A lumbar puncture tests whether there are any cancer cells in the fluid surrounding our brain and spinal cord (cerebrospinal fluid). The lumbar puncture is so called because an insertion is made in the lumbar region of the back (the lower back) by a small hollow needle. The needle goes through the skin and in between the vertebrae until it enters the space that contains the spinal fluid. The needle collects a sample of the fluid and is withdrawn. The fluid is then tested in a laboratory.