Kaposi Sarcoma, from the CureRays guide library.

Kaposi sarcoma is a cancer of blood-vessel-lining cells caused by a herpes virus, appearing as skin lesions and sometimes affecting internal organs; the most important treatment is restoring the immune system, and the tumor itself is very sensitive to radiation.

What it is.

Kaposi sarcoma is an unusual cancer that develops from the cells lining blood and lymph vessels. It is caused by a virus — human herpesvirus 8 (HHV-8), also called Kaposi sarcoma-associated herpesvirus — which most people's immune systems keep in check. Kaposi sarcoma typically appears when the immune system is weakened, so it tends to occur in people living with HIV/AIDS, in those taking medicines that suppress the immune system after an organ transplant, and, in certain populations and older adults, in a slower-growing form. The cancer most often shows up on the skin as purple, red, or brown patches, plaques, or nodules, commonly on the legs, feet, or face, and inside the mouth. It can also involve lymph nodes and internal organs such as the lungs and digestive tract, where it can cause more serious problems like bleeding or breathing difficulty. A defining feature of Kaposi sarcoma is how closely it is tied to the immune system: in many cases, restoring immune function — by treating HIV effectively or adjusting transplant medicines — causes the lesions to shrink or disappear, which is why managing the underlying cause is the foundation of treatment. When lesions need direct treatment, Kaposi sarcoma is notably sensitive to radiation, and other options include local therapies and, for widespread disease, chemotherapy.

How radiation treats it.

Radiation therapy uses focused high-energy beams to damage the DNA inside tumor cells so they can no longer grow and divide. Kaposi sarcoma is one of the more radiosensitive cancers, meaning its lesions respond readily to relatively modest doses of radiation — which makes radiation a valuable and reliable tool for treating individual or grouped lesions. It is especially useful for spots that are painful, bleeding, cosmetically distressing, or interfering with function, such as lesions on the feet that make walking difficult or in the mouth that cause discomfort. Because most Kaposi sarcoma lesions sit in or just under the skin, doctors often use electron beams, which deliver their dose to the skin's depth and then stop, treating the lesion while protecting the deeper, healthy tissue beneath. A focused course typically causes lesions to flatten, fade in color, and stop causing symptoms. It is important to understand that radiation treats the lesions it is aimed at but does not address the underlying cause — the virus held in check by the immune system — which is why radiation is almost always combined with the most important step of all: restoring immune function by treating HIV effectively or adjusting transplant medicines. For widespread disease or internal-organ involvement, chemotherapy works throughout the body, while radiation remains the go-to for controlling specific problem lesions. As with all radiation, treatment is planned to relieve symptoms and control disease while sparing healthy skin and tissue.

The ways we can treat it.

Localized external-beam radiation to skin lesions. Because Kaposi sarcoma is highly radiosensitive, a focused, often short course of radiation to a lesion or group of lesions reliably causes them to flatten, fade, and stop causing symptoms, while sparing surrounding skin.

Superficial electron-beam radiation. Electron beams deposit their energy in the skin and stop, making them well suited to treating skin lesions while protecting the deeper tissue beneath.

Palliative radiation. Radiation quickly relieves pain, bleeding, or swelling from troublesome lesions — for example in the mouth, on the feet, or where lesions interfere with daily function.

Radiation for lymph node or localized internal disease. Focused radiation can be directed at enlarged, symptomatic lymph nodes or a localized internal site to control disease and relieve symptoms, complementing systemic treatment.

Questions we hear often.

Why does treating my immune system help the cancer? Kaposi sarcoma is caused by a virus that a healthy immune system normally keeps under control. When the immune system is weakened — by HIV or by transplant medicines — the virus can drive tumor growth. Restoring immune function with effective HIV therapy, or adjusting transplant medicines when safe, often causes the lesions to shrink or disappear, which is why it is the cornerstone of treatment.

How well does radiation work for Kaposi sarcoma? Very well for the lesions it targets. Kaposi sarcoma is highly sensitive to radiation, so a focused, often short course reliably flattens and fades lesions and relieves pain or bleeding, while sparing the surrounding skin. It is a go-to option for individual problem spots, used alongside treating the underlying immune cause.

Is Kaposi sarcoma contagious? The cancer itself is not contagious. The virus that causes it can be passed between people, but most people who carry the virus never develop Kaposi sarcoma because a healthy immune system keeps it in check. The disease typically appears only when the immune system is significantly weakened.

This guide is informational only. It is not medical advice — please confirm anything here with your care team.