Teaching the immune system to kill cancer is an ongoing concern in laboratories around the world, and the state of the art is pretty effective these days - though in the overregulated world of medicine, a decade or more can separate working methods in the laboratory from working therapies in the clinic, and most of that delay entirely unnecessary. Here is a good example of what is presently possible:
Researchers from UCLA's cancer and stem cell centers have demonstrated for the first time that blood stem cells can be engineered to create cancer-killing T-cells that seek out and attack a human melanoma. ... Researchers used a T-cell receptor from a cancer patient cloned by other scientists that seeks out an antigen expressed by this type of melanoma. They then genetically engineered the human blood stem cells by importing genes for the T-cell receptor into the stem cell nucleus using a viral vehicle. The genes integrate with the cell DNA and are permanently incorporated into the blood stem cells, theoretically enabling them to produce melanoma-fighting cells indefinitely and when needed.
In the study, the engineered blood stem cells were placed into human thymus tissue that had been implanted in the mice, allowing Zack and his team to study the human immune system reaction to melanoma in a living organism. Over time, about six weeks, the engineered blood stem cells developed into a large population of mature, melanoma-specific T-cells that were able to target the right cancer cells. ... The study included nine mice. In four animals, the antigen-expressing melanomas were completely eliminated. In the other five mice, the antigen-expressing melanomas decreased in size.
I'm not overly worried about the cancers that my body is likely to start generating in two or three decades; they will be a risk, but a small risk, more of a financial inconvenience than a genuinely threatening medical condition. By the 2040s this sort of guided approach to eliminating cancer will have long been a mainstream staple in clinics, a mature technology that will benefit from years of refinement, experience, and incremental improvements - and bear in mind that this is just one of a number of different branches of next generation cancer therapy presently under development and achieving similar results.