THE BIOLOGICAL MEASUREMENT OF
SCATTERED RADIATION
CHARLES PACKARD
(From Columbia University, Institute of Cancer Research, F. C . Wood, Director)
When a beam of X-rays enters a tissue a part of it undergoes
a change of direction and an increase in wave length, phenomena
which have been explained by Compton (1). The beam is
made up of bundles of energy or quanta, each of which may have
a mass as great as that of a single electron, although usually
they are much smaller. When a single quantum strikes an
electron in the tissue it moves it from its place and, as a result
of the encounter, may itself be diverted from its original direction. Through what angle it is deflected is a matter of chance;
it may go straight on or it may travel back in the reverse
direction and escape from the tissue. Whenever it is turned
from its course it loses some of its momentum; in other words,
its wave length is increased. The amount of increase depends
on the size of the angle through which it is deflected, but for a
given angle it is always the same, regardless of the nature of
the scattering medium. If the quantum is deflected through
180" this increase is at a maximum and amounts to 0.0484 A.U.
For example, if the primary beam is hard, with a wave length
of 0.1 A.U., then the rays scattered directly back from the
tissue will have a wave length of 0.1484 A.U., that is, they
will be about 50 per cent longer than before. When the primary
beam is very soft, say 1.0 A.U., this amount of increase is
relatively small, from a biological point of view.
The quantum which has thus changed its direction and wave
length proceeds until it strikes another electron, when the
whole process is repeated. Thus the scattered rays, as long as
they remain in the tissue, continue to lose momentum and to
increase their wave length until they escape or are absorbed.
Those which succeed in escaping on the same side as that
373
374
CHARLES PACKARD
through which they entered form a beam which obviously is
very heterogeneous. By measuring such a beam we can gain
some idea of the amount of scattered radiation which is producing the biological changes in the tissue in addition to those
brought about by the primary beam.
Measurement is usually made with a small ionization chamber
connected with an electroscope. The chamber is placed close
to the scattering medium and records the combined intensity
of the primary rays which enter it from one side, and of the
scattered rays entering from the other. When the same
primary beam is measured without scatter, the proportionate
amount of scattered radiation may be found. Water is usually
used as a scattering medium for it is convenient and has almost
the same scattering properties as muscle.
The results obtained by various investigators are so divergent
that there is no agreement as t o the amount of scatter even
under the same conditions of radiation. This is in part due to
the fact that the construction of the ionization chamber is
not yet standardized (2). For example, Grebe and Martius (3),
who used a horn chamber, state that when hard rays are used
(192 KV. 1.0 mm. Zn and 1.0 mm. Al) the maximum scatter
amounts to 104 per cent of the primary beam. Since the
intensity is thus doubled, they believe that a dose of 600 r-units,
measured in air, is equivalent to 1200 units in the patient.
Breitlander (4) on the other hand, found that the scatter
amounts to 43 per cent of the primary beam when the conditions
of dosage are the same as those of Grebe and Martius. Wintz
and Rump (5) tested three different dosimeters and found that
under uniform exposures (180 KV. 0.5 mm. Zn and 3.0 mm. Al)
the scatter varied from 44 to 63 per cent. Schreus (6) also
obtained similar variations with different instruments. On
comparing these with the results of bialogical tests he concluded
that previous estimates of the amount of scatter had been too
high, and that the biological method of measurement is better.
He exposed germinating seedlings to a measured dose of X-rays
without scatter and determined the amount of retard in the
rate of growth of the roots. He then placed other seedlings
375
BIOLOGICAL M E A S U R E M E N T OF RADIATION
on the scattering medium and determined the length of exposure
needed to produce the same amount of retard. The values
which he obtained by a comparison of the dosages under the
two conditions are considerably below those of other workers
who used the ionization method of measurement.
Not only is the construction of the ionization chamber of
great importance; its position in respect to the scattering medium
must be considered. When water is used, the chamber may be
held directly above it, or half immersed, or wholly immersed.
Glocker and Kaupp (7) first stated that the position makes
little difference, but later decided that it is significant (8).
Both views are held by others. The table gives a few of the
measurements made by different investigators. I n each case,
the amount of scatter measured when the chamber is half
immersed is given as 100.
TABLE
1
Above
W immersed
Wholly immersed
(flockcr
KauPP (8)
1
88
~
~-
;:;
Stenstrom Reinhard (9)
y:!
~
1
::::
97
100
102
1
:;
105
1
Klein
Gaertnor (10)
99
100
101
The three sets of figures of Stenstrom and Reinhard were
obtained from three different types of chambers. Since the
difference due to the position of the chamber amounts to
only 2 per cent, according to Klein and Gaertner, and to 31
per cent according to Glocker and Kaupp, it is clear that the
question is not satisfactorily settled. And until it is, the
measurement of scatter from water will remain a matter of
debate.
