This has not been looked at for all types of cancer, but there have been studies of survivors of breast, colorectal, prostate, and ovarian cancers. In these studies, people with higher levels of physical activity after diagnosis lived longer and had less chance of the cancer coming back. Still, more studies are needed to see if exercise has a direct effect on cancer growth.
In the meantime, since physical activity is known to prevent heart and blood vessel disease, diabetes, and osteoporosis, cancer survivors should try to have a physically active lifestyle.