I agree with all the wisdom posted above. The way I see it, the value of the 'phud' portion of MD/PhD is to be mentored in how to identify, investigate, and defend scientific ideas. Interest in the field can certainly help, but isn't dependent on your ability to develop these skills. If you look at some MD/PhDs who have completed all of their training, you can see a decent subsection of them that did their PhD research in one area (say, heart disease) and are doing a residency in another (neurosurgery).
I would say apply broadly to schools that you like for a number of reasons - geography, curriculum, research areas and experts, research culture, life outside of school, etc. 7-9 years is nothing to sneeze at so don't limit yourself to cancer immunology alone. By the time you graduate from Stanfurd (Go Bears!, btw) and even through MS1 and MS2, you may have a different set of interests. Anyhow, I would bet the lionshare of MSTP programs have excellent cancer programs and faculty.
j-weezy - J. Folkman RIP.