Nor can we say that the scatter from the patient will be
approximately the same as that from water, for much depends
on the region which is radiated. Jacobi and Liechti (11) find
that with the same intensity (150 KV. 0.8 mm. Cu) the scatter
from the thorax is 20.5 per cent, while from the vertebral
column it is 36 per cent, and from the abdomen, 28 per cent.
These facts suggest that if a dose is designated as including
376
CHARLES PACKARD
scatter, the ionization chamber must be kept on the patient
so that the exact number of r-units from the primary and
scattered beams can be measured.
The amount of scatter depends on the wave length of the
beam and on the area of the scattering medium. The accompanying table shows both of these points. As the voltage
rises from 60 to 200 KV. the amount of scatter also rises.
Considering the figures in the last column which show the
scatter from a large area, we see that for voltages less than 100
it is about 20 per cent of the primary beam; between 100 and
150 KV. the average is about 30 per cent, while it is approximately 40 per cent for the higher voltages. However, according
to Glocker and Kaupp and also Hess (12), the amount decreases
as the voltage rises above 180 KV.
TABLF:
2
-
Area in sq. om.
Filter
Hess (12). . . . . . . . . . . . . . . . . .
Hess (12) . . . . . . . . . . . . . . . . . .
Jacobi and Liechti (10) . . . . . .
Jacobi and Liechti (10) . . . . . .
0
0.5 A1
1.0 A1
3.0 A1
KV.
-
___
50
-_
60
60
60
77
10
10
10
15
-_ -_
400
-
20
20
19
35
_
_
_
_
_
I
_
_
_
_
_
_
_
_
_
Klein and Gaertner (9) . . . . . .
Glasser and Reittcr (13) , . , . ,
Hess (12). . . . . . . . . . . . . . . . . .
Jacobi and Liechti (10) , . . , , .
Glasscr and Itcitter . . . . . . . . .
Glocker and Kaupp (6) . . . . . .
Hess (12) . . . . . . . . . . . . . . . . . .
1.0 Al
1.0 A1
1.0 A1
0.25 Cu
4.0 A1
3.0 Al
0.5 Zn 1.0 A1
0.8 Cu
Jacobi and Liechti (10) .
Glocker and Kaupp (6) . . . . . 0.5 Cu 1.0 A1
0.5 Zn 1.0 A1
Hess (12) . . . . . . . . . . . . .
Breitlander and Jannscn ii4j 0.5 Zn 1.0 A1
Hess (12). . . . . . . . . . . . . . . . . 1.0 Cn 1.0 Al
Glocker and Kaupp (6). . . . . 1.0 Cu 1.0 A1
Breitlander (4) . . . . . . . . . . . . 1.0 Zn 1.0 Al
Breitlander and Jannsen . . . . 1.0 Zn 1.0 Al
Jacobi and Liechti (10) . . . . . 1.5 Cu 1.0 A1
Glasser and Reittor. . . . . . . . 0.5 Cu 1.0 A1
Glocker and Kaupp (6). . . . . 1.0 Cu 1.0 A1
100
100
110
125
130
135
145
__
150
170
180
182
185
190
192
192
195
200
210
-
10
20
17
20
20
20
19
22
25
18
25
18
30
26
20
36
16
-
32
17
30
4.5
32
25
32
46
38
40
37
35
43
47
52
28
__
Since the wave length is a factor in determining the amount
of scatter, the type of filtration which is used is important.
BIOLOGICAL MEASUREMENT O F RADIATION
377
Klein and Gaertner have shown that the maximum scatter
from an unfiltered beam of 100 KV. is 19 per cent, while it is
38 per cent when 3 mm. of aluminum are used. However, the
figures given in the table make clear the fact that other sources
of variation are not yet controlled. For instance, the results
of Klein and Gaertner, and of Glasser and Reitter (13), both
of whom used 100 KV. and 1 mm. of aluminum are very far
in
SCATTER
A T DIFFERENT
VOLTAGES,
AS DETERMINED
FIG. 1. THEMAXIMUM
BY DIFFERENT
INVESTIGATORS.
which the measurements of a few workers are given. There is
a fair agreement in the results obtained a t 170 to 180 KV.
perhaps because the dosimeters are most nearly correct in this
region.
