Ok sure, if the oncologist says you can do a diet too, go for it. Some diets might make things worse though.

Restricting sugar is one approach. Restricting folic acid is another approach. Restricting protein is a third approach. Different cancers may respond differently to various defecits. I've heard mice given aflatoxen and casein (milk protein) at 15% of their diet get liver cancer, but if protein is restricted below about 8% they don't get cancer even with the aflatoxin in their diet.

If your oncologist doesn't understand dietary impacts on the cancer you have, you should probably find another one.