Elevated cancer incidence in patients with dermatomyositis: a population based study.

To study the relationship of polymositis (PM) and dermatomyositis (DM) and cancer in Finland. The medical records of 175 patients with PM and 71 with DM diagnosed between 1969 and 1985 were reviewed. The patients were followed until death or until December 30, 1990 through the files of Statistics Finland and the Finnish Cancer Registry. Thirty-four cases of cancer were found [standardized incidence ratio (SIR) 2.1, 95% confidence interval 1.4-2.9]. In the PM group the SIR was 1.0 (95% CI 0.5-1.8), and in the DM group 6.5 (95% CI 3.9-10). The relative risk of cancer among DM cases was very high (SIR 26, 12-48) in the first year after DM diagnosis. DM is associated with cancer, but not PM. The risk is high among patients with DM older than 49. Cytotoxic drugs given to DM patients did not increase the risk of malignancies. DM and certain cancers seem to share etiological factors.