Answer: Kaposi’s sarcoma

Histology: The surface epithelium is ulcerated. The submucosa is expanded by a a highly cellular proliferation of spindle cells arranged in ill-defined fascicles. The cells show minimal cytologic atypia, although the mitotic rate is very high. A CD31 immunostain (not shown) is positive in the spindle cells. This cellular background is admixed with extravasated red blood cells and small eosinophilic hyaline bodies that likely represent fragmented erythrocytes. Dilated vessels are noted at the periphery of the lesion.

Discussion: Kaposi’s sarcoma (KS) is proliferation of spindle cells showing endothelial differentiation. Its pathogenesis has long been in dispute, but it is now generally regarded as a malignant neoplasm as opposed to a multicentric hyperplastic process. The human herpes virus (HHV-8) is believed to be a necessary factor in disease development. Once regarded as an oddity that was rarely encountered in the oral cavity, an AIDS-related form of KS has resulted in a dramatic increase in oral KS among the population of HIV-infected individuals. The AIDS-associated form is by far the most likely subtype of KS to be encountered in the oral cavity. KS represents the most frequent oral malignancy seen in association with HIV infection, and it may be the first manifestation of this condition. In the HIV-infected population, oral KS is most commonly encountered in homosexual men with a peak incidence in the 4th decade. The palate is the most commonly involved intraoral site followed by the gingiva and dorsum of the tongue. Multiple oral lesions are common often with concurrent involvement of cutaneous sites and visceral organs.

KS is a non-encapsulated infiltrative lesion. The histologic picture varies with stage of progression. In advanced lesions, the histologic picture is dominated by a highly cellular proliferation of spindle cells separated by slit-like vascular spaces containing red blood cells. The differential diagnosis is contingent on the stage of histologic progression. Due to the subtly of microscopic alterations, early lesions may be easily dismissed as reactive vascular ectasia. As the proliferation of small irregular vessels become more apparent, KS may cause confusion with lobular capillary hemangioma (pyogenic granuloma), bacillary angiomatosis, and even well differentiated angiosarcoma. In advanced stages, the highly cellular proliferation of spindle cells may be mistaken for fibrosarcoma. Unlike KS, the spindle cells of fibrosarcoma are not immunoreactive for fVIII-AG, CD34, or CD31.