Cancer cells often times lose their ability to perform oxidative phosphorylation. This means they can only rely on glycolysis as a sole source of ATP... This makes them EXCESSIVELY glucose hungry.

It's called the warburg effect. I'd have to read up on it and brush up on biochem, but that's the basic principle.

Essentially, cancer should soak up all the harmful sugar before it hits normal cells. This makes it even safer in theory than traditional chemo like methotrexate and such