Patients whose test results indicate a high risk for PCNSL have a CT scan to look for evidence of lymphoma elsewhere in the body and are referred for consultation with a neurosurgeon to discuss undergoing a brain biopsy. The brain biopsy is done in the operating room under general anesthesia. A small cut is made in the scalp and a small opening is made in the skull over the area of the brain to be biopsied. A needle is placed in the opening in the skull and, guided by CT or MRI, moved to the abnormal area of the brain, where a small piece of tissue is removed for study under a microscope.