66 tissue. With “total hody" irradiation of large animals and man, however, the problem cannot be taken care of this simply and new complications enter. Frequently, in practical situations involving hazard evaluation (in the field, and around reactors or other nuclear machines), only the monitored air dose will be known at the time. In addition, with large animals and man, the dose throughout the body frequently is markedly inhomogeneous. With some types of “total body” exposure, portions of the tissues receive but a very small percent of that received by other tissues. Thus two separate problems emerge, (a) for a given monitored air dose, what is the tissue dose and its distribution pattern through large animals and man for different conditions of exposure in the laboratory and in the field, and (6) with large animals, to what degree docs the extent of biological effect vary with different patterns of dose distributionin tissue? Terms used in this report conform to the recommendations of national and international committees [2, 3]. Dose and exposure are used interchangeably. “Free air dose” or “air dose” indicates the dose measured free in air in the absence of animal, phantomor exposure equipment. UROMETRICAL, ENERGY FACTORS - THE SHORTER-TERM BIOLOGICAL HAZARDS OF A FALLOUT FIELD 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 ig 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 circumstance by use of the inverse squarelaw. Dose measured with the dosimeter embedded at any position within the animal or phantom in place for irradiation is termed “tissue dose,” also expressed in roentgens. Thus, ‘entrance tissue dose,” “midline tissue dose,” “exit tissue dose.” Tissue doses are not converted to absorbed dose [2], expressed in “rads,” because of the uncertainty of the conversion factor from tissue dose under some conditions discussed, and because of the con- siderable variation of the conversion factor with different tissues [4, 5}. A wordshould besaid initially regarding the possible application of the large amount of dosimetry data that has been published im con- nection with ¢linical radiation therapy to the problem. Most clinical radiotherapy exposures differ fundamentally from the “total body” exposures considered here in that the object of the one is to obtain localized, circumscribed partial bodyirradiation of a diseased area, while the object of the other usually is to obtein an equal degree of exposures to ali tissues of the body. Theoneusually attempts to narrow the beam bycollimation or by the use of ports; the other requires a beam sufficiently broad to expose the entire irradiated object. Thus, the numerous depth-dose figures published for radiotherapists usually cannot be carried directly to the “total body” exposure situation, although the curves obtained with very large area ports apply approximately in some situations. Since the depth-dose pattern with “total body” irradiation is highly dependent on the precise conditions of exposure, it is not laboratory conditions Listed, and depith-dose measurements were made. This phantom obviously doos not represent exactly the essentially oval configuration of manin cross section in the region of the trunk, but was felt to be a sufficiondy close approximation. A diagram of the exposure conditions for a “point” source is shown in Figure 1 for reference purposes. A target to “skin” distance (TSD) of 100 cm. was used for all exposures unless otherwise indicated. anneAGO CM. wnVy & iB a 6 Figure 1.—-Schematic diagram showing method af exposure of @ Masonite phantom to a “point” source of X- or gamma radiation. Studies showed that lengthening the evlindrical phantom beyond the 26 cm. did notalter the depth-dose curves detectably. The laboratory radiation used for most of the exposures was practical to compile complete tables of depth- eobalt-60 gammarays. As will be seen, high voltage (250 to 2,000 KVP) would have served presented here obviously apply strictly only to the specific conditions employed. of Co® allowed more direct comparison of the dose values for references. The patterns to be as well for most exposures; however, the use exposures in the laboratory, and exposure to geometryeffect. with some exposures not attainable with X-rays (ring, 4 Pi andfield exposures). A description of the cobalt generator used for bilateral cross fire, ring and 4 Pi exposures is givenin references 1 and6. Foressentially all laboratory dosimetry, the same 100 r capacity Victorcen thimble chamber and charger-reader were employed. Fora few field. ring EXPERIMENTAL The exposure geometries considered, all described more fully below, include unilateral, bilateral, multiport, rotational, ring, and “4 Pi” immediate and fallout gamma radiations in the In what follows, each situation is in- vestigated in terms of geometrical considera- tions and the principles of interaction of electro- magnetic radiations with matter. The ex- pected curves are compared with experimen- tally obtained depth-dose patterns. In the experimental work, a cylindrical Masonite (density of 1.1) Masonite phantom 26 cm.long and 26 cm. in diameter, corresponding to a 32-inch waist, was exposed under each of the low dose rate exposures with the bilateral and exposures, a 10 r capacity Vietoreen thimble chamber, intercalibrated with the 100 r chamber, was used. The chambers were embedded in a thin, close-fitting plastic shell which wes, in tur, inserted into closely machined holes drilled in the Masonite phantom. Thus, the phantom was essentially solid during exposure, The same observer took all laboratory measurements. The phantom mensurements in the field were made with thin-walled 67 Sievert-type ionization chambers embedded throughoutthe thickness of the phantom. For measurement of gammaradiation in the fallout field, the chambers were enclosed in sufficient copper to exclude beta radiation, The thimble chamber measurements did not allow accurate characterization of the depth-dose pattern at the surface and just beneath the surface of the Phantom. Since only relative measurements were used in the phantom measurements, absolute calibration of the chambers used was not necessary. annywee we [en We aA,Vn sourcena AAon vA “EFFECT OF RADIATIONS ON MAN Curves were not corrected for inverse square fall off. since it was desired to present depth-dose patierns as actually observed, RESULTS Unilateral exposure--The basie exposure technique, unilateral irradiation, is shown diagramatically in Figure 1. Radiation from the Co™ “point” source traverses air and impinges on the unit-density cylindrical phantom, In Figure 2 are shown curves describing the rate of fall off in dose through the phantom along a diameter parallel to the central axis of the beam. It is useful to attempt to derive the expected curve, since very few experimental depth dose curves for various energy gamma rays are available. The exponential curve “a” indicates the rate of fall off under narrow-beam or “good” geometry conditions, in which no seattered radiation reaches the detectors placed in the phantom. This indicates the rate of fall off of the primary beam uncorrected for inverse square effect. Curve “bh” indicates approximately the rate of fall off in the phantom due to inverse square along with a TSD of 100 em. The measured curve c, ean be regarded as curve a, corrected for scattered radiation and for inverse square (only the primary beam closelyfollows inverse square from target). The amount of scattered radi- ation, or the “build up factor,” has been caleulated using the theories of Spencer and Fano (7, 8} for the infinite medium [9] and for the barrier problem [10], but net for the geometry considered here. The infinite medium build up factors underestimate the rate of fall off