A vegan diet is unique in that it eliminates ALL animal food products. Unlike a vegetarian diet that eliminates meat, a vegan diet takes it a step further and also eliminates dairy products like cheese. Some studies have shown a link to large quantities of animal proteins in the diet to higher rates of cancer. Proponents of the vegan diet say that eliminating the animal protein can virtually stop cancer in its tracks. Since there have been some prominent vegans who have been diagnosed with cancer, this can't be completely true. There are many factors that contribute to cancer (environmental factors, etc), but our goal is to do our best to fight cancer. A balanced vegan diet certainly features many cancer fighting foods, so done properly it can be a great way to battle cancer.
Be careful when choosing this diet. Work with a nutritionist or your doctor.