Note: Descriptions are shown in the official language in which they were submitted.
3LC~80371
The invention relates to a radiation source for an irradiation
apparatus, preferably used for medical purposes, comprising a cylindrical
source-housing, an encapsulated radioisotope centrically arranged in the
source-housing, and a tubular heavy metal lining or cladding disposed between
the cylindrical source-housing and the encapsulated radioisotope.
It is common to obtain radioisotopes - particularly Co 60 and Cs 137
in standardized housings from specialized supply firms for utilization in
irradiation apparatus. Radiation sourcesof this type most often consist of a
double-wall cylindrical housing composed of high-grade sheet steel into which
the radioactive isotope is welded. The diameter of the radioactive filling is
generally in a range of between 10 and 20 mm and it has a height or axial ex-
tent of approximately 30 mm.
A radiation source is installed in the protective housing of an ir-
radiation apparatus in such a manner that the cylindrical wall and the one
circular disc-shaped end of the cylindrical source-housing are surrounded by
heavy metal for the purpose of shielding. Subsequent to opening a correspond-
ing closure system, the radioactive radiation of the radioisotope passes
unweakened through the other (frontal) end of the radiation source and out of
its housing. In the radiation sources of some manufacturers, a shielding-type
of material is also employed within the housing of the radiation source. This
material i9 arranged between the inner capsule containing the radioactive
material and the outer housing of the radiation source, whose diameter has
been correspondingly enlarged, in the form of a tubular shielding member
concentrically surrounding the capsule.
In medical radiation therapy, centers of disease of varying sizes
must beirradiated, whose dimensions most often exceed the dimensions of the
plane of issuance of the radiation source by more than a power of ten. Ad-
justment of the cross section of the irradiation field in the irradiation
apparatus is carried out by means of a radiation diaphragm arranged beyond
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the closure system (with respect to the radiation direction). With the aid of
the radiation diaphragm, the desired field size may be diaphragmed out from
the cone of rays issuing from the radiation source. The field size is here
geometrically defined by the connecting straight line between the radioisotope
and the edges of the diaphragm plates of the radiation diaphragm.
The intensity of the radiation does not immediately drop back to
zero at the border of the irradiation field. On the contrary, it gradually
decreases within a more or less large region. The width of this so-called
"penumbral region" is dependent, among other things, upon the shielding be-
havior of the diaphragm, upon the geometric dimensions of the source, and
upon the geometric interval ratios of source to diaphragm, and diaphragm to
skin surface.
For therapeutic reasons there is a demand for the penumbral region
to be kept as small as possible. In order to bring this about, solutions
have hitherto been sought in the direction of an improvement in the interval
ratios and in a reduction of the source diameter. A natural limit is placed
on improvements in the interval ratios, among other things, due to the relative
adjustability of the patient, the supporting table and the radiator head which
is neaessary in a clinical operation. A reduction in the source diameter,
while maintaining the same specific activity of the radioactive material
employed, means a greater source height and thus greater losses in the dose
rate through self-absorption. At the same time, with greater source-heights
radiation losses are also brought about through the shielding material laterally
surrounding the source. The marginal zones of the irradiation field are again
hereby disadvantageously affected. -
The object which is the basis of the invention consists in pointing
out a way whereby, particularly when employing small source diameters, it is
possible to achieve an improved utilization of the source material, a steeper
drop in intensity beyond the border of the irradiation field, and also achieve
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intensities of substantially constant value within the irradiation field.
In a radiation source of the type initially cited, the inner wall of
the heavy metal lining or cladding is therefore provided with a frustum-shaped
construction, whereby its inner enveloping surface, in an extended line, is
simultaneously tangent to the inner edge of the maximally opened diaphragm
plates of the irradiation apparatus as well as tangent to the edge of the re-
mote end of the radioisotope not facing the diaphragm plates. This solution
postulates the recognition that a major cause of the drop in the dose rate in
the direction of the border of the irradiation field in the known irradiation
apparatus can be found in the fact that, particularly in the case of radiation
sources having a small diameter and a great height (or axial extent), radiation -
quanta issuing from the back or rear regions of the radioisotope which are not
facing the radiation diaphragm must travel appreciable distances through the
heavy metal lining or cladding presently surrounding the encapsulated radioiso-
tope unit in a tubular fashion, if these radiation quanta are radiated in the
direction of the marginal zones of an irradiation field. This heavy metal
lining or cladding shields the radiation more stronely than the material of
the radioisotope itself intervening in the direction of the central ray. More-
over, no additional radiation quanta are further externally fed or strewn into
this solid angle region, in contrast with the solid angle region adjoining the
central ray. By means of the frustum-shaped intermediary space between the
radioisotope unit and the heavy metal lining or cladding, this radiation
deficiency in the marginal zone of the irradiation field is reduced.
