What about the equivalent of find and replace (delete) based on pattern matching? Something along the lines of, extract a sample of the cancer, create a virus that binds to it with high specificity and kills it, then deploy.
I would observe that both cancers and viruses evolve very rapidly in the human body; in many cancer patients, a particular treatment grows less effective over time because the cancer evolves to be less susceptible. Adding a virus that may just quickly evolve to infect all the cells in the body to the mix seems rather risky.
This is basically the plot for the movie "I am Legend", starring Will Smith. In that movie, yeah, things go terribly wrong with this approach. (EDIT: And I mention the movie to say that as a thought experiment this has been done before and drew the same conclusion: This has potential to go very, very wrong.)
It's a general law in fiction, going back to Frankenstein, that any development that challenges the status quo will prove harmful. Mary Shelley didn't call it "The Modern Prometheus" for nothing - the implication is that to challenge our limitations offends the gods, or God, or Nature. It's a great way to make people feel OK about mortality or failure. And a guaranteed trope - when a TV episode or movie starts with a new development in science with great promise you know 100% by the closing credits that the scientist will have been punished and the status quo ante restored.
I would guess that when the first crazy experimentalist tried putting seeds in the ground instead of foraging for fruit, there was a cave painting about how the Spirits would punish the whole tribe for its arrogance.
Well, I'm kind of familiar with that firsthand since my hobbies include getting myself well when doctors claim it cannot be done. So I get called a liar, charlatan, and snake-oil salesman...blah blah blah... by folks who think it just cannot be done. Still, the idea of intentionally introducing a virus to cure cancer doesn't sit well with me. :-)
My mother was working for a (now extinct) biotech company that was developing a cancer 'vaccine' using a retrovirus to do targeted removal of cancer cells. One of the big problems they ran into was that the patient's immune system had a tendency to kill off all of their cancer-killer viruses.
So for a while there, they were seriously working on making a virus that would be completely invisible to your immune system- and this was maybe eight years ago. I'm sure there are dozens of little biotech companies in the bay area alone trying to do the same thing now.
The state of targeted cell killers in the lab and early trials is pretty promising at this time: either viruses, nanoparticles like dendrimers, or enlisting the components of the immune system that already do that job. Even bacteria have been drafted.
The trouble with cancers is that they are basically just you, with a bit of a mutation somewhere. It's hard to find anything that "targets" cancer without targeting your normal cells as well.
I would argue that as soon as we understand how to put "logic" into a virus or delivery mechanism, we will be able to target cancer, and a host of other problems. I would also argue that this will happen.
