Lymphoma is a cancer that forms in the lymphocytes of the lymph system. The lymph system includes the lymphocytes, lymph nodes, lymph vessels and other organs. The lymphocytes circulate through the bloodstream and lymph system to all the tissues in the body. The central nervous system, which is comprised of the brain and spinal cord, receives signals and controls responses for the body.

Central nervous system (CNS) lymphoma is a type of cancer that occurs in the lymphoid tissue in the brain, spinal cord or membranes around the brain and spinal cord (called the meninges). CNS lymphoma should not be confused with cancers that start elsewhere in the body, such as the breast, and metastasize (spread) to the lymphatic system.

When cancerous lymphocytes travel in and out of the central nervous system through the bloodstream and the lymph system, they may cause malignant nerve cells to grow. CNS lymphoma rarely spreads outside of the central nervous system, but the cancer can spread to the eyes, because of their close proximity to the brain. In this case, it is called ocular lymphoma.

CNS lymphoma is a type of non-Hodgkin’s lymphoma. Unlike Hodgkin's lymphoma, which tends to spread from one area to a surrounding area, non-Hodgkin's lymphoma does not spread in a consistent manner. Also, though it originates in lymphocytes, it may be considered to be a type of brain tumor.

CNSL (also known as primary brain lymphoma) is relatively rare, occurring in about 3.5 percent of primary brain tumors, but the number of cases is rising. CNS lymphoma can affect all age groups; however, it mostly occurs in adults.

The incidence of CNS lymphoma has increased in transplant patients and in non-immune-comprised populations for unknown reasons. In these patients, it is slightly more common in men than women. It has decreased among high-risk AIDS patients in the last decade, possibly because of more effective use of immune system medications. Among AIDS patients with CNS lymphoma, a large majority of the patients are male.

Although the prognosis is generally poor for all patients with CNSL, there have been recent advances in chemotherapy and radiation therapy treatments. About 30 percent of people diagnosed of CNS lymphoma can live at least 5 years, according to the American Cancer Society. Prognosis is slightly poorer for patients with AIDS because of additional complications.