Kaposi’s sarcoma is a rare malignancy of endothelial cells seen in unique settings. The classic variant is seen in older patients, most commonly individuals living in the region surrounding the Mediterranean Sea. Kaposi’s sarcoma associated with human immunodeficiency virus (HIV) infection or with acquired immunodeficiency syndrome (AIDS) is seen predominantly in men, and the tumor is thought to be caused by human herpesvirus-8 (HHV8). There is also a variant seen in chronically immunosuppressed patients, such as those who have undergone solid organ transplantation. The African cutaneous variant of Kaposi’s sarcoma is seen in younger men in their third or fourth decade of life. Kaposi’s sarcoma is a locally aggressive tumor that rarely has a fatal outcome. The one exception is the very rare African lymphadenopathic form of Kaposi’s sarcoma, which is distinct from the more common African cutaneous form.
Clinical Findings: The tumors are very similar in appearance across the subtypes of clinical settings. They usually appear as pink-red to purple macules, papules, plaques, or nodules. In the classic form of Kaposi’s sarcoma, the tumors are most often found on the lower extremities of older men. Some tumors in this setting remain unchanged for years, and the patient often dies of other causes. Occasionally, the tumors grow and ulcerate, causing pain and bleeding. The disseminated form of classic Kaposi’s sarcoma can be very aggressive, and patients require systemic chemotherapy.
AIDS-associated Kaposi’s sarcoma is the most common form of the disease. It is most often seen in younger men. In comparison with the classic form, this form usually manifests as purple macules, plaques, and nodules on the head and neck, trunk, and upper extremities. This is an AIDS-defining illness. Patients with AIDS-associated Kaposi’s sarcoma are at a higher risk for internal organ involvement. The small bowel has been reported to be the internal organ most commonly affected by Kaposi’s sarcoma, but it can affect any organ system. Since the advent of multiple-drug therapy for HIV infection, the incidence of AIDS-associated Kaposi’s sarcoma has decreased dramatically.
Tropical African cutaneous Kaposi’s sarcoma is most often seen in younger men. The clinical findings are not much different from those of the classic form of Kaposi’s sarcoma. These patients are much more likely to suffer from severe lower-extremity edema. The tumor also has a higher incidence of bone invasion than the other types. The main difference between the classic and the African forms of Kaposi’s sarcoma is the age at onset. The aggressive form of African Kaposi’s sarcoma occurs in childhood and is often fatal because of its aggressive ability to metastasize. The lymph nodes are often involved before the skin is. The reason the African forms act so differently from each other is poorly understood.
Pathogenesis: The pathogenesis of the classic and African forms of Kaposi’s sarcoma is unknown. The cell of origin of this tumor is believed to be the endothelial cell. Matrix metalloproteinases 2 and 9 have been shown to increase angiogenesis and increase the tissue invasion of the affected endothelial cells. Kaposi’s sarcoma associated with AIDS or other immunosuppressive states is believed to be caused by the action of HHV8 in a genetically predisposed individual. HHV8 is thought to cause dysregulation of the immune response in the afflicted endothelial cells, allowing them to proliferate uncontrolled by normal immune functions.
Histology: Biopsies of Kaposi’s sarcoma show many characteristic findings. The promontory sign is often seen; it is represented by plump endothelial cells jutting into the lumen of the capillary vessel. Many slit-like spaces are also seen. These spaces represent poorly formed blood vessels, which are thin walled and easily compressed. They are filled with red blood cells. The tumor in general is very vascular, with a predominance of vascular spaces and a large amount of red blood cell extravasation into the dermis.
Treatment: For classic Kaposi’s sarcoma, the mainstay of therapy has been localized radiation treatment. Many other treatments have been advocated, including topical alitretinoin, imiquimod, intralesional vincristine, and interferon. Systemic chemotherapy for disseminated and aggressive forms is indicated and is usually based on a regimen of either vinblastine, paclitaxel, bleomycin, or pegylated liposomal doxorubicin.