There are things to watch out for like raw food that could harbor bacteria, especially if you are on chemo, and the amount of antioxidants you eat, again if you are on chemo, need to be monitored.
However, while there is DEFINITELY something to eating a better diet effecting your rates of developing some cancers, there is absolutely no such thing as an anti-cancer diet or anything that you can eat that will kill the cancerous cells. There are a lot of books, but the science is clear - there is no special anti-cancer diet, sorry.
What you can do depends on what you are up for. My husband was in the 'I have cancer, screw my diet' camp through his treatments, and I was not going to argue with him - he went with comfort food. A lot of people do the reverse and feel better for it, so talk to your oncologist and maybe a nutritionist, let them help guide you depending on your treatment, and a healthy diet is always a good plan long-term.