A Beam That Spares Healthy Cells express
Two cities: one with neat, straight streets, the other with a tangle of alleys. If you send a powerful flood of water through both, in the first the water quickly drains away through channels, while in the second it swamps everything. That's roughly how tissues behave under ultrafast radiation therapy, known as FLASH.
In healthy tissue, cells are arranged in an orderly fashion, like houses along regular streets. When radiation hits, charged particles are created that quickly find each other and form a dense 'electron-hole liquid.' This liquid is viscous and prevents the energy from spreading — harmful chemical reactions are slowed, and the tissue stays intact.
In a tumor, however, cells are jumbled, like shanties in an old city. There, charges don't merge into a calm liquid; instead, they immediately trigger a chain of reactions, producing free radicals — aggressive molecules that destroy cancer cells. So the tumor dies, while healthy tissue survives.
Now it's clear why very high doses and dose rates are needed to protect healthy tissue. If you irradiate too slowly, harmful radicals accumulate in healthy tissue, and then it suffers too. But with ultrafast irradiation, the liquid in healthy tissue manages to 'lock up' the energy, preventing it from doing damage.
Free radicals aren't always bad: they're exactly what kills cancer cells in FLASH therapy, while in healthy tissue their production is suppressed.