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staining for vimentin. However, the tumor cells of KS do not always show positive staining for
factor VIII-related antigen. Radiation or chemotherapeutic effect on KS may produce
involutional changes including loss of atypical spindle cells, absence of vascular spaces, fibrosis,
and extensive hemosiderin deposition. Flow cytometry of KS indicates that most are diploid, but
a few demonstrate DNA aneuploidy. Mitotic counts are higher in more advanced stages of
disease.[964] Using the polymerase chain reaction to detect human herpesvirus-8 (HHV-8) will
help in distinguishing the lesions of KS from other neoplastic spindle cell proliferations in
cytologic samples.[965]
The histologic appearances of KS can be helpful in assessment of prognosis in patients
with AIDS. The appearance of an initial lesion on the lower extremities, presence of spindle-cell
nodules, nodular form, absence of hemosiderin, and absence of irregular vascular spaces are all
associated with increased survival. Nodular KS is associated with a 30-month survival, while
patients with patch or plaque lesions survive for half this time or less. These findings are similar
to survival curves with classic and endemic KS.[966]
A diagnosis of KS in fine needle aspiration (FNA) cytology specimens can be
challenging. Cytologic features of KS seen in FNA specimens may include tissue fragments of
overlapping spindle cells, loosely cohesive clusters of spindle cells, individual cells, bare oval
nuclei with fine chromatin, prominent nucleoli, elongated cytoplasm with vacuoles, and
metachromatic background stroma on May-Grünwald-Giemsa (MGG) stain. Nodular spindle
cell vascular transformation as well as mycobacterial spindle cell pseudotumor seen in lymph
node can have similar features on FNA. Demonstrating the presence of HHV-8 may help to
distinguish KS from other spindle cell proliferations.[409]
Histologic variations of KS have been described. Lesions of KS may contain acid-fast
bacilli in patients infected with Mycobacterium avium-complex (MAC). These proliferations
must be distinguished from the uncommon “mycobacterial pseudotumor” that contains MAC-
infected macrophages forming a spindle cell proliferation. A fascicular arrangement of spindle
cells with slit-like spaces, lack of granular eosinophilic cytoplasm, and presence of mitoses are
features more consistent with KS. Additional KS patterns may include glomeruloid,
telangiectatic, ecchymotic, lymphangioma-like, verrucous, keloidal, micronodular, bullous,
myoid nodular, and pigmented. An anaplastic variant of KS is associated with a highly
aggressive course poorly responsive to therapy. The presence of CD31 and CD34 and the
absence of staining for CD68 and S100 by immunohistochemistry favors KS.[426,960,967,968]
Differentiation between granulation tissue and KS may be a diagnostic problem, although
the cells of the latter should show atypism. Non-neoplastic lesions that may partially mimic KS
include: bacillary (epithelioid) angiomatosis, capillary hemangioma, sclerosing hemangioma,
resolving dermal fasciitis, pyogenic granuloma, and papular angioplasia. Other vascular tumors
resembling KS include spindle cell hemangioma and Kaposiform hemangioendothelioma.
Other spindle cell neoplasms resembling KS include fibrohistiocytic tumors (cellular,
angiomatoid and atypical variants of fibrous histiocytoma) dermatofibrosarcoma protuberans,
spindle cell melanoma, cutaneous leiomyosarcoma, amelanotic melanoma, and spindle cell
squamous cell carcinoma.[960]
Antiretroviral therapy can be accompanied by regression of KS lesions. Features of
regression of lesions include flattening, reduction in size, and change from a purple-red
appearance to an orange-brown macule. Microscopic features include greater circumscription of
nodular lesions that are less cellular and are enveloped by a densely sclerotic stroma. KS lesions
that have undergone complete regression show an absence of spindled cells, a modest increase in