Biological and technical principles of total body irradiation for bone marrow graft conditioning.

The aims of total body irradiation, namely immunologic suppression of the hematopoietic tissues and eventually eradication of leukemia, require a large dose which may cause detrimental effects on healthy tissues. The main problem is raised by the interstitial pneumonitis and a relative protection of the lung is advisable. A partial reduction of the dose to the lung may cause an important increase of the number of surviving cells in the cell population the irradiation is aiming aiming at. Technical problems are raised to keep this increase to an acceptable value. A relative protection of the critical tissues can be achieved by a convenient time distribution of the dose, since the repair of sublethal injuries taking place between the sessions of a fractionated irradiation or during the course of low dose rate irradiation is larger for intestine, lung..., than for bone marrow and probably leukemic cells. Animal experiments have provided radiobiological data for estimating the therapeutic benefit related to the fraction number or to the dose rate.