211At-BC8-B10 Followed by Donor Stem Cell Transplant in Treating Patients With Relapsed or Refractory High-Risk Acute Leukemia or Myelodysplastic Syndrome

OUTLINE: This is a dose-escalation study of astatine At 211 anti-CD45 monoclonal antibody BC8-B10. PREPARATIVE REGIMEN: Patients receive astatine At 211 anti-CD45 monoclonal antibody BC8-B10 infusion over 6-8 hours on day -8, fludarabine intravenously (IV) over 30 minutes on days -6 to -2, and cyclophosphamide IV over 1 hour on days -6 and -5. Patients also undergo TBI on day -1. TRANSPLANT: Patients undergo peripheral blood stem cell (PBSC) or bone marrow transplant on day 0. GVHD PROPHYLAXIS: Patients receive cyclophosphamide IV over 1-2 hours on days 3-4, mycophenolate mofetil IV or PO three times daily (TID) on days 5-35, and tacrolimus IV over 1-2 hours (changed to PO once tolerated) on days 5-180 with taper beginning on day 84 per physician discretion. Patients also begin granulocyte colony-stimulating factor (G-CSF) IV or subcutaneously (SC) on day 5 to continue until absolute neutrophil count (ANC) \> 1000/mm\^3 x 3 days. Patients undergo bone marrow biopsy and aspiration and blood sample collection throughout the study. After completion of study treatment, patients are followed up at day 100, and at 6, 9, 12, 18, and 24 months.