Foamy Cell Angiosarcoma Resembling a Xanthomatous Histiocytic Infiltrate Without Evidence of Vascular Malignancy.

Angiosarcoma is a rare and aggressive malignancy of endothelial cells with multiple subtypes. Foamy cell angiosarcoma is a rare variant in which endothelial cells demonstrate "foamy" cytoplasmic change. We present the case of a 59-year-old male who presented with progressive erythema and swelling of the midface and bilateral eyelids. Two biopsies obtained 3 months apart showed an infiltrate of foamy mononuclear cells in the deep dermis, resembling a xanthomatous histiocytic process. Clinical work-up for disorders including Erdheim-Chester disease, Langerhans cell histiocytosis, and necrobiotic xanthogranuloma was negative. Nine months later, a third set of biopsies was performed showing a similar infiltrate of foamy histiocyte-like cells within the deep dermis. However, there was also a dermal proliferation of irregular vascular spaces lined by atypical endothelial cells, diagnostic of angiosarcoma. Subsequent immunohistochemical stains demonstrated expression of CD31 and ERG within the foamy cells in both sets of biopsies, strongly suggesting endothelial lineage and supporting a diagnosis of foamy cell angiosarcoma. CD34 was negative. This case represents a very unusual presentation of angiosarcoma and a diagnostic conundrum. In cases such as these, especially when features of a vascular proliferation are absent, ERG appears to be the most useful marker for differentiating foamy cell angiosarcoma from histopathologic mimickers.