Background: Kaposi s sarcoma herpes virus (KSHV) causes several kinds of cancer, Kaposi sarcoma (KS), a form of Multicentric Castleman s Disease (MCD) and a type of lymphoma known as Primary Effusion Lymphoma (PEL). These cancers can occur alone or at the same time in the same patient. MCD can cause a lot of symptoms and problems with various organs in the body, making patients feel quite unwell. If unrecognized, the disease can be fatal. Medications such as rituximab alone or in combination with chemotherapy may help treat MCD but there is little known about the long term effects and the natural course of MCD.

Objective: To better understand the biology of KSHV-MCD to help identify how this disease causes illness and how cancer treatments known to be effective in MCD may help patients with this condition. This study also aims to help identify ways to treat the disease by providing other standard cancer treatments that would be useful to use to treat MCD based on what we know about this condition.

Eligibility: People 18 years of age and older with KSHV-MCD.

Design: Participants will be screened with: Medical history Physical exam CT scan Blood and heart tests Participants will have an initial evaluation. This will include: Review of participants symptoms and ability to perform their normal activities Blood and urine tests Imaging studies such as CT and PET scans. Participants may have a contrast agent injected into their arm. Photographs to document skin lesions Optional skin biopsy. For this, a small piece of the skin will be removed. Optional lymph node needle biopsy Optional samples of the fluid in the space around the lungs, intestines, or heart Optional sample of the liquid that surrounds the brain and spinal cord Saliva samples DEXA scan to examine the bones Questionnaires Optional limb measurements or cognitive tests Physicians will give participants recommendations about treatment. After their initial evaluation and any treatment, participants will have additional visits. These will occur every 3 months for the first year, then every 6 months for the second year, and then once a year for up to 1 year.