The area which is radiated also determines the amount of
scatter. This was recognized by radiologists long before the
explanation for the phenomenon was known; experience showed
that to produce an erythema in a small area required a much
378
CHARLES PACKARD
longer exposure than is needed for a large area. The relation
between area and scatter is shown in Fig. 2. The curves are
plotted from averages of many observations and are therefore
only approximations. Maximum scatter from soft radiation is
attained when the area of the medium is from 100 to 150 sq. cm.
38 30
v)
60-90 KV
-
/------
L
FIG.2. THE:RELATION
BETWEEN
VOLTAGES AND THE
L
I
AMOUNTOF SCATTEF~
AT DIFFERENT
AREAO F THE SCATTERING MEDIUM.
THE
Beyond this there is little, if any, increase. At high voltages
the maximum is reached when the area is from 300 to 400 sq. cm.
The latter size of field is therefore a convenient one for experimental work.
EXPERIMENTAL
I have determined the amount of scatter from a large area of
solid paraffin by means of a Victoreen dosimeter and by Drosophila eggs. Two types of ionization chamber can be used with
this instrument,-% large one which gives the number of runits delivered per minute, and a small one which records
the total number of r-units delivered during exposure. The
former is sometimes more convenient but since it registers the
intensity during the brief period when the observation is made,
it may not show accurately the dosage given over a period of
many minutes, for in this time the output of the machine
BIOLOGICAL M E A S U R E M E N T O F RADIATION
379
may vary. A variation is commonly found during the first
few minutes of a run, for as the tube warms up, its output
increases decidedly even when the voltage and milliamperage
remain steady. When using this large chamber I have always
made the observations after the machine has become thoroughly
heated, and have repeated the test several times.
The biological measurement of scattered radiation consists
of exposing Drosophila eggs to beams of various voltages,
both with and without scatter from solid paraffin. The flies
lay their eggs on small pieces of filter paper which are placed
on a band of gauze supported by a wooden frame. The scatter
from this is negligible. The ionization chamber is placed on
the same level with the eggs. All parts of the instrument
except for the chamber itself are protected from stray radiation.
Since the dosimeter does not measure soft radiation (60 KV.)
correctly, I estimated the proper length of exposure necessary to
give 180 r-units by the biological method (16). I n the tests a t
100 KV. I gave the eggs 180 r-units, measured by the dosimeter, noting the time needed for the dose. In the other tests
I exposed the eggs for definite periods, ranging from 5 to 20
minutes.
To measure the amount of scatter I inserted under the gauze
strip a circular block of paraffin having an area of about 340
sq. cm. and a thickness of 8 cm. This fitted closely against
the gauze. When the slips of paper with their eggs are placed
in position they are thoroughly wetted so as to form a water
contact between them and the paraffin. They are now given
doses of the same length as in the previous tests without scatter.
After the eggs have been radiated they are allowed to hatch
in a moist chamber a t room temperature. The percentage of
hatching eggs is a measure of dosage. I have already shown
that these eggs are in themselves good ionization chambers
for they are independent of wave length, within a very wide
range. The proportion that lives after a definite dose, carefully
measured by means of an open ionization chamber, is remarkably
constant. This is seen in the upper two sections of Table 3.
The dose, without scatter, was 180 r-units. I n the first
380
CHARLES PACKARD
experiment when the voltage was 60 KV. the average of many
separate tests, which varied but little among themselves, is
49.1 per cent: in the second experiment with 100 KV. it is
49.9 per cent. Previous experience has shown that with this
dose 50 per cent should hatch (15). The data obtained on
TABLE
-
__
__ __
-
Alive
Dosage
180 r-
180 r-
3
___
__.
Dead
Per Cen
Alive
Per Cen
Corr.
R Units
Scuttcr
462
840
35.5
36.6
224
No scuttei
728
799
47.6
49.1
182
Scatter
306
599
33.8
34.8
232
No scattei
463
494
48.4
49.9
181
Per Cent
Ion.
Scatter
Test
-~
23
21
___-
28
27.3
__
__ __
__
Scatter
1,033
145
87.7
90.4
93
No scatter
-____
1,131
102
91.7
94.5
66
Scatter
646
541
54.4
56.0
164
No scatter
496
169
75.5
78.0
120
68
240
19.7
20.3
322
255
547
31.7
32.7
241
5 min.
10 min.
15 min.
Scatter
41
-_
37
----
_-
33
No scatter
37.5
__ __
37.0
~
10 min.
Scatter
554
No scatter
694
Scatter
295
No scatter
45 1
162
55.1
56.8
76.4
78.8
509
36.7
37.8
220
477
419
53.2
54.8
167
Scatter
232
763
23.3
24.0
290
No scatter
442
710
38.8
40.0
211
38.4
15 min.
__-__ ____
214
___
---
117
---
33
20 min.
