eeeea ce eee Figure 7 shows the influence of exposing depth-dose and biological effect. a Masonite phantom to 2000 kVp x ray from @ single direction than when the Tne next exposure is bilateral with half of the dose given to each side (33). yrtality of had exposure, count im the irth, and the fe Figure 4 Ue is attenuated by decreased dose as the beam is attenuated, is very uniform deposition of phantom. of In it the unilateral inverse square and Thus the bone marrow of large animals being exposed would have a progressively there owest white uTe falls off as the case of absorption so that the exit dose is about 45% of the entrance dose. fifth < ance of the dose In the surface dose. expressed as percent of a lealiie e 3 plots The dose in the phantom was measured by Sievert fonization chambers and is 5 causes of count a gies: it Sahpe aon , ee bo ree ai his this ne fraction of Se Subsequent : great. However, with bilateral exposure energy The biological consequences of throughout the tissue equivalent the different dose pattern are It is of considerable importance to bear these differences in mind when evaluating therapy of radiation injury and trying to. make an anima} experimentation as comparable as possible to an assumed real-life human exposure, Figure 8 shows a comparison of bilateral exposure to 4 Pi exposure This situation is fallout irradiation with exposure approaching 4 from @ planar source, yas MOTE amy cin with 8 Lin was mos t-4] Se mortality et al- (32) ys followed cillin and . Pi important when trying its wide in energy and range source. Since to evaluate fallout the the hazards of radiation radiativn is delivered the usual narrow beam geonetry is not applicable, such a diffuse 360 degree field, the decrease of duse with deptin is less pronounced than that resulting from a bilateral exp sure in In tissue to an x-ray beam because fallout from inverse square is ain efrect neutralized. For the same euergy, the dose at the center of the body is approximately “50% higher thau would result from a given air dose with narrow beam geometry. Figure 8 further illustrates the depth-dose curve from an experimental situation using spherically oriented cobalt-60 sources with a phantom placed at their center, compared with a conventional bilateral depth-dose curve obtained with a single Cobalt source (34). tase, In the tdeeer the air dose is usually measured at the point subsequently occupied the center of the proximal surface of the source. the patient or animal with respect For the field case, all surfaces are "proximal" in individual is the same. It is the this air dose which is measured by field struments; it does not bear the same relationships as the surface dose the depth dose as air dose measured in a "point source” beam in the -10)1- er (33).