dermatofibroma with fat invasion and foamy histiocytes

DERMATOFIBROMA

Dermatofibromas vary considerably in cellularity and relative proportion of fibroblasts to histiocytes from case to case and, sometimes, within a given lesion. The same can be said for the degree of vascularity. This particular case is moderately cellular and extends into the subcutis. The pattern of infiltration of the subcutis is one of encroachment with rapid termination rather than the 'Swiss cheese' type of asymmetrical, patchy, dissecting pattern of dermatofibrosarcoma protuberans (DFSP). Foamy histiocytes, when present such as in this case, further distinguish this tumor from DFSP. As has been mentioned, DFSP is not an ordinary fibrohistiocytic tumor and a histiocytic component with or without hemosiderin deposition should raise the possibility that a suspected DFSP is probably a dermatofibroma variant. Atypical giant cells (monster cells) are not indicative of malignancy. Many of the cells in a dermatofibroma are factor XIIIa positive and CD-34 negative.

Note on terminology: As a matter of personal preference, I use 'dermatofibroma' for the inclusive term that includes dermatofibroma, histiocytoma, sclerosing hemangioma, nodular subepidermal fibrosis, and intradermal benign fibrous histiocytoma.

Scan power view of the top of the specimen. Epidermal hyperplasia is present in this example, but this is not present in all dermatofibromas, particularly those that spare the superficial dermis. There is a central depression in the outline of the skin, and this is seen in a few dermatofibromas.

Scan power view of the bottom part of the specimen. There is partial replacement of the subcutis by the dermatofibroma, but the infiltration, as seen in the lower right side of the lesion, rapidly terminates.

Medium power view of the interior of the lesion. There is a moderately cellular storiform figure, but the other features in this lesion exclude the possibility of DFSP.

High power view of above.

A medium power view from another part of the lesion. Numerous histiocytes are present in this field. This number of histiocytes would not be characteristic of a DFSP.

High power view of above. There is a classical Touton giant cell, a degenerating cell resembling a 'monster' cell, and foamy mononuclear histiocytes in this field. Classical Touton giant cells are not common in dermatofibromas, but the other features in this case are not those of a xanthogranuloma.

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	Scan power view of the top of the specimen. Epidermal hyperplasia is present in this example, but this is not present in all dermatofibromas, particularly those that spare the superficial dermis. There is a central depression in the outline of the skin, and this is seen in a few dermatofibromas.
	Scan power view of the bottom part of the specimen. There is partial replacement of the subcutis by the dermatofibroma, but the infiltration, as seen in the lower right side of the lesion, rapidly terminates.
	Medium power view of the interior of the lesion. There is a moderately cellular storiform figure, but the other features in this lesion exclude the possibility of DFSP.
	High power view of above.
	A medium power view from another part of the lesion. Numerous histiocytes are present in this field. This number of histiocytes would not be characteristic of a DFSP.
	High power view of above. There is a classical Touton giant cell, a degenerating cell resembling a 'monster' cell, and foamy mononuclear histiocytes in this field. Classical Touton giant cells are not common in dermatofibromas, but the other features in this case are not those of a xanthogranuloma.