Hi Chris, I don't know how it will come about, but when you look at the discovery of penicillin, or the discovery of cowpox/smallpox vaccine, you see that a scientific breakthrough can suddenly and unpredictably happen. To the above I would also add the discovery that some stomach ulcers are caused by bacteria and are now treatable, and vaccination against HPV virus, has started to prevent cervical cancer (I'm not citing this as a cure for cancer, just as an unpredictable piece of science).
A big problem with treating cancer is that they are our own cells, so an attack on those cells will result in collateral damage. An interesting article I read on immunotherapy said a trial was having some success against lung cancer, but the damage to healthy lung cells made it impractical. I thought, yes but a similar approach to prostate cancer would be much more profitable, as we certainly don't care if we lose a few healthy prostate cells on the way (some people even choose prostatectomy).
From what I understand of genetics, some cancers are caused by just a few genes getting mutated, could we just turn those genes back off (but only for the cancer cells not the cells we need to keep dividing normally). Could we train the immune system to spot cancer cells wherever they are hiding and attack them? I think the answer two these two questions and perhaps some I haven't thought of is, yes one day. Probably not in our lifetime but probably within the next generation or two.
A story I always think of when discussing curing disease is from a medical school in the USA in the 1950s. An experienced heart surgeon said to the medical students "today 60% of Americans die of heart disease", one of them said "that is terrible we need to do something about that", the heart surgeon said "what would you prefer them to die of?"