204 ATOMIC MEDICINE dose absorbed by the tissue. At relatively recent meetings (1956-1962) of the International Commission on Radiological Units ICRU), while it was stated that the roentgen (svmbol r) be retained as the unit of measurement of gamma and x-radiation, it was recommended that when ever feasible “the dose be expressed in terms of the quantity of energy absorbed per unit mass (ergs per gram) of irradiated material at the place of interest.” The unit is the ‘rad, 100 ergs per gram, which replaced the older “rep”? which could be assigned values ranging from approximately &0 to 100 ergs per gram. The rad is not readily determined for gammaorx-radia- tion; however, tables for its estimation from exposure dose in tissue are providedin reports of the Commission. The conditions under which the exposure dose is measured shotild be designated as free in air, at skin surface, or at x-centimeters depth, and the data such as seatter, half-value layer, target-to-skin distance, and KVP =hould be included. In commenting on the Commission’s recommendations, Failla stated that ‘no physical unit canfulfill the ideal requirements of making all biological effects of ionizing radiation appear independent of wave length or more generally, independ- ent of specific ionization,” and that ‘tin the present state of our knowledge no chemical or biological unit can fulfill the ideal requirement either.” For the purposes of correlating dose with effects on the whole body, depth dose and distribution of the absorbed energy becomes of great importance, as shown by Tullis ef al. and Bond et al. Further, no single physical unit can satisfactorily characterize the total dose because of differences in relative absorption by different tissues. It is essential to attempt quantification of the biological response, as well as the physically measured dose, when trying to correlate effect with dose. The biological measure may include the LDs9 and the slope of the dose mortality curve, asin all toxicological studies, Other biological measurements of dose are xplenic-thymic weight decrease (Carter ef al.), body weight (Chapman and Jerome), gut weight (Conard), the hematological response, survival time, iron uptake by erythrocytes (Hennesseyet al.) and histological changes. Quantitative measures of biological effect useful in mammalian radiology have heen reviewed byStorer ef al. 9.2 Classification of Radiation Injuries Radiation injuries can be divided into two general categories, early and late injuries. The carly type results from brief intense exposure; the late type either from exposure to large single doses, or from prolonged exposures of lower intensity. The following is an outline of the two categories, only ar i the first of which is dealt with extensively in this chapter: 9.2.1. The Barly Injurtes Produced by Brief Exposure to Large Doses of Radiation.