Once you’ve been diagnosed with cancer, there are mostly no specific things you must or mustn’t eat. Having said that, the same things that are good for you in preventing cancer are good once you’ve got cancer. So more fruit, more vegetables, less meat, more fish.
There are some very specific cancers where the treatment means you have to be on a special diet. For example, if you had pancreatic cancer and an operation to remove your pancreas, you are likely to have trouble digesting fatty foods, and you will need to avoid those.
Similarly for some mouth and throat cancers, you may have trouble swallowing, very dry, very bulky food, and you may need to modify the composition of your diet.