lo EXPERIMENTAL SIMULATION AND CEOMETRY FACTOR In such a diffuse field, the decrease of dose with depth in tis- sue is less pronounced than that resulting from a bilateral exposure to an Xeray beam and the relationship to airedose differs as noted in the two cases, The result is that, for a given energy, the dose at the center of the abdomen is considerably higher than a given proximal air-dose would imply for the narrow-beam or point-source case, Figure 7,1 illustrates the depth dose curve in a 36-cn diameter cylindrical masonite phantom from an experimental simulation of the fieldpee (reference 13) using a spherically oriented group of 36 Co™ sources, The phantom was placed at the center of the assembly. This is compared to a conventional bilateral depth-dose curve measured in the same phantcm and obtained with a single Co source, Both are normalized to air~dose, but the average air-dose at all points later occupied by the phantom surface is implicit for the diffuse case, while the proximal air-dose is used in the Lilateral case, Figure 7.2 is a similar comparison for 200-KVP, 0.S=mm, copper-= filtered Xerays, with the diffuse geometry that of a plane rather than spherical source assembly. This wes produced in this case by rotation of the phantom and ion chamber in the beam of a stationary Xeray unit. The useful beam angle of the unit was wide encwgh to include the whole phantom, The averape air-dose around the circumference was here used for the diffuse geometry and the proximal airedose again in the bilateral exposure, It is evident that for both these energies (the effec- tive energy of the X-ray beam being atout 90 KV), the diffuse-narrow beam depth dose ratio for either 2 m radians (plane) or hw steradians (volume) diffuse geometry is almost the same, That is, the midline dose is about 50 percent higher and the Seem dese is 35 percent higher than the same airedose (measured proximally) woulc imply in the narrow beam bilateral exposure, It is therefore assumed that this approximate factor will apply throughout the field exposures, On this tasis the air-dose values calculated from the survey meter the situation to that of a bilateral exposure to a source with the same energy distribution but using a point source geometry and a proximally measured air-cose, Alternatively, if a point source of higher energy, say C060, were used bilaterally in the same way to simulate a field exposure to only the higher gamma components, then the meter energy correction factor woulc be unity, In this case, to specify a bilateral exposure yielding a midline dose equal to that with diffuse geometry, the point source air-dose should be the diffuse field air-cose meas- ured with the meter anc multiplied by (1.09 x 1.5) only. The doses are discussed further in Chapter 8, s2. ei9 age eee apereerenee readings (Table 8.1) should be multiplied by 1,5 in order to compare