Fig. 42.2. The histology of the hemangiosarcoma of Kaposi. Multiple spindle-shaped cells and reticulin fibers suggest a fibrosarcoma. The spindle cells interweave and sweep in bundles separated by vascular slits.

The geographic distribution of the disease in malarial areas is very similar to that of Burkitt's lymphoma, and the predilection of the Kaposi sarcoma in the HIV-negative for the lower extremities is unusual in any form of malignancy. It is also strange for any malignant neoplasm to undergo spontaneous regression. A herpes-type virus was isolated from Kaposi tissue in 1972 and, later, antibodies to human herpes virus 8 (HHV8) have been found in Kaposi sarcoma patients everywhere, with or without AIDS. Herpes 8, now called Kaposi Sarcoma-associated Herpes Virus (KSHV), is the principal causative factor in the development of Kaposi sarcoma.. The occurrence of Kaposi sarcoma in patients on immunosuppressive therapy may also be significant in view of the frequency of the sarcoma in areas where there is chronic immunodeficiency from malaria. HIV-negative African patients who suffer from Kaposi's sarcoma have been shown to have impaired immunity.

It must be emphasized that the histology is identical whether the disease is seen in the tropics or the temperate climates and with or without AIDS (Fig. 42.2). The majority of lesions are multiple and perivascular in distribution, occurring along the course of the veins. The larger nodules may have foci of central necrosis. Some tumors reach a considerable size, 10 cm (4 inches) or more across, and are ulcerating and necrotic, with secondary infection. The majority are small subcutaneous nodules, distributed over the limbs, particularly the legs. Histologically, they are composed of vascular slits with free anastomoses; the space is filled by well-differentiated spindle-shaped cells and reticulin fibers. The histological appearance is often confused by concomitant inflammatory or granulomatous reaction. The nodules are in direct continuity with supplying arterioles and draining venules, and there is direct connection with well-formed, dilated lymphatics (these findings may be demonstrated radiologically).