Iodine I 131 Monoclonal Antibody 3F8 in Treating Patients With Central Nervous System Cancer or Leptomeningeal Cancer

Inclusion Criteria: * Patients must have a histologically confirmed diagnosis of a malignancy known to expressGD2. Such tumors include medulloblastoma/primitive neuroectodermal tumor of the CNS, high grade astrocytomas, malignant glioma, neuroblastoma, retinoblastoma, ependymoma, rhabdoid tumors, sarcomas, melanoma or small cell lung carcinoma. For patients with other tumor types, GD2 expression must be confirmed by immunohistochemical staining and assessed by the Department of Pathology using prior frozen tissue, bone marrow or CSF cytology (send to Research Lab). * Patients must have CNS/ leptomeningeal disease including high risk medulloblastoma, or a CNS/leptomeningeal malignancy which is refractory to conventional therapies, or for which no conventional therapy exists, OR a recurrent brain tumors with a predilection for leptomeningeal dissemination (medulloblastoma, PNET, rhabdoid tumor). * Patients must have an absolute neutrophil count (ANC) \> 1000/ul and a platelet count \> 50,000/ul. * Patients may have active malignancy outside the central nervous system. * Patients who have a programmable shunt will not be excluded. * Both pediatric and adult patients of any age are eligible. * Patients or a legal guardian will sign an informed consent form approved by the IRB and obtained by the Principal or a Co- Investigator before patient entry. Minors will provide assent. Exclusion Criteria: * Patients with obstructive or symptomatic communicating hydrocephalus. * Patients with an uncontrolled life-threatening infection. * Patients who are pregnant: Pregnant women are excluded for fear of danger to the fetus. Therefore negative pregnancy test is required for all women of child-bearing age, and appropriate contraception is required during the study period. * Patients who have received cranial or spinal irradiation less than 3 weeks prior to the start of this protocol. * Patients who have received systemic chemotherapy (corticosteroids not included) less than 3 weeks prior to the start of this protocol. * Severe major organ toxicity. Specifically, renal, cardiac, hepatic, pulmonary, and gastrointestinal system toxicity should all be less than or equal to grade 2. Patients with stable neurological deficits (because of their brain tumor) are not excluded. Patients with \<= 3 hearing loss are not excluded. * Patients must have no rapidly progressing or deteriorating neurologic examination. * Patients who have already received \>45 Gy to the craniospinal radiation or \>72 Gy focal brain radiation.