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Kaposi sarcoma

What is Kaposi sarcoma?

Kaposi Sarcoma (KS) is a type of cancer made up of abnormal blood vessels. It is part of a group of cancers called sarcomas. Most of these tumours start in the skin and it is not uncommon for a person to have more than one tumour. Kaposi sarcoma can also develop inside the mouth and anywhere along the digestive tract including the small intestines, colon, rectum, and anus. Over time the tumour cells can spread to lymph nodes and the lungs.

What causes Kaposi sarcoma?

Kaposi sarcoma is caused by a virus called human herpesvirus-8 (HHV8). We do not fully understand the connection between HHV8 and Kaposi sarcoma, but it seems likely that both a weakened immune system and the virus contribute to the disease. Importantly, most people who are infected with HHV8 will not develop Kaposi sarcoma.

Types of Kaposi sarcoma

There are four types of Kaposi sarcoma called classic, iatrogenic, African, and AIDS-related. The type depends on the setting in which the disease develops.

  • Classic type – This is the most common type of Kaposi sarcoma. These tumours typically develop in older men of Mediterranean or Central/Eastern European ancestry. Most patients with classic Kaposi sarcoma develop multiple red or purple tumours on their feet or the lower part of their legs. The tumours often grow slowly at first and many patients do not seek medical care for months or years.
  • Iatrogenic type – This type of Kaposi sarcoma occurs in patients who have a weakened immune system (immunosuppressed) often due to medical treatment such as chemotherapy or anti-rejection medications following organ transplantation. For some patients with iatrogenic type Kaposi sarcoma, the tumours disappear once the normal function of the immune system has been restored. Regression is a word pathologists use to describe a tumour that decreases in size or disappears without treatment.
  • African type – This type of Kaposi sarcoma tends to occur in young children and middle-aged adults in equatorial Africa, and is known to develop quickly. Patients develop skin tumours that very quickly spread to lymph nodes and other organs.  African KS and is not associated with HIV infection. Due to its aggressive clinical nature, this form of KS is often fatal.
  • AIDS-related type – This type of Kaposi sarcoma is the most aggressive type because it quickly spreads to involve the mouth, throat, stomach, bowel, lymph nodes, and other organs. The tumour typically starts on the face, genitals, or on the lower extremities including the feet. Unlike the other types of Kaposi sarcoma, this type almost always develops in a person infected with both HIV and HHV8. Restoration of normal immune system function improves outcome and survival for patients with AIDS-related Kaposi sarcoma.

Kaposi sarcoma

How do pathologists make this diagnosis?

The diagnosis is typically made after a small tissue sample is removed in a procedure called a biopsy. A second surgical procedure is usually then performed to remove the entire tumour. The diagnosis can only be made after a piece of the tumour is examined under the microscope by a pathologist.

Pathologists describe Kaposi sarcoma as a spindle cell lesion because most of the tumour cells are long and thin. The tumour cells are often organized in long bundles which pathologists describe as a fascicular pattern. When the skin is involved, the tumour almost always is found in the dermis, a layer of tissue just below the surface of the skin.

The tumour cells in Kaposi sarcoma connect to form small abnormal-looking blood vessels. For this reason, many pathologists believe that the tumour cells are related to the cells that normally line the inside of blood vessels. These cells are called endothelial cells.

As the tumour grows, the abnormal blood vessels in Kaposi sarcoma merge to create larger blood-filled spaces (see picture below). Red blood cells commonly spill out of these spaces into the surrounding tissue. Pathologists describe these red blood cells as extravasated because they are outside the blood vessel. Hemosiderin, a dark pigment leftover after red blood cells break down, may also be seen.

Kaposi sarcoma

Human herpesvirus-8

Most pathologists will perform a test called immunohistochemistry to look for HHV8 in the tumour cells. Tumour cells that produce HHV8 will be described as positive while those that do not will be described as negative. The image below shows a tumour that is positive for HHV8 by immunohistochemistry. The brown cells are producing HHV8.

Immunohistochemistry HHV8


A margin is the normal tissue that surrounds a tumour and is removed with the tumour at the time of surgery. If no cancer cells are seen at the cut edge of the tissue the margin is called negative. If cancer cells are seen at the very edge of the cut tissue the margin is called positive. A positive margin is associated with an increased risk that the tumour will come back in the same location after surgery (local recurrence).


by Brian A Keller MD PhD and Bibianna Purgina MD FRCPC (updated September 13, 2021)
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