The central nervous system (CNS) is made up of the brain and spinal cord. A lymphoma is a cancerous tumour of the lymph cells, which are part of the body's immune system and help to fight infection.
Some lymphomas in the head will have spread from lymphomas in other parts of the body. Lymphomas may also start in the brain and are called primary CNS lymphomas. Most of those affecting the brain are high-grade non-Hodgkin lymphomas. The most common site for a CNS lymphoma is the cerebrum, but it can also spread to other parts of the brain.
It is common for more than one tumour to be present.