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.
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.
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.
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.
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.
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).