To examine the temporal relationship between malignancies and idiopathic inflammatory myopathies (IIMs) and its impact on mortality.

A retrospective cohort for IIM patients was conducted using the Korean National Health Insurance Service databases. We observed more than 5 years before and after the diagnosis of IIM (2002~2016) to identify IIM patients who developed any malignancy and classified these patients into two groups: the cancer-associated myositis (CAM) group, who developed malignancy within 3 years before or after the diagnosis of IIM and the cancer-not-associated myositis (CNAM) group, who developed malignancy beyond 3 years of IIM onset. The survival rates of the two groups were compared.

We identified 1072 incident cases of IIM between 2007 and 2011. A total 225 patients of these patients were diagnosed with malignancy. The development of malignancy was frequent within 1 year before and after the time of IIM diagnosis. The common sites of malignancies in the CAM group differed from those in the CNAM group: the lung, hematologic malignancy, and the liver were common in both groups, but thyroid and oropharynx followed them in CAM while prostate, stomach, breast, and thyroid followed them in CNAM. CAM patient mortality was worse compared with CNAM patients (log-rank test, p < 0.01).

Among IIM patients with malignancy, common sites of malignancy were different between the CAM and CNAM groups, and patients with CAM had poor prognosis compared with CNAM patients.