Abstract
Research evaluating the issue whether psychological factors influence the course of cancer has produced inconsistent results. In breast cancer, which has the largest body of research to it, studieshave shown positive and negative or missing relations between emotional distress and survival. The inconsistency of results may in part be explained by methodological flaws of most studies. To guide the evaluation of the quality of investigations, methodological prerequisites are presented covering the following issues: prospective design, correct definition of the endpoint under study, homogeneous sample, adjusting for biomedical prognostic factors, and adequate statistical analysis. Furthermore, possible mechanisms mediating the link between psychological factors and cancer progression are highlighted. When interpreting results, a clear distinction between causal risk factors and prognostic markers predicting, but not influencing, the course of cancer is crucial. This issue can only be solved on the basis of experimental intervention studies evaluating the impact of the modification of psychological factors on the course of the disease. At present, the results of intervention studies do not favour the interpretation of psychological factors influencing the course of cancer.
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