Background: An association between polymyositis and cancer was first proposed in 1916, but the existence of the association has been disputed. An association between dermatomyositis and cancer is better accepted, but its magnitude is not known.
Methods: We undertook a study to provide accurate estimates of the risk of cancer in patients with dermatomyositis or polymyositis. We studied the incidence of cancer and the rate of mortality from cancer in a population-based cohort of 788 patients with dermatomyositis or polymyositis in Sweden from 1963 through 1983. The results were compared with those for the general population.
Results: Among the 396 patients with polymyositis, 42 cancers were diagnosed at the same time or after polymyositis was diagnosed in 37 patients (9 percent). The relative risk of cancer was 1.8 (95 percent confidence interval, 1.1 to 2.7) in the male patients and 1.7 (95 percent confidence interval, 1.0 to 2.5) in the female patients. Eighty-four males and 85 females died, and in 24 of these cases (14 percent) cancer was the principal cause of death. The mortality ratio (the rate of mortality from cancer in these patients as compared with that in the general population) was 0.90 (95 percent confidence interval, 0.6 to 1.4). Among the 392 patients with dermatomyositis, 61 cancers were diagnosed at the same time or after dermatomyositis was diagnosed in 59 patients (15 percent). The relative risk of cancer was 2.4 (95 percent confidence interval, 1.6 to 3.6) in the male patients and 3.4 (95 percent confidence interval, 2.4 to 4.7) in the female patients. Fifty-seven males and 110 females died, and in 67 of these cases (40 percent) cancer was the principal cause of death (mortality ratio, 3.8; 95 percent confidence interval, 2.9 to 4.8).
Conclusions: The risk of cancer is increased in patients with polymyositis or dermatomyositis. In patients with dermatomyositis there is also a higher rate of mortality from cancer.