One of the most impactful things to look into would be a ketogenic diet. It is used in conjunction with the standard of care for cancer (chemo, radiation, surgery, etc.). I work in clinical research and almost exclusively write on cancer trials. Very few individuals are taking or are allowed to take supplements.

Dr Colin Champ is board certified radiation oncologist at the University of Pittsburgh. He has done a lot of research in area of nutrition (keto diets) and cancer. You can find him on You Tube and on several iTunes podcasts.

Here is one of his talks:

https://youtu.be/ot96y5-D_K0