Cell Population Evolution

We have learned that evolution occurs at the population level, and that natural selection occurs at the individual level. The evolution of an individual is not possible, but what about the evolution in an individual? The evolution of populations of an individual's cells can lead recovery like in the case of the young boy who suffered from adenosine deaminase deficiency, but the more common case presented in the textbook is the development of cancer.
Cancer cells arise from a cell that has a high number of mutations, according to the Cancer Genome Project cancer cells on average have 60 or more mutations. The older a cancer cell is the more mutations it will accumulate because the tumor population will begin from a single cell with low genetic diversity. The cancer cells develop their mutation through microevolution but after the mutations are present there are environmental pressures and other natural forces that act as natural selective forces inside the body. The cancer cells can undergo its own form of genetic drift called metastasis, which is when a cancer cell migrates to another part of the body. We have looked at another case where there has been an evolution inside of a person when looking at HIV, and like with HIV some scientists say that future treatments of cancer need to anticipate the evolution of the cancer cells to the selective force or treatment. The authors of one paper say that the best treatment will use the natural selection process to direct the cancer away from a resistant form.
http://www.sciencedaily.com/releases/2013/01/130122101454.htm
http://www.nature.com/scitable/topicpage/cell-division-and-cancer-14046590
http://www.sciencedaily.com/releases/2012/06/120621101905.htm