 |
Description  |
|
|
FIELD OF THE INVENTION
This invention relates to intracavitary hyperthermia applicators for use in
nearly closed-end cavities such as the nasopharynx and the cervix.
BACKGROUND OF THE INVENTION
Interstitial insertion of catheters to apply therapeutic radiation and
hyperthermia is commonplace. Several miniature microwave antenna designs
are known. Conventional antenna designs are typically 1 to 2 mm in
diameter and 5 to 7 mm in length and are operated at frequencies of from
about 300 MHz up to about 2450 MHz.
Locally induced microwave hyperthermia for cancer therapy permits
flexibility in treatment procedures for tumors of irregular volume and for
tumors located deep within the body. Production of adequate thermal field
distribution in superficial, accessible and deep-seated tumors is an
important consideration. Limited depth of energy penetration has
restricted the use of prior art antennas primarily to the heating of well
localized tumors extending to depths of up to a few centimeters. Tumors in
hollow viscera or cavities such as the oesophagus, cervix and prostate are
amendable to treatment with intracavitary hyperthermia techniques.
Interstitial hyperthermia techniques are employed for accessible tumors of
large volume. A major limitation of prior art interstitial devices is
maximization of thermal energy along the sides rather than at the tip of
the applicator.
One prior art approach to enhancing the heating at the tip of the
applicator is described in Lin, et al., Int. J. Hyperthermia, 3:37-47
(1987). The antenna described operates at 2450 MHz.
A 915 MHz applicator having a diameter in excess of 1 cm and hence too
large for use in the treatment of nasopharyngeal cancer is described in
Abstract Ce-9, p. 43, "Abstracts of Papers for the Thirty-Sixth Annual
Meeting of the Radiation Research Society, Eighth Annual Meeting of the
North American Hyperthermia Group", Philadelphia, Pa., Apr. 16-21, 1988,
and was shown at that meeting.
SUMMARY OF THE INVENTION
It is an object of this invention to provide a hyperthermia applicator for
treating cancers and other abnormal tissues, in which thermal energy is
concentrated near the applicator tip.
It is also an object of the invention to provide a hyperthermia applicator
of a size appropriate for the treatment of nasopharyngeal cancer which
provides a high concentration of energy at the tip.
Another object of the invention is to provide an intracavitary hyperthermia
applicator which has low power requirements and may be operated from
portable machines.
A further object of the invention is to provide an intracavitary
hyperthermia applicator which has an improved heating pattern as compared
with prior art devices.
These objects are achieved by a microwave antenna, for example, a coaxial
cable, having an inner conductor and an outer conductor, and in which the
outer conductor is terminated in spaced relation to the end of the inner
conductor and may be slotted at a location spaced from the end, defining a
junction. A first sleeve of conductive material is shorted to the outer
conductor at a predetermined spaced location rearwardly from the junction,
and a second sleeve of conductive material is shorted to the outer
conductor spaced rearwardly from the first sleeve. The sleeves extend
forwardly in coaxial relationship with the cable end, but terminate at
mutually spaced positions and all are spaced from the distal end of the
inner conductor of the cable. A conductive body is affixed to the distal
end of the inner conductor, and the entire active portion of the antenna
is encapsulated in an insulating material such as epoxy. The dimensional
relationships of the terminal ends of the sleeves and inner and outer
conductors, their radial spacing from one another, the lengths of the
sleeves, the location of the junction, and the geometry of the conductive
body at the distal end of the inner conductor are all selected to achieve
optimum results.
The outer diameter and the length of the applicators of the invention are
selected to accommodate the bodily cavity in which a cancer may appear.
Outer diameters may, for example, range from about 0.5 centimeters for
nasopharyngeal cancer therapy to about 5 centimeters for cervical cancer
treatment. Applicator lengths may correspondingly range from about 4.5 to
about 10 centimeters.
