This is something to be debated, I think. I have read quite a bit about the acid/alkaline debate and to me it seems what matters is the effect the food you eat has on your body once you've consumed it, rather than how acidic/alkaline a food is itself. This is what people mean when they they refer to the alkaline ash of a food. From what I have read, oranges have an alkaline ash, as do most fruits.
I dont think Sugar from fruit is a bad thing if it is the only source of glucose you are giving to the cancer cells. I have heard of all kinds of fruit juice fasts being effective against cancer - but these are fasts when that's the only glucose the cancer gets. Along with the glucose the cancer cells are getting potent anti-cancer nutrients. Of course it is best to be stategic about the fruits you consume; picking the ones with the most anti-cancer activity will be more beneficial, obviously.
Anyway, I am by no means an expert but I can tell you I have read a lot and this is what I understand to be true.