Intensity-Modulated Radiation Therapy, Etoposide, and Cyclophosphamide Followed By Donor Stem Cell Transplant in Treating Patients With Relapsed or Refractory Acute Lymphoblastic Leukemia or Acute Myeloid Leukemia

RATIONALE: Giving intensity modulated radiation therapy (IMRT) and chemotherapy, such as etoposide and cyclophosphamide, before a donor stem cell transplant helps stop the growth of cancer cells. It also helps stop the patient's immune system from rejecting the donor's stem cells. When the healthy stem cells from a donor are infused into the patient they may help the patient's bone marrow make stem cells, red blood cells, white blood cells, and platelets. Sometimes the transplanted cells from a donor can make an immune response against the body's normal cells. Giving IMRT together with chemotherapy before transplant may stop this from happening. PURPOSE: This phase I/II trial is studying the side effects and best dose of intensity-modulated radiation therapy (IMRT) when given together with etoposide and cyclophosphamide followed by donor stem cell transplant and to see how well they work in treating patients with relapsed or refractory acute lymphoblastic leukemia (ALL) or acute myeloid leukemia (AML).