Background Pembrolizumab, an anti-programmed cell death-1 protein monoclonal antibody, is effective for patients with advanced non-small-cell lung cancer. However, immune checkpoint inhibitors such as pembrolizumab induce various immune-related adverse events, involving the lung, liver, gastrointestinal, endocrine system, and skin. Intralymphatic histiocytosis (ILH) is a rare, chronic cutaneous disorder with a reactive inflammatory component, which often occurs in patients with rheumatoid arthritis. Case presentation We present a 67-year-old man with lung adenocarcinoma who developed ILH associated with pembrolizumab treatment. He was treated with palliative thoracic radiotherapy for superior vena cava syndrome. Subsequently, he received four cycles of pembrolizumab. Approximately 2.5 months after the initiation of pembrolizumab, he developed erythema on the trunk of his body. Based on findings of skin biopsies, he was diagnosed with pembrolizumab-induced ILH. Moreover, the upregulation of tumor necrosis factor-α was observed during pembrolizumab therapy. Conclusions This is the first report of ILH induced by pembrolizumab in a patient with lung adenocarcinoma.

A 49-year-old man living on an island was admitted to our hospital in January with antibiotic-resistant intermittent fever, cough, and hypoxemia. Based on his medical history, imaging findings, and a positive test for anti-Trichosporon asahii antibody, we diagnosed summer-type hypersensitivity pneumonitis. A literature review of reported cases of summertype hypersensitivity pneumonitis diagnosed in seasons other than summer showed that it is common in hot and humid environments as well as in warm regions. The possibility of summer-type hypersensitivity pneumonitis should be considered even in winter.