A case of the resected lymphohistiocytoid mesothelioma: BAP1 is a key of accurate diagnosis

Background: Malignant mesothelioma (MM) is a well-known malignant tumor that occurs in the pleura and is histopathologically classified into three subtypes. Lymphohistiocytoid mesothelioma (LHM) is considered a variant of epithelioid MM, and few cases have been reported. First case of LHM was reported by Henderson et al. in 1988. It is difficult to precisely diagnose LHM, and it is often misdiagnosed as reactive mesothelial cell proliferation. Case report: An 82-year-old man, with the smoking history of nine pack-years, was referred to our Department due to an abnormal shadow and pleural effusion in the left lung field on the chest X-ray imaging. His occupation was a teacher through his life without any asbestos exposure. Computed tomography (CT) and ¹⁸F-fluorodeoxyglucose-position emission tomography showed a tumor which was suggestive of malignancy on the left chest wall, with the possible invasion into the left 2nd to 4th ribs. He underwent a CT-guided biopsy and a thoracentesis, but the tumor was shown to be a benign tumor indicative of a reactive mesothelial cell proliferation. Then, he underwent a surgical resection and the tumor was suspected of liposarcoma macroscopically. Histological and immunohistochemical findings were suggestive of mesothelial lesion, such as nodular histiocytic or mesothelial hyperplasia. However, loss of BAP1 and no p16 homozygous deletion in the tumor cells led to the diagnosis of LHM, not a benign lesion.