If you've been diagnosed with breast cancer, don't give up your exercise program. If you don't exercise, now would be an excellent time to start. An article from Huliq.com says:

....women who had done a lot of exercise before their cancer had no fewer symptoms than those who had not done much exercise. However, the biggest difference in cancer-related symptoms and quality of life was seen in those who either maintained or increased their activity levels over the cancer experience. Increased physical activity, especially post cancer, was consistently related to better everyday functioning and reduced fatigue and bodily pain.

SAWF News adds this:

An added bonus of this study was that it emerged that those who maintained vigorous activity levels reported less weight gain and cared more about their appearance. Since excess body weight is associated with increased recurrence of breast cancer and lower survival rates this can only be a good thing.