-37
33
36.1
the number of r-units which kill different proportions of eggs
has been put in the form of a curve from which one can determine
the dosage which must have been received when any percentage
hatches after exposure (16). This method of estimating dosage
has been used in calculating the amount of scatter from paraffin.
BIOLOGICAL MEASUREMENT O F RADIATION
381
Thus in the upper section of the table, the dose given, without
scatter, estimated by the biological method, is 182 r-units.
With scatter it is 224 units. The amount of scatter is therefore
224
-182 r
1 = 23 per cent scatter
Tests with the ionization chamber give an average value for
the scatter of 21 per cent, a fairly close agreement.
I n the table, the first two columns show the dosage either in
r-units, measured by the dosimeter, or in length of exposure.
I n the fourth column are the actual number of eggs, alive or
dead after exposure, with and without scatter. Next appear
the percentages of eggs hatching and the corrected percentages.
The latter are obtained by dividing the former by 97, which is
the proportion of eggs which hatch in the unradiated controls.
The results of the biological tests are shown in Fig. 1 where
they may be compared with those of other investigators who
used physical means of measurement and other scattering
media. There is a fair agreement a t 60 and 190 KV.; elsewhere
the differences are greater. The values found by the biological
method are close to the average of all the other values.
T h e dotted line in the figure shows the results of ionization
measurements on the amount of scatter from paraffin. They
agree well with the biological data except a t the highest voltage
where they are noticeably smaller.
All the curves in the figure, with one exception, show a
tendency to become horizontal or to fall at the high voltages.
The significance of this phenomenon is not clear. Theoretically,
the amount of scatter increases as the wave length becomes
shorter. The total amount from highly filtered gamma rays
of radium should therefore be very large. Probably the decrease
is due t o the fact that the scattering medium is not made
deeper when the short rays are used. If its thickness should
be varied so that the total absorption remains the same for all
wave lengths, then the proportion of rays scattered backward
should remain unchanged. Another explanation is that as the
rays grow harder the ratio between the forward and backward
382
CHARLES PACKARD
scatter changes, the former increasing and the latter decreasing.
Some criticism has been raised against the use of paraffin as
a scattering medium on the ground that the intensity of scatter
is greater than from water or from tissue. Jacobi and Liechti
find that with low voltages it is almost twice as great, while
with higher voltages it is only slightly more than that from
water. The data obtained in the present experiments show
that the scatter from paraffin when the lowest voltage is employed is the same as that found by Jacobi and Liechti while
at the higher voltages it is uniformly lower than their values.
SUMMARY
1. The amount of radiation scattered backward from paraffin
has been determined by both biological and physical methods.
The two agree closely, the latter giving slightly lower values.
2. With increasing voltages, the scatter increases from 23 per
cent a t 60 KV. to 37 per cent at 190 KV.
3. At higher voltages it decreases somewhat.
REFERENCES
1. COMPTON,
A. H.: Bndiology, 1924, iii, 479,
2 . BI~AUN,
It., AND H. IiUSTNlCR: Strahlentherapie, 1929, xxxiii, 551.
3. GREIIE:, I,., AND 11. MARTIVS: Strahlentherapie, 1925, XX, 128.
4. BREITLANUEE,
Ii. : Strahlentherapie, 1926, xxiii, 79.
5. WINTZ,€I., A N D W.RUMP:Htrahlentherapie, 19’26, xxii, 451.
6. S ( x ~ ~ ~ ~€I.
: nT.:
s , Klin. Wchnschr., 1926, ii, 1762.
7. GLOCKER,R., A N D E. I c . 4 ~ 1 Strahlentherapie,
~:
1927, xxiv, 517.
8. GLOCKERIt., AND E, RAUPP:
Btrahlentticrapie, 1927, xxvi, 156.
9. STENSTROM,
W., AND M. I ~ N H A: R
Strahlentherapie,
D
1920, xxiii, 88.
10. I ~ L E AND
I N (:AICRTNER: Fortschr. n. d. Gcb. d. Itbntyenstrahlen., 1927, xxxv, 492.
11. JACOB],H., A N D A. LIECHTI:
Str:ihlenthcrapie, 1928, xxvii, 711.
12. HESS,P.: Strahlcnthcrapie, 1928, xxvii, 146.
13. (;LASHER, O., AND C. S. REITTER:
Am. J. Roentgenol., 1926, xvi, 43.
14. BREITLANDFR,
I<., A N D K. JL4NNSEN: Strahlentherapie, xxii, 1.
15. I’ACKAED,
C.: 6.Cancer Ites., 1927, xi, 1.
16. I’ACKARD,
C.: J. Cancer Ites., 1927, xi, 282.