Pathology of Aneurysmal Variant of Dermatofibroma (Aneurysmal Fibrous Histiocytoma)
Aneurysmal fibrous histiocytoma
differ from the classical cutaneous
in both their clinical presentation and pathologic features.
Clinically, they may be larger than the usual cutaneous fibrous histiocytoma, are blue, black, or dark red, and have a cystic consistency.
They are most commonly located on the extremities and may be associated with symptoms of pain and rapid growth.
The clinical diagnosis of fibrous histiocytoma is rarely considered in the differential diagnosis, which may include malignant melanoma, hemangioma, neurofibroma, and nonspecific cyst.
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Histopathology section of Aneurysmal Variant of Dermatofibroma
Histologically, the lesions are characterized by the presence of large, blood-filled tissue spaces.
Most cases show degree of epidermal hyperplasia, as seen in common fibrous histiocytoma.
These spaces lack an endothelial lining, being surrounded and lined by histiocytes, many of which contain hemosiderin pigment, fibroblasts, and foam cells.
The solid portions of the tumor have the usual features of a cutaneous fibrous histiocytoma.
Despite the presence of prominent secondary changes due to haemorrhage, many cases show cellular polymorphism, hyalinized collagen bundles surrounded by tumour cells in the periphery of the lesion.
The tumour is usually positive for vimentin.
There is rare focal smooth muscle actin positivity.
CD68 is positive in some reactive macrophages only.
Stains for CD31, CD34, desmin and factor XIIIa are usually negative.
1) The presence of extravasated erythrocytes in combination with a spindle-cell stroma may lead to an erroneous diagnosis of Kaposi's sarcoma. Kaposi's Sarcoma is positive for CD34 and lacks fibrohistiocytic cells.
2) Another important differential diagnosis is spindle cell angiosarcoma.
Spindle cells in aneurysmal fibrous histiocytoma is negative for CD31 and CD34.
Some histiocytes may be CD31 positive.
3) Malignant melanoma is excluded due to absence of an overlying in-situ component. Immunohistochemistry reveals that S100 protein is negative.
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