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subcutaneous adipose tissue. A promontory sign may be seen, as in the patch stage. There are
few mitotic figures and no significant nuclear or cytological pleomorphism. The initial
confluence of the spindle cell proliferation within the more superficial dermis progresses to
involve the deep dermis and/or subcutis. So-called autolumination may be seen in which an
erythrocyte is contained within a clear paranuclear vacuole in the cytoplasm of a spindled
endothelial cell. There can be extracellular hyaline globules. There is a chronic inflammatory
cell infiltrate with numerous plasma cells admixed with hemosiderin laden macrophages and free
hemosiderin granules. The lesions may show small clusters of capillary proliferations with
rather plump endothelial cells. In these cases, differential diagnosis from chronic vascular stasis
changes (stasis dermatitis) of the lower legs may be difficult. In stasis dermatitis, newly formed
capillaries are located close to the epidermis, are surrounded by an edematous to fibrotic dermis,
and are often accompanied by hemosiderin granules.[960]
The patch and the plaque stages of KS both show chronic inflammatory infiltrates which
may be perivascular and/or diffuse and of varying severity. These infiltrates consist of lymphoid
cells, plasma cells, and some macrophages. As a result of these infiltrates, early lesions of KS
may resemble granulation tissue. However, the presence of atypical spindle cells, large
protruding endothelial cells, extravasated erythrocytes, hyaline globules, and hemosiderin
pigment should suggest a diagnosis of KS.[960]
The nodular, or tumor, stage of KS is characterized by the appearance of grossly visible
red-purple nodules of varying sizes, usually 0.2 to 2 cm, on the skin. A solitary nodule may be
present, but more likely there are multiple nodules that in severe cases may become confluent
over a wide area. Microscopically, nodules are relatively circumscribed with a variable cellular
proliferation of neoplastic spindled cells arranged in fascicles. Spindle cells are numerous and
tend to be more monomorphic. Erythrocyte extravasation is pronounced, and hemosiderin
pigment is abundant. Autolumination is present. Neoplastic cell phagocytosis of red blood cells
leads to intracytoplasmic slits and formation of erythrophagosomes that form the hyaline
globules seen by hematoxylin-eosin staining. The entire nodular lesion appears as a mass,
though it rarely has discrete borders, and infiltration around adjacent adnexal structures or into
underlying adipose tissue is common. The overlying epidermis is usually intact, but an ulcerated
nodular lesion may be mistaken for a pyogenic granuloma. The periphery of some nodular
lesions may show more dilated vascular spaces resembling a cavernous hemangioma.[960,962]
Advanced Kaposi's sarcoma can be associated with chronic lymphedema. The clinical
stage of KS may be difficult to assess when lymphedema is present. Grossly, the appearance
may resemble a fibroma-like nodule. Microscopically, this variant of KS is characterized by
epidermal hyperplasia, hyperkeratosis and fibrotic protuberances with a verrucous pattern of the
skin surface. A transdermal increase in blood vessels, thick-walled lymphatic vessels, and
fibroblasts may also be seen. This KS-associated lymphedema may be the result of lymphatic
obstruction, venous obstruction, protein-rich interstitial fluid, tissue hemosiderin, and/or
subcutaneous infection.[963]
Immunohistochemical staining for endothelium-associated CD34 or CD31 antigens may
provide a useful marker for identification of both vascular and spindle cell components of KS
and help to confirm a diagnosis in some cases. The lymphatic endothelium does not contain
CD34, but does mark with D2-40. The endothelial nuclei are positive for HHV-8 latent nuclear
antigen 1 (LNA-1).
Factor VIII-related antigen will be found in the vascular portions of KS, but rarely in the
spindle cell components. All KS cells will demonstrate positivity with immunohistochemical