Kaposi Sarcoma

Overview
  • Kaposi Sarcoma (KS) is a type of malignant vascular tumor with a complex epidemiology. The disease is found in four distinct populations and manifests in each as a slightly different clinical variant. Below we discuss variants of KS, theories as to their pathogenesis, and describe their common morphological features.
Variants
  • Classic KS:
    • Occurs in elderly men, mostly among those with Ashkenazi Jewish or Mediterranean descent. Tumors mostly localize to skin and subcutaneous tissue of the legs with slow progression proximally.
  • Endemic (African) KS:
    • Lesions generally tend to involve visceral structures and present with concomitant lymphadenopathy. Disease is highly aggressive and affects the same population as that prone to Burkitt Lymphoma.
  • Transplant-associated KS
    • Transplant-associated KS occurs in the context of severe iatrogenc immunosuppression following solid organ transplant. Lesions can be superficial or visceral and may metastasize widely.
  • AIDS-associated KS:
    • Lesions are observed in AIDS Patients and are usually disseminated widely throughout the body but patients usually die of another complication of AIDS.
Pathogenesis
  • The precise pathogenesis of KS is not well-understood; however, it appears that viral infection, especially with HHV-8 may play an etiological role. Titers to HHV-8 are detected in nearly 1-2% of the general population and thus infection with this virus combined with environmental or genetic factors present in the specific prone populations above may be responsible for the disease. In general, most of the proliferative cells seen in lesions are not neoplastic and represent reactive proliferations; however, late in the disease some of these cells may transform to a malignant phenotype.
Morphology
  • Overview
    • The lesion of KS usually progresses through three morphologically distinct stages. In Classic KS, progression is slow whereas in the other variants progression can be rapid.
  • Patch Stage
    • The lesions consist of irregular and dilated blood vessels with a mild infiltrate of mononuclear cells, resulting in red, superficial patches. These lesions are usually found in the skin and expand over time. Importantly, the patch stage can be distinguished from Bacillary Angiomatosis, which also occurs in AIDS Patients, since that has a more neutrophilic infiltrate.
  • Raised Plaque Stage
    • Lesion still contains irregular, dilated blood vessels but are now lined by 'spindle cell's which are of unknown origin but are probably derived from endothelial cells. At this point the proliferative spindle cells are likely not neoplastic but are prone to transformation
    • The mononuclear inflammatory infiltrate is still present and lesions form a raised plaque.
  • Nodular Phase
    • Lesions become a more homogeneous sheet of 'spindle cells' which begin looking more dysplastic. Some scattered small blood vessels can still be seen together with an interspersed mononuclear inflammatory infiltrate. Grossly, these lesions appear as nodules on the skin but at this point tumors can be observed in other organs and viscera.