Bone Marrow Transplantation/adverse effects; Dementia/etiology; Demyelinating Diseases/etiology; Parkinson Disease, Secondary/etiology; Adolescent; Anemia, Aplastic/complications; Anemia, Aplastic/surgery; Anemia, Aplastic/therapy; Antineoplastic Combined Chemotherapy Protocols/therapeutic use; Biopsy; Bone Marrow Purging; Child; Combined Modality Therapy; Demyelinating Diseases/cerebrospinal fluid; Demyelinating Diseases/drug therapy; Demyelinating Diseases/pathology; Female; Gliosis/cerebrospinal fluid; Gliosis/etiology; Gliosis/pathology; Humans; Immunoglobulins, Intravenous/therapeutic use; Leukemia/complications; Leukemia/drug therapy; Leukemia/surgery; Magnetic Resonance Imaging; Male; Methylprednisolone/therapeutic use; Parkinson Disease, Secondary/cerebrospinal fluid; Parkinson Disease, Secondary/drug therapy; Parkinson Disease, Secondary/pathology; Transplantation, Autologous; Transplantation, Homologous