TBI during BM and SCT: review of the past, discussion of the present and consideration of future directions.

TBI has been used widely in the setting of BMT over the past 3 decades. Early research demonstrated feasibility and efficacy in the myeloablative setting, in preparation first for allogenic BMT and later for autologous stem cell rescue. As experience with TBI increased, its dual roles of myeloablation and immunosuppression came to be recognized. Toxicity associated with myeloablative TBI remains significant, and this treatment is generally reserved for younger patients with excellent performance status. Reduced intensity conditioning regimens may be useful to provide immunosuppression for patients who are not candidates for myeloablative treatment. Efforts to reduce toxicity through protection of normal tissue using methods of normal tissue blocking and use of TLI, rather than TBI, continue. In the future, modalities such as helical tomotherapy, proton radiotherapy and radioimmunotherapy, may have roles in delivery of radiation to the BM and lymphoid structures with reduced normal tissue toxicity. With further investigation, these efforts may expand the therapeutic ratio associated with TBI, allowing safer delivery to a broader range of patients.