In this 20-minute TED talk, Dr. William Li discusses the growth of new capillaries in the body (called angiogenesis), especially how tumors can remain microscopic and isolated until capillaries form nearby and feed them. While there are successful anti-angiogenesis drugs out there, similar chemicals that work to inhibit capillary growth are found in ordinary foods.
Here’s Dr. Li’s slide listing some of the foods that contain the anti-angiogenesis chemicals. There’s really only one or two unusual items on that list. Adding or increasing such food items in your diet can help keep microscopic cancer cells at bay.
Unfortunately, this doesn’t really help much against existing malignant tumors. The better news is that the foods taste good (olive oil & garlic; blackberries & dark chocolate!?) For a guy like me, I can include such a diet with medical treatment without depriving myself. I’m fighting off melanoma that escaped once already, but currently have no detectable tumors.
WRT the OP, you can use the diet to attract your dad’s focus, while knowing that there is scientific backing to the approach and it shouldn’t interfere with your doctors’ medical plans for you.