 I. To evaluate the maximum tolerated dose MTD and the transplant-related mortality TRM and toxicity of delivering 131I-BC8 iodine I 131 monoclonal antibody BC8 anti-cluster of differentiation CD45 antibody at a starting dose of 22 Gy to the normal organ receiving the highest dose in combination with the non-myeloablative regimen of fludarabine fludarabine phosphate FLU, 2 Gy total body irradiation TBI, cyclosporine CSP, mycophenolate mofetil MMF, and human leukocyte antigen HLA-matched related or unrelated allogeneic hematopoietic stem cell transplant HSCT in patients 16 to 50 years old who have advanced acute myeloid leukemia AML or high risk myelodysplastic syndrome MDS. 