l u1 Lv a EFFECTS OF EXPOSURE GEOMETRY therapy, and the recent trend toward following these recommendations has aided in eliminating apparent discrepancies in quantitative comparisons of small animal radiation data. With large animals and man, however, added problems enter that make the situation more difficult. In the first place, under many practical exposure conditions, only fhe monitored air dose will be available or readily calculable at the time. Thus, in monitored exposures or in radiation accidents around reactors or other nuclear machines, or after exposures to the initial or to fallout y-radiations from an atomic bomb, only the dose measured in air will be available. Second, sinee the dose delivered to different tissues with “whole-body” exposures of large animals can vary quite markedly under some conditions of exposure, as will be seen, it is frequently not possible to characterize an exposure with @ stngle tissue dose, Unlike the situation of relatively uniform dose distribution in small animal exposures, it becomes very difficult to decide what is the locus of prime interest in the large animal exposure situation, and which of the myriad possible ‘‘tissue doses’? to use. Similar considerations enter in the sterilization of bulky foodstuffs with ionizing radiations (6). For these reasons it appeared useful to investigate the depth-dose patterns under the various exposure geometries, and to compare the several dose distributions with large animal exposure data as obtained from the published literature. Terms used in this report conform to the recommendations of national and international committees (4, 9). Dose and exposure are used interchangeably. “ree air dose” or “air dose” indicates the dose measuredfree in air in the absence of animal. phantom, or exposure equipment. Unless otherwise specified, this refers to the dose as it has been conventionally measured at a point in space corresponding to the proximal skin surface (the side nearest the radiation source) of the animal or phantom when it is later put in place for irradiation. This is termed more explicitly the “entrance air dose” and is expressed in roentgens. Air doses at other points in space are easily approximated under most circumstances by use of the inverse square law. Dose measured with the dosimeter embedded at any position within the animal or phantom in place for irradiation is termed ‘“tissuc dose,” also expressed in rocntgens; thus, “entrance tissue dose,” ‘midline tissue dose,” “exit tissue dose.’’ Tissue doses are not converted to absorbed dose (4), expressed in ‘rads,’ because of the uncertainty of the conversion factor for tissue dose under some conditions discussed, and because of the considerable variation of the conversion factor with different tissues (7, 8). 3 See references 4,7, and 8. Tissue dose refers only to the ionization measured by the detector embedded in the material being irradiated and usually does not indicate accurately the absorbed dose, i.e., the energy per unit mass deposited in the irradiated material, here tissue or unit density material. Over muchof the range of radiation energies usually of interest in large = sactind p.D GF Ca? animal work, from 250 kvp to 1.5 Mev or higher, the tissue dose will be equal to the absorbed dose in lean tissue, expressed as rads (100 ergs/gm), to within 10% or better. Much larger discrepancies occur in bone.