1 Intermittent fasting is healthy for your gut and your metabolisms (Insulin, cortisol etc...)
2 Ketogenic diet is a potent mechanism to starve cancer cells. Indeed cancer cells are the more sensitive to glucose deprivation. So you can expect to damage primarily these cells.
A large number of studies tend to back such approach, but they are either indirect or anecdotal. Nevertheless all cancer patient tend to survive longer with a normal BMI.
One must discuss this issue with an oncologist and diet should be very well designed to avoid any weakening of the body inflammatory or immune response or any loss of muscle mass.
We urgently need RCT on this subject and as far as I know one is on the go.