Has physical exercise ever been medically or scientifically proven in large studies to prevent any form of cancer?

Several studies: Have led to the observed link between physical activity and a reduced risk of cancers of the prostate, lung, and uterus. But realize this effect is difficult to study as so many factors contribute to the incidence of cancer in a person. If anything it's likely a small effect but any help is good and exercise benefits you in some many other ways.