Cancer needs sugar. One of the primary ways they detect cancer is via a PET-CT with a radioactive glucose tracer. It lights up where glucose uptake is the highest. This article talks about starving cells going into defensive repair mode, but also about how energy-hungry cancer cells are.

I had cancer (lymphoma) and did three years of chemo (mostly maintenance/preventative, as it was wiped out in the first couple months). I actually gained an enormous amount of weight, because the steroids I was on (and all the weed I smoked) made me ravenously hungry. If I end up in that situation again, I'm going to make a concerted effort to dramatically reduce my calorie intake, and in particularly carbs, because I've read a lot about the potential benefits of starving the cancer out.