Cancer: is there any large study scientific proof that regular exercise, in moderation, lessens the possibility of the diagnosis of cancer?

Indirect evidence: More data is availiable for benefit of exercise in cancer survivors (esp breast and colon).

No good studies: The difficulty in evaluating the impact of exercise on disease incidence and outcome is that people who exercise also tend to have other habits, especially not smoking and being healthier to begin with. The cardiovascular benefits seem clear, but exercise would seem unlikely to keep cells from mutating and i retain a pathologist's scepticism about improved immunity protecting from most cancers.