Psychological stress and breast cancer incidence: a systematic review.

Objective: Breast cancer is the world's leading cause of cancer mortality in women. Stress is an imminent risk factor with a documented negative impact on neuro-endocrine and immune system. Numerous epidemiological studies have investigated the link between stress and cancer, reporting contradictory results from no association to a close causal link. The impact of the topic and the lack of conclusion compelled this systematic review.

Methods: A systematic review was carried out, including all literature studies from 1966 to 2016, investigating the relationship between stress and the occurrence of breast cancer. Of the 1813 articles identified in the PubMed/Medline database, 52 were eligible and included in the analysis.

Results: A number of 17 retrospective, 20 limited prospective and 15 prospective studies were analyzed. The number of patients exceeded 29,000, for a total number of more than 700.000 women recruited from hospital, screening cohorts or population registers. We identified 26 positive articles linking personal traits, stressful events and breast cancer, 18 negative articles that did not confirm their hypothesis and 8 articles that could not be classified. Facing heterogeneity, all possible misguiding factors such as: study design, information gathering, stress type, moment of exposure, individual susceptibility and personality, were discussed independently.

Conclusions: Qualitative analysis of articles has revealed a possible association between stress and cancer, especially regarding stressful life events. In the absence of a meta-analysis and taking into account the methodological heterogeneity of the studies, the results are difficult to interpret and the role of chance is difficult to exclude.