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Total body irradiation: the lung as critical organ.

The pathogenesis of interstitial pneumonitis (IP) after chemoradiotherapy and subsequent bone marrow transplantation is not completely understood. Total body irradiation (TBI) with a dose of about 10 Gy significantly contributes to this very serious complication. The radiation induced morphological alterations especially of the crucial targets, namely the pneumocytes type II and capillaries are described. Investigations in animals and in humans reveal that pneumonitis occurs over a very short range of doses. In humans a single dose of 9.3 Gy given at a high dose rate leads to an incidence of 50%. Reduction of the dose by shielding the lungs, fractionation of irradiation and decreasing the dose rate are considered to be the main possibilities which diminish the risk of pneumonitis after TBI. The role of further factors involved in the development of IP such as cytomegalovirus, age, reduced lung volume, immunological mechanisms and others is discussed. Especially chemotherapeutic agents may increase lung morbidity by interacting with TBI.