Beam parameters that switch the FLASH effect on
In plain words
The effect depends on how fast the dose arrives, how it is split into pulses and how big each pulse is. A usable formula connecting these to the sparing does not exist.
Precise statement
Establish the sparing factor as a function of mean dose rate, dose per pulse, pulse width, pulse repetition frequency and total dose, for fixed tissue and endpoint, and locate the thresholds. Current working values of mean dose rate above 4000 rad/s, total delivery under 200 ms and dose per pulse above about 100 rad are empirical. An answer is a parametrized response surface with stated validity ranges.
What would settle it
A systematic animal dose-response series in which each beam parameter is varied independently at constant total dose with a common endpoint.
Status in the literature
Unverified note
Reported thresholds differ between electron, proton and photon beams and between tissues as of 2025.