First of all, cancer can be prevented with a healthy lifestyle. That includes good nutrition, exercise, rest, and relieving stress. Vitamin and mineral supplements are just that --> supplements to be used in conjunction with your healthy nutrition plan. As insurance, take a quality multi-vitamin/mineral complex and Omega-3/fish oil. For days you're not getting in enough greens or fruit, take a greens/berry complex. I like the NOW Foods brand as they are not only economical, but they don't contain all the ingredients that come with allergies. Most of their products also don't contain preservatives.
On another note, I had cancer when I was 24/25 years old. Doctors said that I had 5 years. However, through good nutrition and a healthy lifestyle, I was able to overcome. It's now been 20 years. What do doctors truly know?!