Interesting idea: introduce a different form of cancer into a patient to weaken the deadlier cancer...

I actually tried something like this. I introduced the same tumor that a rat was bearing, with the introduction of a human major histocompatibility protein complex. The MHC is often something that the immune system recognizes for xenograph rejection. At any rate, the theory was that if we reintroduced the same tumor cells with an easy marker for rejection, the immune system might translate the rejection to the other tumor cells. It didn't work in that experiment, however that doesn't mean it can't.

I am convinced that the approach described is probably a good way to develop strategies. Except for some unique tumors that are the result of some very specific molecular changes, you have to consider the environment, because these are heterogeneous competing organisms that adapt to, and change, their environment.

As an aside, I used to work in stroke research. My mentor once wondered if we could introduce a brain tumor into the focus of the stroke. Brain tumors release a lot of growth factors, and spur new blood vessel growth. The tough part would be getting the tumor to stop when you wanted it too. You could give it a suicide gene, but there's always the chance it could get away from you. Probably a very tough sell for any sort of trial: "You had a stroke. Ok, now we are going to give you a brain tumor."