Depending on the type of cancer, there are correlations with diet changes and slowing growth of cancer. Causation isn't known yet, with the speculations about reasons veganism works I've seen all refuted. Low carb effects seems about increasing ketones and reducing glucose levels in the blood, which I think is being studied with some lung cancers. Type of cancer is important, with different diet changes positively correlated different with cancers not returning (and the wrong change could possibly even promote growth, as different cancers 'feed' on different things). Prostate, breast and lung cancer I'm aware of there being these correlations.