It is a well-known fact that exercise can help prevent breast cancer.  What is not as widely recognized is that regular exercise can mitigate the symptoms of cancer treatment.  This includes

• Increased functional capacity
• Increased mobility and range of motion (ROM) post-mastectomy
• Decreased body fat
• Increased lean muscle mass
• Reduced loss of bone mineral density
• Decreased nausea and fatigue
• Improved natural defence mechanisms
• Improved sense of control
• Improved mood
• Improved self-esteem

A systematic meta-analysis published in the Journal of the Canadian Medical Association as far back as 1996 analyzed data from 14 studies, and found that the benefits of exercise were positive even when statistical significance was not achieved. (Outcomes were quality of life, cardiovascular fitness, physical functioning, fatigue, body composition, and adverse treatment effects.) In addition to the aforementioned benefits, physical activity may increase the probability of recovering from the disease itself.  A 2005 study of 3000 breast cancer patients found that just one hour of walking per week significantly increased the likelihood of making a full recovery. 

References 

• American Cancer Society (2005).  Exercise Can Improve Breast Cancer Survival.  American Cancer Society: Cancer.org.
• McNeely et al. (2006).  Effects of exercise on breast cancer patients and survivors: a systematic review and meta-analysis.  Journal of the Canadian Medical Association.