Depression can increase the likelihood of dying from cancer, while cutting out feelings of helplessness reduces both depression and physical inflammation in the body, thus could greatly increase chances of survival.

A new analysis of studies that have been published on this subject shows that people suffering from chronic depression have a roughly 39% higher risk of dying from their cancer, regardless of the form that cancer has taken. That increase in risk in the range of the increase in risk of breast cancer that has been linked to post-menopausal hormone replacement therapy (1, 2), although it is far lower than the risk increase linked to tobacco (roughly 700% - 1000%).
In fact, as I wrote in Anticancer: A New Way of Life, it is not so much the stress in our environment that expose us to a higher risk of illness in general (and cancer in particular), but rather the feeling of helplessness that becomes durably rooted when we don't know how to deal with stress, or when there is no one around to give us a helping hand. Over periods of several months or years, this feeling of helplessness becomes manifest in depression and in abnormal secretions of cortisol and adrenalin. This stimulates inflammation and reduces the immune system's capacity to fight disease.
Fortunately there are several ways to leave this helpless feeling behind, as it is most often an illusion of helplessness. In another very recent study, a researcher in psycho-immunology at Ohio State University, Barbara Andersen, looked at women who had been treated for stage 2 or 3 breast cancer. Those who were initially depressed -- but who participated for a period of one year in a group in which they learned to practice a daily relaxation exercise, to change their diet, to do more physical activity, and to ask their friends and family for help --  considerably reduced their risk of depression. They also manifested a significant reduction in the level of inflammation in their bodies, and an increase in the activity of their immune systems.

After 11 years of follow-up of the large group of all women in this study, the women who were treated for breast cancer and who received this training -- whether or not they were initially depressed -- benefited from a reduction in mortality of almost 70% compared to women who had the same medical treatment but who received no support in modifying their lifestyle and leaving helpless feelings behind them (3, 4).
In cancer, we should be suspicious of false hopes. But these studies once again remind us that we should also be suspicious of the false hopelessness that overpowers us when we are not informed of all the practical ways in which we can look after ourselves to prevent or overcome cancer.

[1] In 2002, a large study concluded that there was more risk of breast cancer in women using estrogen-progestin hormone replacement therapy after menopause. The increase in risk was estimated at 24%; in other words, there were 8 more cases per year for every 10,000 femmes treated with estrogen and progestin for a year, compared to 10,000 women taking a placebo.
Rossouw JE, Anderson GL, Prentice RL, et al. Risks and benefits of estrogen plus progestin in healthy postmenopausal women: Principal results from the Women’s Health Initiative randomized controlled trial. Journal of the American Medical Association 2002; 288(3):321–333.
[2] Depression as a predictor of disease progression and mortality in cancer patients: a Meta-Analysis.
Cancer. 2009 Sep 14

[3] A Psychological Intervention Reduces Inflammatory Markers by Alleviating Depressive Symptoms: Secondary Analysis of a Randomized Controlled Trial
Psychosomatic Medicine 71:715-724 (2009)
[4] Andersen, B.L., et al., Psychologic Intervention Improves Survival for Breast Cancer Patients: A Randomized Clinical Trial.
Cancer, 2008. 113: p. 3450-3458