How effective is alternate medicine in the cancer prevention?

It's not. If a "natural healing" or "conspiracy buff" person is trying to sell you something to prevent cancer, be aware that you are being played for a sucker. There's no pill that will make you immune to lung cancer from smoking, or breast color or other cancers from dumb luck. Don't pay people money to reward lying -- when their stuff fails, they blame you ("You ate GMO food"). Be savvy.
No trials. I am not aware of any controlled trials that have addressed this issue.
Not too much. Although alternative medicine is very popular & has demonstrated benefits for many conditions, it has a very limited role in cancer prevention & in general has no effect. Insofar as its techniques enable someone to reduce known risks -such as smoking or excessive alcohol consumption - it will be beneficial. Don't let alternative med delay ca screening, which can be life-saving.
Unproven method. First choice in prevention is by proven methods, healthy stress free good living good diet, good environment avoiding industrial carcinogens ionizing radiation exposure regular exercises avoid smoking excessive drinking, etc when all these things are done then the ' alternate medicine ' to prevent cancer as supplement. Or complement).