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