-7e2
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 X-ray beam and the relationship to air-dose 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-can diameter
cylindrical masonite phantom from an experimental simulation of the
fiela geometry (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 phantcm surface is implicit for the diffuse case, while
the proximal air-dose is used in the bilateral case,
Figure 7.2 is a similar comparison for 200eKVP, 0.S-mm, copper-
filtered X-rays, with the diffuse geometry that of a plane rather than
spherical source assembly. This was 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 enawgh to include the whole
phantom, The average air-dose around the circumference was here used
for the diffuse geometry and the proximal airedose again in the bilate
eral exposure,
It is evident that for both these energies (the effece-
tive energy of the X-ray beam being atout 90 KV), the diffuse-narrow
beam depth dose ratio for either 2 7 radians (plane) or kw steradians
(volume) diffuse geometry is almost the same, That is, the midline
dose is about 50 percent higher and the 5ecm dose is 35 percent higher
than the same air-dose (measured proximally) woulc imply in the narrow
beam bilateral exposure, It is therefore assumed that this approximate
factor will apply thrcuzhout the field exposures,
On this tasis the air-—dose values calculated from the survey meter
readings (Table 8.1) should be multiplied by 1.5 in order to compare
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-dose. 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-dose meas-
ured with the meter anc multiplied by (1.09 x 1.5) only.
The doses are discussed further in Chapter 8,
as
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