Miliary pulmonary metastases from a clinically occult pleural mesothelioma

Mesothelioma is a rare neoplasm of the serosal membranes. Signs and symptoms of a pleural effusion typically herald discovery of the tumor. We report a case of miliary metastatic mesothelioma involving both lungs in a 54-year-old man who presented with right-sided chest discomfort, numerous pulmonary nodules detected by computed tomography of the chest, and absent pleural effusion. Immunohistochemical and electron microscopy studies performed on wedge biopsies of parenchymal pulmonary nodules led to the diagnosis of metastatic mesothelioma. Subsequent pleural evaluation and biopsy of pleural thickening noted at a site of prior chest wall trauma identified the primary neoplasm and confirmed the diagnosis as malignant epithelioid mesothelioma. The histologic appearance of discohesive epithelioid cells in a distinctly myxoid background was the clue in this case leading to the consideration of metastatic mesothelioma and a thorough immunohistochemical evaluation of the tumor. This case shows that mesothelioma may metastasize throughout the lungs in a miliary pattern and the metastases may be clinically detected before the primary pleural tumor. Metastatic mesothelioma is a consideration for metastatic pulmonary tumors of unknown origin.