Subsets of Idiopathic Inflammatory Myositis Enriched for Contemporaneous Cancer Relative to the General Population

Objective This study investigates cancer risk in idiopathic inflammatory myopathy (IIM) relative to the general population. Methods We conducted a single‐center, retrospective cohort study of IIM patients and malignancy. Myositis‐specific and ‐associated autoantibodies were determined by Euroimmun line blot, enzyme‐linked immunosorbent assay, and immunoprecipitation. We calculated standardized prevalence ratios (SPRs) and adjusted for calendar year, age, sex, race, and ethnicity by comparing observed cancers in IIM patients versus expected cancers in the general population using the Surveillance, Epidemiology, and End Results registry. Results Of 1,172 IIM patients, 203 (17%) patients with a cancer history were studied. Over a median follow‐up of 5.2 years, the observed number of IIM patients diagnosed with cancer was increased 1.43‐fold (SPR 1.43 [95% confidence interval (95% CI) 1.15–1.77]; P = 0.002). Within 3 years of IIM symptom onset, an increased SPR was observed for anti–transcription intermediary factor 1γ (anti‐TIF1γ)–positive patients for ovarian and breast cancer (ovarian SPR 18.39 [95% CI 5.01–47.08], P