Psychoneuroimmunology explores the connection between the mind, body and immune system. There have been several studies that support the idea that the connection between the brain and immune system affect the quality of life, and possibly the morbidity and mortality rates of cancer patients.
According to one study, considerable work over the past decade has shown how psychological processes can impact pathways implicated in cancer progression. Furthermore, immune system dysregulation may have major implications for fatigue and depressive symptoms among cancer survivors. 
In the article, Psychoneuroimmunology and cancer: fact or fiction? by Kiecolt-Glaser JK1 and Glaser R., they state:
“There is substantial evidence from both healthy populations as well as individuals with cancer linking psychological stress with immune downregulation. This discussion highlights natural killer (NK) cells, because of the role that they may play in malignant disease. In addition, distress or depression is also associated with two important processes for carcinogenesis: poorer repair of damaged DNA, and alterations in apoptosisA type of cell death in which a series of molecular steps in a cell lead to its death. This is one method the body uses to get rid of unneeded or abnormal cells. The process of apoptosis may be blocked in cancer cells. Also called programmed cell death.. Conversely, the possibility that psychological interventions may enhance immune function and survival among cancer patients clearly merits further exploration, as does the evidence suggesting that social support may be a key psychological mediator. These studies and others suggest that psychological or behavioral factors may influence the incidence or progression of cancer through psychosocial influences on immune function and other physiological pathways.” 
Psychological stress has been found to be the main contributor in affecting immune response. This is explored in an article in the Journal of the National Cancer Institute, Psychologic Stress, Immunity, and Cancer by Sheldon Cohen and Bruce S. Rabin. They explore five areas that stress can affect cancer patients:
- Psychologic stress can alter immune function.
- The immune system plays a role in regulating tumor growth.
- Immune changes under stress are of the type that would influence tumor growth and metastasisThe spread of cancer cells from the place where they first formed to another part of the body. In metastasis, cancer cells break away from the original (primary) tumor, travel through the blood or lymph system, and form a new tumor in other organs or tissues of the body. The new, metastatic tumor is the same type of cancer as the primary tumor. For example, if breast cancer spreads to the lung, the cancer cells in the lung are breast cancer cells, not lung cancer cells..
- Immune changes under stress are of the magnitude that would influence tumor growth and metastasis.
- Stress-reduction interventions will influence the progression of the disease.