I think the problem is that cancer cells eat
and produce waste pretty much like non-cancerous
cells, so it's not easy to tell them apart.
Cancer cells 'just' have the problem that they
don't know when to stop dividing.
But, also, scientists are working hard to try
to find specific chemicals that are produced by
cancers. Is there a blood test for prostate
cancer now? Maybe you'll find a test for some
specific chemical produced by a cancer.