So cancer is not like flue its like virus. So its not necessarily impossible but you need a commanality that always works in all cases for the immune system to be trained on and it can't be something that is in regular cells. What becomes problematic is cancer are mutations that cause the out of control growth. Some diseases are misformed proteins that fit into normal protein places. This makes something that attacks it without attacking signs of the normal protein hard. This is just example of how the mechanisms work and why its tough. Now im not sure if crispr may allow for more direct fine tuning of the immune system as we should be able to make more exacting matches than what training it from vaccines do. But that would still be a ways out. For now we are creating vaccines against specific types of cancer and we might be able to make treatments for individuals specific cancer. Personalized medicine. The greatest road block with that will actually be the way we fund medicine. Unfortunately I can see it being like the de vinci robotic surgery things where a company makes a device to analyze a sample and make a custom treatment but its essentially rented to the hospitals who can use it so many times then it goes back for "refurbishment" or such.