Light micrograph section through human brain tissue showing a central nervous system (CNS) tumour. A CNS tumour begins when healthy cells within the brain or the spinal cord change and grow out of control, forming a mass. A cancerous tumour is malignant, which can grow and spread, whereas a benign tumour can grow but will not spread. A CNS tumour is particularly problematic because a person's thought processes and movement can become impaired. Tissues around the tumour, that are often vital to the body's functioning, can also be affected.