For example, a first form of the invention is designed as a small
applicator for use in nasopharyngeal cancer therapy. This applicator has
an outer diameter of about 0.75 cm and a length of about 5.5 cm. The
sleeves comprise copper foil or screen, and the conductive body comprises
brass. The cable is preferably Micro coax, semi-rigid UT-250-A or
RG-58A/U, and the sleeves are soldered to the outer conductor. When
operated on a muscle phantom at 915 MHz, return losses of 10-15 DB are
obtained, and maximum heatings of 1.3 and 0.85 degrees C/W-min. 1.15 cm
apart are achieved at the tip and at the junction, respectively.
In a second form of the invention, the applicator is designed for the
treatment of cervical cancer. This applicator comprises RG-9/U coaxial
cable and has first and second coaxially arranged sleeves shorted at their
rearward ends in spaced locations on the outer conductor of the cable,
similarly to the nasopharyngeal form of the invention. However, the second
or outer sleeve is spaced a greater distance radially from the first
sleeve, and the conductive body at the distal tip of the inner conductor
is a spiral. This applicator has an outer diameter of about 3 cm and an
overall length of about 6.5 cm. Further, this applicator has distinct
heating at the tip when operated at 915 MHz on a muscle phantom, with a
maximum rate of heating of 0.25 degrees C/W-min.
Applicators of the invention described above can be effectively operated at
915 MHz with a 30 watt power generator, thus facilitating use with
portable equipment at small rural hospitals or clinics. Deeper penetration
is obtained at 915 MHz than at 2450 MHz, rendering the applicators more
clinically useful than prior art designs.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other objects and advantages of the invention will become
apparent from the following detailed description and the accompanying
drawings, in which like reference characters designate like parts
throughout the several views, and wherein:
FIG. 1 is a longitudinal sectional view of a nasopharyngeal form of
applicator according to the invention.
FIG. 2 is a view depicting the terminal exterior of the distal end cap of
the applicator.
Computer generated FIGS. 3A, 3B, 3D and 3E show a thermographic temperature
elevation of the applicator as shown in FIG. 1 operated at 915 MHz on a
muscle phantom.
FIG. 3C illustrates the relationship of the applicator tip to the maximum
heating peak shown in FIG. 3B.
FIG. 4 is a longitudinal section of an intracavitary applicator for
treatment of cervical cancer.
Computer generated FIGS. 5A, 5B, 5C and 5D show a thermographic temperature
elevation of the applicator shown in FIG. 4 operated at 915 MHz on a
muscle phantom.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring more specifically to the drawings, a first form of intracavitary
hyperthermia applicator in accordance with the invention is indicated
generally at 10 in FIG. 1. This form of the invention is relatively small
in size and is suitable for use in the treatment of nasopharyngeal cancer.
The device is also useful in the treatment of other conditions, such as
sinusitis, for example.
The applicator 10 comprises a length of Micro coax semi-rigid cable,
UT-250-A or RG-58A/U, having an inner conductor 11 and a tubular outer
conductor 12 spaced from the inner conductor by a layer of electrically
non-conductive insulating material 13, such as, for example,
polytetrafluoroethylene (Teflon, a trademark of DuPont). The inner and
outer conductors have proximal and distal ends, and the distal end of the
outer conductor is terminated short of the distal end 14 of the inner
conductor. The outer conductor is also slotted at 15, spaced a distance
"C" from its distal end 16. The slot has a longitudinal axial dimension
"B".
A first sleeve 17 extends coaxially with the outer conductor and is
maintained in radially outwardly spaced relationship thereto by a layer of
non-conductive insulating material 18. The distal end 18 of the sleeve 17
extends beyond the end of the outer conductor by a distance "D", and is
connected or shorted to the outer conductor at a location spaced a
distance "A" rearwardly from the slot 15.
A second sleeve 19 extends coaxially with the first sleeve and is
maintained in radially outwardly spaced relationship thereto by an
insulating layer 21. The second sleeve is connected or shorted to the
outer conductor 12 at a location spaced a distance "A.sub.1 " rearwardly
of the point of connection of the first sleeve, and extends forwardly over
the first sleeve a distance "F".
A cap 30 of conductive material, preferably brass, is affixed to the distal
end 14 of the inner conductor 11 in transverse relationship to the axis of
the applicator. The cap 30 has a diameter "G" approximately the same as
the diameter of the second sleeve, and has a plurality of radial holes 31
extending from the center through the outer periphery at 90 degree
intervals.
