RATIONALE: Giving chemotherapy drugs, such as busulfan and etoposide, and intensity-modulated radiation therapy 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 intensity-modulated radiation therapy together with busulfan and etoposide before a transplant may stop this from happening.