These lymphomas are often linked with lymphomas of the brain, which are known as primary central nervous system (CNS) lymphomas. For more detailed information on the treatment of CNS lymphomas, see Non-Hodgkin Lymphoma. Because these cancers are rare, they have been hard to study. Most often, doctors treat these cancers with external radiation therapy, chemotherapy (chemo), or a combination of the two. The radiation therapy may be given only to the eye, or it may also include the brain and spinal cord. But it can also cause side effects, leading to problems with thinking, concentration, and memory. It can also be given directly into the eye (intraocular chemo), which gets higher doses of the drug to the tumor. Methotrexate is usually the main chemo drug used.