Anatomic Pathology: Skin Pathology

• Kaposi sarcoma can be classified into four clinical subtypes: classic, African (endemic), AIDS-associated (epidemic), and a variant associated with immunosuppressive therapy.

• HHV-8, originally known as Kaposi sarcoma–associated herpesvirus, has been demonstrated in all types of Kaposi sarcoma and is considered to be the etiologic agent in Kaposi sarcoma.

• Classic Kaposi sarcoma most commonly occurs in men of eastern European or Mediterranean origin and manifests as slowly developing vascular nodules on the lower extremities. AIDS-associated Kaposi sarcoma differs from the classic type in that it more commonly demonstrates rapid progression; visceral involvement; and an atypical distribution affecting the trunk, arms, and mucosa.

• The characteristic histopathologic examination of Kaposi sarcoma demonstrates a dermal proliferation of abnormal vessels with “jagged” borders that tend to separate collagen bundles. Normal adnexal structures and preexisting blood vessels often protrude into newly formed abnormal vessels, which is termed the promontory sign. Plasma cells, extravasated erythrocytes, and hemosiderin deposition are also typically found. The nodular stage of Kaposi sarcoma demonstrates a well-defined nodule composed of vascular spaces and spindle cells that replace the dermal collagen. The vascular lumina lie closely together in a “back-to-back” arrangement.

• Immunohistochemical demonstration of HHV-8 nuclear staining is the most useful tool in the diagnosis of Kaposi sarcoma and in distinguishing it from other vascular tumors. Other vascular markers that may be positive in Kaposi sarcoma but are not as specific are CD31, CD34, D2-40, Ulex europaeus, and factor XIIIa.