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In order to be effective in the treatment of disease, especially neoplastic disease, radiation necessarily needs to be given in large doses. When treating tumors with high radiosensitivity in tissue of relatively low radiosensitivity, few complications of therapy occur. Frequently, however, the differential of sensitivity to radiation between the neoplasm and normal tissues exposed is small and the incidence of radiation injury to normal tissues becomes significant. Nowhere is this more apparent than in treatment of neoplasms during which the alimentary canal is heavily radiated. The following discussion will describe the pathologic changes that occur in the various units of the exposed alimentary tract and in various components of those units during and especially months or years after radiation therapy. The usual rapid and effective regeneration of the rapid-renewal cell system after injury will be noted. The serious delayed manifestations of injury to the fibrovascular tissues with resulting delayed complications of ulcerations, stenoses, fistula formation, and obstructing adhesions will be emphasized. Brief discussions of several other organ systems injured by radiation will be presented.The development of second neoplasms as a result of radiation will be briefly mentioned and discussed from three standpoints: very low levels, low levels, and high levels of radiation.

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