Are you sure about this? There is data to back it up in The China Study https://en.wikipedia.org/wiki/The_China_Study and mentioned in the Forks over Knives documentary. That's more about prevention, but it has a measurable impact on cancer rates, I don't see how that could be classified as no impact.
If diet stopped cancer, then the patients who eat less would see noticeable effect. This has never happened, except for maybe Otto Warburg, but even that is disputable[1].