In a preferred construction, the dimensions of the various components of
the applicator illustrated in FIGS. 1 and 2 are as follows: A=1 cm;
A.sub.1 =1 cm; B=0.5 cm; C=1.5 cm; D=0.3 cm; E=1.2 cm; F=3.8 cm, and
G=0.75 cm. The first and second sleeves are soldered to the outer
conductor and comprise copper foil or screen. A layer 40 of epoxy is used
to encapsulate the entire active section of the applicator. With the
stated dimensions and when operated at 915 MHz and covered with a rubber
finger cot or the like, this applicator produces a computer generated
thermograph heating pattern as shown in FIGS. 3A, 3B, 3D and 3E. FIG. 3C
shows the relationship of the applicator 10 to the tip of the maximum
heating peak shown in FIG. 3D.
FIG. 3A depicts heating gradient lines. Maximum heating is shown at the
point marked "+". In FIG. 3B, the peak, reflecting a maximum heating of
about 1.3 degrees C. per minute watt (.degree.C./W-min), corresponds to
the point "+" in FIG. 3A. In FIG. 3D, the peak again shown at a maximum
heating of 1.3 degrees C. per watt minute, corresponds to the "+" on FIG.
3A, as does the maximum peak in the three-dimensional FIG. 3E.
The second form of the invention is indicated generally at 50 in FIG. 4.
This form of the invention comprises a length of RG=9/U coaxial cable
having an inner conductor 51 and a tubular outer conductor 52 separated
from the inner conductor by a layer of insulation 53. The outer or distal
end 54 of the outer conductor is terminated short of the distal end 55 of
the inner conductor by a distance "D" plus "E", and is slotted at 56 at a
location spaced a distance "C" from its distal end. The slot has an axial
dimension of "B".
A first sleeve 60 extends coaxially over the outer conductor and is
maintained in radially outwardly spaced relationship thereto by a length
of electrical insulating tubing 61, such as Tygon. The first sleeve
extends at its distal end 62 a distance "D" beyond the distal end of the
outer conductor, and is shorted or connected to the outer conductor at a
location spaced a distance "A" rearwardly of the slot 56.
A second sleeve 70 is connected or shorted to the outer conductor at a
location spaced a distance "A.sub.1 " rearwardly from the point of
attachment of the first sleeve, and extends coaxially forwardly over the
first sleeve a distance "F". The second sleeve is maintained in radially
outwardly spaced relationship to the first sleeve by a layer of insulation
71, such as Styrofoam or the like, for example.
A body 80 of conductive material is secured to the forward or distal end of
the inner conductor, and in the form of invention shown, comprises a
spiral. The spiral has a diameter "G" approximately the same as the
diameter of the second sleeve.
In a preferred construction of this form of the invention, the first and
second sleeves are made of brass and the dimensional relationships are as
follows: A=1 cm; A.sub.1 =2 cm; B=0.5 cm; C=1.5 cm; D=0.3 cm; E=1.2 cm;
F=4 cm, and G=3 cm. When operated at a frequency at 915 MHz on a muscle
phantom and with the stated dimensions as covered with a rubber finger cot
or the like, this applicator produces a computer generated thermograph
heating patterns and shown in FIGS. 5A, 5B, 5C and 5D.
FIGS. 5A, 5B, 5C and 5D are analogous to FIGS. 3A, 3B, 3D and 3E. The point
of maximum heating appears at the place marked "+" in FIG. 5A. As appears
from FIGS. 5B and 5C, a maximum heating of about 0.25 degrees C. per watt
minute is obtained.
The first form of the invention described above is particularly suited to
treatment of nasopharyngeal cancer, while the second form is best suited
for treatment of cervical cancer. However, as noted, either form of the
invention could be used in the treatment of other illnesses.
Although the invention has been described with reference to a particular
embodiment, it is to be understood that this embodiment is merely
illustrative of the application of the principles of the invention.
Numerous modifications may be made therein and other arrangements may be
devised without departing from the spirit and scope of the invention.
* * * * *
|
|
 |
|
 |
Description |
|
