To examine the association between idiopathic inflammatory myopathy (IIM) and cancer before and after IIM diagnosis.
We used prospectively collected nationwide register data to design a case-control study to investigate the occurrence of cancer before IIM, and a cohort study to investigate the occurrence of cancer after IIM. Patients diagnosed with IIM between 2002 and 2016 in Sweden, were compared to the general population. The association between cancer and IIM was estimated before and after IIM diagnosis via logistic regression and Cox regression models, respectively.
We included 1419 patients with IIM and 7045 individuals from the general population. The overall odds of cancer before IIM diagnosis were increased in IIM compared to the general population, adjusted odds ratio (AOR) 1.5, 95% confidence interval (CI) 1.3-1.8. This association was also noted after IIM diagnosis, adjusted hazard ratio (AHR) 1.7 (95% CI 1.4-2.0), or one additional cancer in every 125 IIM patients per year. Colorectal (AOR 2.1), lung (AOR 5.4) and ovarian (AOR 7.0) cancers were associated with IIM before diagnosis. Oropharyngeal (AHR 9.1) and cervical (AHR 3.8) cancers, malignant melanoma (AHR 3.2) and non-melanoma skin cancer (AHR 3.1) were associated with IIM after diagnosis. Adenocarcinomas were associated with dermatomyositis before diagnosis and squamous cell cancers after IIM diagnosis. Lymphatic hematopoietic cancers were associated with IIM both before and after diagnosis.
The cancer types that occur before IIM diagnosis differ from the ones that occur after diagnosis. This may have an impact on screening decisions for IIM.