FLASH ultra-high dose-rate radiotherapy

6 problems

• Conformal FLASH delivery to deep-seated tumours

  Open

  Fast dose is easy to produce near the skin but hard to shape around a deep tumour. The accelerator and delivery physics for doing both at once is unsettled.

• Beam parameters that switch the FLASH effect on

  Open

  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.

• Does the FLASH sparing effect occur in human normal tissue

  Open

  The tissue-protecting effect of ultra-fast radiation has been shown mostly in mice and other animals. Whether it happens in people at clinically useful doses has not been measured.

• Can radiolytic oxygen depletion quantitatively explain FLASH sparing

  Contested

  One idea is that a very fast dose uses up the oxygen in tissue faster than blood can resupply it, and oxygen-starved tissue is harder to damage. The question is whether the numbers work.

• Inter-track radical recombination rates at FLASH dose rates

  Open

  At very high dose rates the chemical fragments made by different particle tracks overlap and can react with each other before reaching biological targets. The size of this effect is unknown.

• Physical mechanism of normal-tissue sparing at ultra-high dose rate

  Open

  Healthy tissue survives better when the dose arrives in a flash rather than slowly. The task is to find the physical or chemical process responsible.