Total body irradiation (TBI) in conjunction with high dose chemotherapy is part of the preparative regimen used to ready a patient for a bone marrow transplant. It is used to treat patients with leukemias, lymphomas and other hematologic conditions. The entire body is irradiated by a beam of high-energy photons from a linear accelerator in order to eradicate any remaining cancer cells and cause depression of the immune system (immunosuppression). This is required to help reduce the risk of transplant rejection. Even though the entire body is irradiated, one uses custom fabricated blocks of "cerrobend" to shield the lungs and prevent normal tissue damage. The treatment is typically done once a day, but may be modified by the physician if the need arises.