The National Cancer Institute has a good piece that summarizes the affect of psychological stress on cancer patients.  In a nutshell, psychological stress does not cause cancer directly, but…

it does have an impact on the overall state of health (physical and mental) of a patients and deleteriously affects the way cancer patients cope with cancer – http://www.cancer.gov/cancertopics/factsheet/Risk/stress?utm_content=sf25524591&utm_medium=spredfast&utm_source=twitter&utm_campaign=National+Cancer+Institute&cid=sf25524591.

Research has shown that people who experience intense and long-term (i.e., chronic) stress can have digestive problems, fertility problems, urinary problems, and a weakened immune system. People who experience chronic stress are also more prone to viral infections such as the flu or common cold and to have headaches, sleep trouble, depression, and anxiety.  Psychological Stress and Cancer – National Cancer Institute

To the extent that chronic stress leads to GERD (gastro-esophageal feflux disease), which leads to Barrett’s Esophagus (epithelial metaplasia of the lower esophagus), which leads to cancer, chronic stress can have INDIRECT effects in “causing cancer.”  The same can be said about chronic stress leading to increased smoking, which leads to lung cancer and bladder cancer and other cancers.

It is important for patients with cancer to manage their stress.  People who have cancer may find the physical, emotional, and social effects of the disease to be stressful. Those who attempt to manage their stress with risky behaviors such as smoking or drinking alcohol or who become more sedentary may have a poorer quality of life after cancer treatment. In contrast, people who are able to use effective coping strategies to deal with stress, such as relaxation and stress management techniques, have been shown to have lower levels of depression, anxiety, and symptoms related to the cancer and its treatment. However, there is no evidence that successful management of psychological stress improves cancer survival.  Psychological Stress and Cancer – National Cancer Institute