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).