Immune checkpoint inhibitor therapy in a patient with small cell lung cancer and anti‐transcriptional intermediary factor 1‐γ antibody‐positive dermatomyositis: A case report

Autoimmune diseases (ADs) are closely related to cancers; 30% of dermatomyositis (DM) cases are associated with malignancy. In lung cancer patients accompanied by DM, the most frequent cancer type is small cell lung cancer (SCLC). Anti‐transcriptional intermediary factor 1 γ (anti‐TIF1γ) antibody is a promising marker for the assessment of cancer risk in DM patients. The recent use of immune checkpoint inhibitors (ICIs) for extensive‐stage SCLC has improved patient outcomes. However, clinical trials of ICI excluded most patients with ADs because of the increased risk of toxicity. Nevertheless, recent evidences suggest that ICI may be appropriate for AD patients. A 76‐year‐old man diagnosed with extensive‐stage SCLC and anti–TIF1γ Ab‐positive DM developed limb weakness and typical skin manifestations of DM. Positron emission tomography‐computed tomography showed diffuse uptake in all muscles. The results of a nerve conduction study and electromyography were consistent with acute myopathy. Electron microscopy showed tubuloreticular inclusions in endothelial cells. He was treated with corticosteroids for DM and chemotherapy with atezolizumab for SCLC. Despite concerns regarding the use of ICI because of DM, atezolizumab was administered under close observation. After treatment, tumor size decreased and his symptoms improved significantly. We believe that the response of SCLC to chemotherapy including ICI, had a positive effect on the improvement of DM. Clinicians should consider ICIs for SCLC patients with DM and carefully monitor the patient's symptoms during treatment.