There have been many studies documenting that exercise increases survival and quality of life in people with cancer. A study published in JAMA in May of 2005 showed that just 3-5 hours of walking at 2-3 mph reduced deaths by 50% for women with hormone sensitive breast tumors. There was an absolute 6% decrease in mortality at 10 years. A second study published in January of 2014 in the Journal of Physical Activity and Health included more than 1000 men with a variety of cancers and looked at the effect of burning 12,000 calories per week vs less than 2500 calories per week doing exercise and found about a 50% reduction in mortality. A 175 pound man walking briskly for 30 minutes per day would burn 4200 calories per week. So jogging, tennis, basketball, etc would easily burn 12,000 calories.
A new study published from the University of Pennsylvania in the American Journal of Physiology looked at a mouse model where they were innoculated with melanoma cancer cells. They found that combining exercise with chemotherapy shrunk the tumors significantly more than those with chemo alone.