Total body irradiation (TBI) is often part of a transplant preparative regimen—treatment used to prepare a patient for stem cell transplant. TBI gives a dose of radiation to the whole body. It can destroy cancer cells throughout the body. It also destroys the immune system so that it will not attack the donor's cells during the transplant.

TBI can reach cancer cells within scar tissue or other areas of the body that chemotherapy may not reach (due to a blood/brain barrier, for example). However, the dose of radiation must be low enough that the body's healthy cells can recover. For this reason, TBI alone cannot be used to destroy large numbers of cancer cells. Instead, the transplant preparative regimen uses TBI along with high-dose chemotherapy. Some preparative regimens use only chemotherapy and do not include TBI.