This trial is testing how well total body irradiation works as a way to prepare for hematopoietic cell transplantation in patients with myelodysplastic syndrome or acute leukemia. Total body irradiation can help lower the relapse rate, but there are some fatal side effects, like damage to normal organs that's irreversible, and graft-versus-host disease. With total body irradiation, radiation is directed at the entire body in an attempt to suppress the immune system, prevent rejection of the transplanted bone marrow and/or stem cells, and wipe out any remaining cancer cells. Intensity-modulated radiation therapy is a