Diffuse intrapulmonary mesothelioma mimicking pulmonary lepidic adenocarcinoma: a rare case report and review of the literature.

Mesothelioma, with various clinical manifestations, radiological features, and histomorphological types, can be divided into epithelioid, sarcomatoid, and biphasic types, according to their histomorphological characteristics. There is a rare growth pattern of pleural mesothelioma: diffuse intrapulmonary mesothelioma (DIM), with a distinctive pattern of predominantly intrapulmonary growth, has no or minimal pleural involvement, and simulates interstitial lung disease(ILD) clinically and radiologically. A 59-year-old man presented to the hospital with recurrent pleural effusions for 4 years and a history of asbestos exposure. Computed tomography (CT) showed bilateral pure ground-glass opacity lesions, and the tumor cells showed a lepidic growth pattern pathologically. Immunohistochemical staining was positive for CK, WT-1, calretinin, D2-40, CK5/6, and Claudin4, while TTF-1, CEA, EMA, CK7, CK20, and other epithelial markers were negative. BAP1 loss its expression, and MTAP was positive in cytoplasm. CDKN2A was negative tested by Fluorescence in situ hybridization (FISH). The final diagnosis was DIM. In conclusion, we should recognize this rare disease to avoid misdiagnosis and delayed treatment.