Cancer is a difficult disease to cure. Our current systems of classification for it are based on the symptoms rather than the cause. Yet the actual causes vary so much that it is difficult to generalize treatment anyway.
In essence, cancer is a disease of sociopathic cells. These cells no longer follow the rules of the cellular environment around them - they don't care about their neighbors and they don't listen to the advice that staying low is better for society (i.e., the body) as a whole.
Hey, I write a blog, not a webcomic, okay?
Now, anthropomorphizing aside, human cells are complex little things. The problem with finding a simple cure for cancer is that each instance of cancer may involve a different set of proteins becoming disabled or overactive. Cells have redundancies and backup pathways for most of their processes, but sometimes, even the backups break down. Add to that the existing genetic variation between individuals and you have a mess more complicated than a tesseract. The best we can do is try to prevent it by living healthily.
But now we may have a new weapon in our arsenal: a cancer vaccine. It is still being studied, but I can talk about the possibilities.
It won't prevent cancer in healthy people. That would be amazing, but you would have to correctly predict what type of cancer they were about to get and give them a vaccination specific for the markers those cells would express. And how would you know if it worked? Maybe you just cured something that wasn't even going to happen in the first place.
What it can do is prevent relapses. Once somebody has been diagnosed with cancer and treated with drugs and/or chemo, adding the vaccine helps the immune system do its part in the fight in getting rid of the few remaining cells. It helps to highlight parts of the cancer cells that are not normal so that our white blood cells know that these targets can be attacked. If you can catch the micrometastases before they grow back into full-fledged tumors, you can theoretically nip the problem of relapse in the bud and be fully cancer-free.
There's a lot of possibilities here, and I hope this avenue of research proves fruitful.