Answer: Malignant mesothelioma

Histology: Most of the lesion consists of a cellular spindle cell lesion with a collagenous appearance. The nuclei are fairly uniform and ovoid with visible nucleoli. Mitotic figures are present yet not frequent. In areas, the lesion dissects through the fat leaving entrapped adipocytes. Within the fat, focal glandular structures were identified. The biphasic nature of this lesion composed of epithelial and spindle cell components is diagnostic of a biphasic malignant mesothelioma. Stains for cytokeratin and calretinin were strongly positive both within the spindle cell component as well as within the tubular structures.

Discussion: In contrast to reactive pleural fibrosis, this lesion lacks a significant inflammatory component. Reactive pleural fibrosis may demonstrate mesothelial hyperplasia consisting of tubules. However, these tubules are seen closer to the pleural surface and associated with inflammation. The histology of cellular fibrous tissue dissecting the adipose tissue would also not be expected in a reactive process. The presence of keratin within the spindle cell component does not distinguish reactive pleural fibrosis from malignant mesothelioma as reactive pleural “fibrosis” results from a proliferation of submesothelial cells that also express keratin. A fibromatosis would have a much more uniform growth appearance consisting of long sweeping fascicles of fibrous tissue. The presence of a glandular component rules out fibromatosis. Epithelioid hemangioendothelioma is a low-grade vascular malignancy that lacks a prominent cellular spindled cell component. Rather, it typically consists of epithelioid appearing cells often with a single cytoplasmic vacuole set in a dense hyalinized fibrotic background. Gland formation is not identified. Epithelioid hemangioendotheliomas can express keratin in addition to vascular markers.