No research shows that stress causes cancer, but stress can cause other health problems (National Cancer Institute)
http://www.nci.nih.gov/cancertopics/life-after-treat...

Though many illness may be made worse by stress, there is no evidence that stress causes cancer. There may be an elevated risk of developing cancer when the immune system does not work properly, and stress decreases the activity of the immune system; however, the decrease of immune system activity by stress is not severe enough to increase the risk of developing cancer. It is also important that individuals with cancer realize that their stress did not contribute to the development of cancer.
http://healthylifestyle.upmc.com/StressHealth.htm