Interleukin‐5‐producing malignant pleural mesothelioma with eosinophilic pleural effusion

Malignant tumors are often associated with eosinophilic pleural effusion. Here, we encountered a case of interleukin‐5 (IL‐5)‐producing malignant pleural mesothelioma with eosinophilic pleural effusion. The patient was a 50‐year‐old male. He had a history of a cough for several weeks and had visited a local doctor. Left pleural effusion was noted, and the patient was referred to our hospital. He was diagnosed with malignant pleural mesothelioma by pleural biopsy, with eosinophilic pleural effusion. IL‐5 in the pleural effusion increased, and tumor cells were IL‐5‐positive by immunostaining. There have been few reports of IL‐5‐producing tumors, and this is the first report of IL‐5‐producing malignant pleural mesothelioma. Host‐tumor cell interactions cause eosinophilic pleural effusion. In patients with eosinophilic pleural effusion, malignant pleural effusion should be considered. It is necessary to clarify the pathophysiology of malignant tumors and eosinophils.