Stress and breast cancer: a systematic update on the current knowledge.

A vast body of research has been carried out to examine the relationship between psychological stress and the risk of breast cancer. Previous reviews on this issue have mainly focused on stressful life events and have included both prospective and retrospective studies. The results from these reviews have revealed conflicting data. We evaluate whether stressful life events, work-related stress, or perceived global stress are differentially associated with breast cancer incidence and breast cancer relapse in prospective studies. Systematic and explicit methods were used to identify, select, and critically appraise relevant studies. The substantial variability in the manner in which stress was conceptualized and measured did not allow for the calculation of a quantitative summary estimate for the association between stress and breast cancer. Despite the heterogeneity in the results obtained, it is concluded that stress does not seem to increase the risk of breast cancer incidence. Whether stress affects the progression of breast cancer is still unclear. Studies with more thorough adjustment for confounding factors and larger studies on stress and breast cancer relapse are required to address this issue.