. The tumor response is the same. The damage to surrounding healthy tissue is dramatically reduced. The effect is reproducible across multiple tissue types and animal models. The mechanism has been enigmatic.
Shvydka, Karpov, and Gupta (arXiv:2603.15913) propose that the answer is structural order. Normal tissue is ordered — cells arranged in regular patterns, extracellular matrix organized, vasculature systematic. Tumor tissue is disordered — cells disorganized, architecture chaotic, vasculature tortuous.
At ultra-high dose rates, the radiation generates enormous charge densities in tissue. In ordered tissue, these high charge densities form an electron-hole liquid — a condensed phase where charges are bound together and cannot diffuse freely. Free radical production, which causes biological damage, is suppressed because the charged species are trapped in the liquid phase. The tissue is shielded by its own order.
In disordered tumor tissue, the same charge density cannot form a condensed phase because the structural disorder provides efficient energy relaxation pathways. The charges dissipate into free radicals normally. The tumor receives the full biological dose.
The threshold behavior follows: below a critical dose rate, charge densities never reach the condensation threshold, and normal tissue gets damaged like tumor tissue (standard radiotherapy). Above it, the electron-hole liquid forms only in the ordered tissue, creating selective protection. The dose rate is not just a delivery parameter — it is a physical switch between two different radiation-matter interaction regimes, and the switch position depends on tissue structure.