A precise support mounting of the encapsulated radioisotope unit in
the source-housing is achieved if the intermediary space between the encapsul-
ated radioisotope unit and the specially shaped heavy metal lining or cladding
is filled with a material which is readily permeable to radiation, in accordancewith the invention. A good thermal conduction from the radioisotope to the
source-housing is hereby ensured, particularly when a material is used possess-
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ing good heat-conducting properties, such as an aluminum form-body. In
addition, a sufficiently stable and reliable support mounting of the radio-
isotope in the source-housing is an essential requirement for the transport-
ability of the radiation source.
In a further development of the invention, a particularly expedient
construction of the radiation source is provided if the encapsulated radio-
isotope unit is supported in a centered fashion in the source-housing at its
end facing the diaphragm plates by an apertur~d disc composed of a material
readily permeable to radiation, and at its opposite end directly by the heavy
metal lining or cladding. This type of construction results in especially low
radiation absorption values for the marginal or peripheral radiation. It can
also be manufactured with a sufficient degree of mechanical stability.
In another embodiment of the invention, the encapsulated radio-
isotope unit can be supported in a centered fashion in the source-housing at
its end facing the diaphragm plates by an impression in the base of the
cylindrical source-housing which is adapted to the shape of the encapsulated
radioisotope unit, and at its opposite end directly by the heavy metal
lining or cladding. By proceeding in this manner, the apertured disc
mentioned in the above sample embodiment can be dispensed with.
According to the broadest aspect of thc invention there is provided,
in an irradiation apparatus for medical purposes including source receiving
means and diaphragm means providing a maximum opening angle (~), a radiation
source in said receiving means having an elongated encapsulated radioisotope
extending axially therein, and means providing substantially unobstructed
radiation paths which extend laterally and frontally from substantially the
entire periphery of the encapsulated radioisotope at acute angles of sub- -
stantially one half the maximum opening angle (~/2).
Other objects, features and advantages of the invention will be
apparent from the following detailed description, taken in connection with
the accompanying sheet of drawings.
Figure 1 shows a simplified sectional illustration of a radiator
head with an installed radiation source;
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Figure 2 presents a diagram showi?lg the applied dose rate J in
dependence upon the distance from the central ray;
Figure 3 presents a section of a radiation source in which the en-
capsulated radioisotope unit is supported in a centered fashion at its one
end by the heavy metal lining or cladding, and at its other end by an
apertured
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disc composed of a material readily permeable to radiation;
Figure 4 presents a section of a radiation source in which the en-
capsulated radioisotope unit is supported in a centered fashion at its one
end by the heavy metal lining or cladding, and at its other end, by a special
shaping of the base of the source-housing; and
Figure 5 presents a section of a radiation source in which the encap-
sulated radioisotope unit is provided with a slightly conical construction.
Figure 1 is a simplified illustration of a section of a radiator
head 1 of an irradiation apparatus not further illustrated for purposes of
clarity. A radiation source 3 is introduced in a source-wheel 2 which is
constructed as a seal or closure and is rotatable in the radiator head. On
radiator head 1, two diaphragm plates 4, 5, which are adjustable at right
angles to one another, of a not further illustrated radiation diaphragm 6,
can be recognized. The one diaphragm plate 4, which is adjustable in the
plane of projection, is shown in two different opening positions.
Radiation source 3 consists of a cylindrical high-grade steel source-
housing 7. In the source-housing, a radioisotope 10 having a diameter of 10 mm
and a length of 30 mm i.9 welded into a high-grade steel capsule 9, and is cen-
trically disposed in relation to symmetry axis 8 of the source-housing.
Capsule 9 of radioisotope 10 is surrounded by a specially shaped
generally tubular heavy metal lining or cladding 11 resting against the inner
wall of source-housing 7. The inner wall 12 of this heavy metal lining or
cladding 11 has a sloping configuration to provide a progressively increasing
interior cross section in a frustum-fashion with the maximum open cross section
at the frontal end of the radioisotope 10 which faces in the direction of
radiation diaphragm 6. The lining 11 thus leaves a free intermediary space
between itself and capsule 9 of radioisotope 10. This intermediary space is
filled by an adapted aluminum form-body 13. In an extended line, the inner
wall 12, bordering on the form-body 13, of this generally tubular heavy metal
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lining or cladding 11 is tangent to the inner edge of the maximally opened - -
diaphragm plates 4, 5, of the radiator head as well as tangent to the remote
peripheral edge (not facing radiation diaphragm 6) of radioisotope 10. A
tangent of this type, which simultaneously also represents a marginal ray
when the radiation diaphragm is opened to its maximum, is illustrated in
Figure 1 by reference numeral 14. Consequently, in the sample embodiment of
Figure 1, due to the rectangular diaphragming, the inner wall 12 of the heavy
metal lining or cladding 11 likewise has a rectangular cross section in a
plane normal to the symmetry axis 8 of source housing 7.
