Pseudomesotheliomatous Epithelioid Angiosarcoma in One Dog: a Pleural Lesion that Mime Malignant Pleural Mesothelioma

Adrian Gal


The pleural surface is often involved in neoplasm that have metastasized from other
organs, especially from pulmonary tumors by rupture of visceral pleura and seeding of pleural
cavity. Mesothelioma seems to be the only primary tumor of the pleura. Regarding
mesothelioma, this is a rare neoplasm of thoracic, pericardial, and peritoneal mesothelium
being most commonly encountered in calves. In most cases, mesothelioma is formed by two
components, a cellular component, which structurally and functionally consists of epithelial
and endothelial cells of mesodermal origin. The second component is connective vascular
tissue, whose structure includes fusiform cells, collagen fibers and capillaries (1).
This is a case presentation of a diffuse pleural tumor that mimes malignant pleural
mesothelioma in one 13 years old German Sheppard female dog. The subject had been
presented to the clinicians due to some respiratory disturbances with sanguinolent pleural
effusion in both hemithorax. The necropsy revealed large quantities of sanguinolent exudate
in thorax (about two liters) associated with diffuse thickening of the parietal pleura. Excepting
this, there were obvious some small polypoid and pedunculated gray-reddish lesions attached
to the parietal pleura. Histologically, the general features of the tumor didn’t indicate
mesothelioma but a very rare type of angiosarcoma (pseudomesotheliomatous epithelioid
angiosarcoma) recently described only in humans (2). The tumor was characterized by diffuse
thickening of the parietal pleura that presents polymorph endothelial cells generating
numerous microvessels. The polypoid structures of the tumor consist of numerous endothelial
tumoral cells that generate microvessels. Interestingly there were identified some endothelial
cells that have epithelioid feature mimicking mesothelial cells. These polygonal epithelioid
cells show rudimentary vascular differentiation. Some of the poorly differentiated epithelioidlike
endothelial cells generate some papillary projections into the vessels’ lumen, or are free
into some tumoral vessels indicating micrometastasis. The neoplastic epithelioid-like
endothelial cells are organized in some papillary intravascular structures or they are disposed
in solid clusters, tumoral cells showing large vesicular nuclei with occasional prominent
nucleoli and abundant acidophilic cytoplasm. Excepting the part of the tumor that indicate
diffuse pseudomesotheliomatous (epithelioid) angiosarcoma, there were identified some
limited zones with fibrosis of the parietal pleura associated with some large non-tumoral
vessels. At the surface, these non-tumoral areas were covered by mesothelial cells comparing
with tumoral part of the pleura where mesothelial cells were present only here and there.
We conclude that pseudomesotheliomatous angiosarcoma should be added to the list of
pleural tumours capable of simulating malignant mesothelioma, this case being (according to
our knowledge) the first one described in veterinary canine literature.

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