From The National Cancer Institute: http://www.cancer.gov
Lymphoma is a disease in which malignant (cancer) cells form in the lymph system. The lymph system is part of the immune system and is made up of the lymph, lymph vessels, lymph nodes, spleen, thymus, tonsils, and bone marrow. Lymphocytes (carried in the lymph) travel in and out of the central nervous system (CNS). It is thought that some of these lymphocytes become malignant and cause lymphoma to form in the CNS. Primary CNS lymphoma can start in the brain, spinal cord, or meninges (the layers that form the outer covering of the brain). Because the eye is so close to the brain, primary CNS lymphoma can also start in the eye (called ocular lymphoma).
From Wikipedia: http://en.wikipedia.org/wiki/Primary_central_nervous_system_lymphoma
A primary central nervous system lymphoma (PCNSL), also known as microglioma and primary brain lymphoma,  is a primary intra cranial tumor appearing mostly in patients with severe immunosuppression (typically patients with AIDS). PCNSLs represent around 20% of all cases of lymphomas in HIV infections (other types are Britt's lymphomas and immunoblastic lymphomas). Primary CNS lymphoma is highly associated with Epstein-Barr virus (EBV) infection (> 90%) in immunodeficient patients (such as those with AIDS and those iatrogenically immunosuppressed), and does not have a predilection for any particular age group. Mean CD4+ count at time of diagnosis is ~50/uL. In immunocompromised patients, prognosis is usually poor. In immunocompetent patients (that is, patients who do not have AIDS or some other immunodeficiency), there is rarely an association with EBV infection or other DNA viruses. In the immunocompetent population, PCNSLs typically appear in older patients in their 50's and 60's. Importantly, the incidence of PCNSL in the immunocompetent population has been reported to have increased more than 10-fold from 2.5 cases to 30 cases per 10 million population. The cause for the increase in incidence of this disease in the immunocompetent population is unknown.
American Uveitis Society
Primary central nervous system (CNS) lymphoma is a rare cancer that involves the central nervous system (brain, spinal cord, one or both eyes, and/or the coverings of the brain and optic nerve, also known as the meninges). The designation as a "lymphoma" reflects the fact that the cancerous cells are lymphocytes, a type of white blood cell. Primary CNS lymphoma affects all age groups, but is most commonly diagnosed in persons who are over 50 years of age. In addition, individuals who are immunosuppressed (have reduced functioning of the immune system) such as patients with AIDS or those taking certain drugs after organ transplantation, appear to be at increased risk.
I consider Primary Central nervous system lymphoma an orphan cancer. Lymphoma is a cancer that begins in the lymphocytes of the immune system. It is considered a blood based cancer even though it arises from the lymphatic system. http://en.wikipedia.org/wiki/Lymphatic system). Even stranger, the brain does not receive any direct support from the lymphatic system as it is protected from a system called the "Blood-Brain barrier" which works much like a baby's placenta to keep unwanted bacteria and toxins out of the brain. http://en.wikipedia.org/wiki/Blood-brain barrier, unlike other brain tumors, because of its diffuse nature, surgery is not often considered an option for CNS lymphoma. The treatment methods used for other forms of lymphoma don’t work well for CNS lymphoma.
There are, however, treatment protocols available today that are effective in treating CNS Lymphoma. The standard "core" treatment involves "High Dose Methotrexate chemotherapy often in conjunction with various other agents: Vinchristine, Procarbazine, Temodar, and Cytarabine. The high dose delivery often requires in-hospital treatment that lets the eight hour infusion do its thing, keeping the patient in hospital a total of 24 hours or more. Chemotherapy may be followed by radiation focused directly on the tumor(s) or delivered across the whole brain. Recent advancements in treatments include the use of biologically engineered chemotherapy agents and tightly focused radiation delivered in a "3D" manner with many points of radiation from different angles delivered to a single point inside the brain.
Effective treatments for PCNSL DO exist TODAY!