My understanding is that cancer is generally an opportunist, and while many cancers prefer glucose, it will use whatever energy source it has available, whether glucose or fats.

Usually it's sugars (glucose) but apparently there are some cancers that actually feed primarily on fat.

From what this article is saying, it sounds like pancreatic is one that reliably feeds ONLY on sugars, so a nearly carb free, very high fat diet would always be beneficial when treating pancreatic cancer

That apparently is not necessarily true for all cancers though.