Clinical, histopathological features and malignancy frequency of patients with idiopathic inflammatory myopathy

Purpose: Idiopathic inflammatory myopathy (IIM) group diseases are strongly linked to cancer. This study aims to describe the clinical and histopathological features of IIM patients diagnosed in our clinic, evaluate the frequency and subtypes of accompanying malignancies, and examine the relationship between pathological staining patterns and IIM subgroups. Materials and Methods: This retrospective study included 74 patients with IIM diagnosed between 2010 and 2023 who had a muscle biopsy at the time of diagnosis. Age, gender, imaging, electromyography, and muscle biopsy results were obtained using the hospital's electronic system. Results: Of the 74 patients with IIM, 45 are polymyositis (PM), 27 are dermatomyositis (DM) and two are inclusion body myositis (IBM). Malignancy developed in 12 (16%) of 74 IIMs, two (16.6%) of whom had malignancy before the diagnosis of IIM (one breast cancer, one thyroid papillary cancer). Malignancy was diagnosed in five patients (41.6%) in the PM group (the most common breast cancer), five patients (41.6%) in the DM group (the most common endometrial cancer, and two patients (16.8%) in the IBM group. It was found that positive staining with membrane attack complex was higher in the DM group compared to the PM group. Conclusion: Cancer screening should be performed at regular intervals in IIM patients. Breast and endometrial cancer screening should be prioritized in female patients, as well as lung cancer screening in male patients. Large cohort studies are needed to explain the relationship between pathological staining patterns and IIM subtypes.