Seeing things.

Many patients claim they see flashes of light (aka phosphenes) while receiving radiation. Suuuuure, whatever they say (wink, wink). What patients may actually be “seeing” is Cherenkov light, produced when a charged particle passes through a medium at a speed that is faster than the “phase velocity” of light in that medium. In other words, charged particles produced during ionizing radiation pass through the vitreous humor at speeds exceeding the propagation speed of light in that viscous medium. This creates a “photonic boom” (think sonic boom but with light) that is absorbed by the medium and then emitted as a burst of light. But don’t take our word for it. It can be detected emitting from the pupil with cameras. In fact, the spectrum of Cherenkov light is in the blue-green range—explaining reports of blue flashes. TBL: Photonic booms generated within the vitreous explain the flashes of light experienced by patients receiving radiation near the eyes. | Tendler, Int J Radiation Oncol Biol Phys 2019