It's interesting that the core of this effect - that large doses delivered on timescales of a second or less significantly reduce normal tissue toxicity - was actually observed over 40 years ago, e.g.:
Some of the authors of that work were still active when the "Flash" concept came around again, and to hear them tell the story they tried to get funding to test it in tumour-bearing mice, but both their lab heads and external funders were sure the effect would be the same in tumours as normal tissue, and so wouldn't fund the work.
They eventually moved on to other things, and it needed a few decades for someone with enough soft money to give it a go and kick off this new research field.
https://radiation-research.kglmeridian.com/view/journals/rar...
Some of the authors of that work were still active when the "Flash" concept came around again, and to hear them tell the story they tried to get funding to test it in tumour-bearing mice, but both their lab heads and external funders were sure the effect would be the same in tumours as normal tissue, and so wouldn't fund the work.
They eventually moved on to other things, and it needed a few decades for someone with enough soft money to give it a go and kick off this new research field.