Incidence of Malignant Disease in Biopsy-Proven Inflammatory Myopathy: A Population-Based Cohort Study

BACKGROUND: The validity and magnitude of an association between myositis and malignant disease continue to be debated. Such issues as the legitimacy of a myositis diagnosis and distinction among myositis subgroups in previous population-based studies remain unresolved. OBJECTIVE: To determine the risk for malignant disease in patients with biopsy-proven inflammatory myopathies. DESIGN: Population-based, retrospective cohort study. SETTING: Victoria, Australia. PATIENTS: 537 patients in whom a biopsy-positive idiopathic inflammatory myopathy was first diagnosed from 1981 through 1995. MEASUREMENTS: Standardized incidence ratios were calculated to compare the incidence of malignant disease in patients with inflammatory myopathy and the general population. RESULTS: A total of 116 cases of malignant disease were found in 104 patients. Seventy-four cases were identified concurrently with (within 7 days) or after diagnosis of myositis. The highest risk for malignant disease was associated with dermatomyositis (standardized incidence ratio, 6.2 95% CI, 3.9 to 10.0). The risk was also increased in polymyositis (standardized incidence ratio, 2.0 CI, 1.4 to 2.7), although the relative risk for malignant disease in dermatomyositis compared with polymyositis was 2.4 (CI, 1.3 to 4.2). An increased risk for malignant disease was also found in inclusion-body myositis (standardized incidence ratio, 2.4 CI, 1.2 to 4.9). The excess risk for malignant disease diminished with time (standardized incidence ratio, 4.4 CI, 2.7 to 7.1 in the first year; 3.4 CI, 2.3 to 5.1 between 1 and 3 years; 2.2 CI, 1.3 to 3.9 between 3 and 5 years; and 1.6 CI, 1.0 to 2.6 beyond 5 years P for trend, 0.002). CONCLUSION: The risk for malignant disease is increased in biopsy-proven dermatomyositis and polymyositis and also appears to be increased in inclusion-body myositis.