Another factor is that cancer cells act more aberrantly under stress. Fasting (not sure if this also applies to keto) creates resources competition by which cancer cells neither compete as effectively and make theme-selves more apparent to the body’s immune system. Lot’s of anecdotal information here with some evidence but as a hypothesis with reasonable with reasonable biological justification it makes enough sense to me that I’d be personally comfortable taking some of these risks for my self or my family.
I wouldn't expose my family to my own pet theories. I'd choose to listen to the oncologist. There is a downside to fasting as well. Removing the pleasure and normal lifestyle may isolate the patient from social interactions that involve food. Those social interactions could be really important to personal well being and staving off depression.