In the operating position, the symmetry axis 8 of the source-housing
7 coincides with the central ray, illustrated in Figure 1 by the broken line
15, of the cone of rays 16 issuing from maximally opened radiation diaphragm 6. :
In addition, it is clearly apparent from Figure 1 that the radiation quanta :
which originate from the rear or back region of radioisotope 10 (said region
not facing radiation diaphragm 6) and which are aimed at the marginal region
of the diaphragmed irradiation field 17, are no longer weakened by the heavy
metal lining or cladding due to the frustum-shaped construction of the inner
wall 12 of the heavy metal lining or cladding 11. As a consequence, the dose :
rate in this marginal region of irradiation field 17 is increased as compared
with the conditions prevailing in conventional purely tubular constant cross
section heavy metal linings or claddings of the radiation source. In Figure 1,
the broken lines additionally represent the penumbral region 18 outside the .
diaphragmed irradiation field 17 in which, as a consequence of the shape of ~;
the path for the laterally emanating photons from radioisotope 10, a relatively
sharply decreasing radiation dose is obtained; that is a steeper drop in
radiation intensity is realized beyond the boarder of the irradiation field 17.
Figure 2 illustrates in a diagram the schematic characteristic of the
applied dose rate J in dependence upon the distance from the central ray 15.
The continuous line 19 illustrates the characteristic of the dose rate when the
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inventive radiation source 3 is used. The broken line 20 illustrates the
characteristic of the dose rate when a conventional radiation source is used
with a heavy metal lining or cladding surrounding the encapsulated radioiso-
tope in a tubular fashion. A comparison of the curves 19 and 20 demonstrates
that the dose rate in the diaphragmed irradiation field 17 can be strongly
adapted or conformed to the value in the region of central ray 15, as a
consequence of the particular shaping of the heavy metal lining or cladding 11.
By way of contrast, the width dimension of the so-called penumbral region 18,
which is dependent upon the diameter of radioisotope 10, cannot be influenced
thereby.
Figure 3 illustrates a different radiation source 21 in which the
encapsulated cylindrical radioisotope unit 22 is guided or secured by the
heavy metal lining or cladding 23 at its remote end not facing the radiation
diaphragm, and is guided in a central hole of a light metal disc 24 at its
opposite (frontal) end. The outer diameter of the light metal disc 24 is,
in turn, guided or secured by the inner wall of source-housing 25. This
method of construction has the advantage that the Porm-body 13, composed of a
material readily permeable to radiation, employed in the sample embodiment of
Figure 1, which fills out the intermediary space between capsule 9 of the
radioisotope 10 and the heavy metal lining or cladding 11, may be eliminated,
and an air path substituted therefore. A further approximation of the dose
rate in the marginal region of irradiation field 17, to that prevailing in the
region of central ray 15 is thereby effected.
Figure 4 illustrates a slightly modified radiation source 26 as
compared with the sample embodiment of Figure 3. Here, too, the encapsulated
radioisotope unit 27 is guided or secured at its end not facing radiation
diaphragm 6 by heavy metal lining or cladding 28, which, in turn, rests against
the exterior wall of housing 29 of radiation source 26 However, at its end
facing the radiation diaphragm, the encapsulated radioisotope unit 27 is guided
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or secured in a recess 32 which is centrically impressed in the base of
housing 29 of radiation source 26, and whose inner dimensions are adapted
to the outer dimensions of capsule 30 of radioisotope 31. This embodiment
of radiation source 26 is distinguished as compared with that of Figure 3 by
an even more favorable characteristic of the dose rate curve (that is a
relatively more constant intensity J over the region 17, as compared to curve
19, Figure 2), because the absorption of the apertured disc 24 is eliminated.
Finally, Figure 5 illustrates a radiation source 33 in which
encapsulated radioisotope unit 34 is tapered in the direction of radiation
diaphragm 6. The tapering angle B; i.e., the angle between the tangent on the
outer circumference of the encapsulated radioisotope unit 34 and its symmetry
axis 35 cannot be permitted to be greater than half the opening angle a
(Figure 1) of the cone of rays 16 when radiation diaphragm 6 is opened to its
maximum. Here, also, the encapsulated radioisotope unit 34 is guided by a
heavy metal lining or cladding 36 at its end not facing radiation diaphragm 6
and it is guided by an aluminum apertured disc 37 at its opposite end. In
turn, the apertured disc 37 illustrated here, as in the sample embodiment of
Figure 3, reæts against the inner side of housing 38 of radiation source 33.
By way of variation, the encapsulated radioisotope unit 34 could also be
supported in the manner illustrated in Figure 4, i.e., without an apertured
disc. Due to the conical shape of the encapsulated radioisotope unit 34, the
width of the penumbral region 18 (Figure 1) may be reduced. This becomes ;
apparent if one imagines radiation source 3 (Figure 1) to be replaced by
radiation source 33, and then correspondingly adapts the marginal rays illus-
trated by broken lines.
It will be apparent that many further modifications and variations
may be made without departing from the scope of the novel concepts and
teachings of the present invention.
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