Synchronous tri-cavitary biphasic malignant mesothelioma in a dog presenting with dyspnea

Malignant mesothelioma is an uncommon tumor that has a challenging diagnosis and a tendency for late diagnosis. A 7-year-old intact male dog presented with respiratory distress. Thoracic radiography revealed pleural effusion and bone reactions at multiple ribs. A computerized tomography scan with rib biopsy was suggested as a diagnosis plan. Unfortunately, the dog died. Gross pathological findings demonstrated multifocal to coalescing, pinkish-white firm masses scattered over the parietal pleural surface, extended to the serosa of the diaphragm and the pericardium, making them adhere to each other and causing the coalescence of a white irregular mass at the diaphragm. Multiple white to pink nodules were also noted at the intercostal muscle, mediastinum, and peritoneum. The histopathologic findings indicated a diffuse nodular structure of neoplasm in the pleura, mediastinum, pericardium, peritoneum, and lungs. The nodules were mainly composed of neoplastic epithelioid cells mixed with a minor component of atypical spindle-shaped cells. A biphasic malignant mesothelioma was considered according to gross appearance, location of lesions, and histopathological findings. The histopathological characteristics of this tumor were confirmed by immunohistochemical staining of vimentin, cytokeratin, and desmin. This report describes the pathomorphological, histopathological, and immunohistochemical features of tri-cavitary biphasic malignant mesothelioma in a dog.