Can diet help prevent cancer?

Yes: Dietary changes (increased fruits & vegetables, increased lean meats, less red meats, less fat, reduced alcohol use) can help reduce the incidence of cancers. Colon cancer, stomach cancer, breast cancer are some examples where the rates of new and recurrent cancers are affected by diet changes.

Some small effect: It's impossible to do retrospective epidemiologic studies without generating huge amounts of junk data. The studies of "diet and cancer" have been so massively contradictory and inconsistent over my four decades in medicine that i feel comfortable encouraging otherwise-healthy folks to eat what they please and keep up the vigilant cancer screening that's been such a benefit to us.