Exercising after completing chemotherapy boosts immunity by replacing senescent NK cells with vibrant, healthy NK cells that can fight against the progression of cancer according to an October 2012 article presented at the Integrative Biology of Exercise in October of 2012. This study out of the University of Nebraska studied people who participated in a 12 week exercise program and looked at NK cell activity before and after the 12 week period.
Previous studies have repeatedly shown the exercise can decrease the chance of developing several cancers that include prostate, breast, and colon cancer. It can also improve the prognosis of people with cancer and can decrease the chance of cancer recurrence.
You would think that oncologists would be more interested in promoting lifestyle medicine than chemotherapy, radiation, and surgery because it often works better and is far less toxic.