 |
Description  |
|
|
TECHNICAL FIELD
This invention relates to a microwave antenna for treatment of deep-seated
cancerous tumors by hyperthermia and other biomedical and geological
applications involving the use of dielectric heating for treatment of
materials.
BACKGROUND ART
The use of elevated temperatures, i.e., hyperthermia, to repress tumors has
been under continuous investigation for many years. When normal human
cells are heated to 41.degree.-43.degree. C., DNA synthesis is reduced and
respiration is depressed. At about 45.degree. C., irreversible destruction
of structure, and thus function of chromosome associated proteins, occurs.
Autodigestion by.the cell's digestive mechanism occurs at lower
temperatures in tumor cells than in normal cells.
In addition, hyperthermia induces an inflammatory response which may also
lead to tumor destruction. Cancer cells are more likely to undergo these
changes at a particular temperature. This may be due to intrinsic
differences, between normal cells and cancerous cells. More likely, the
difference is associated with the low pH (acidity), low oxygen content and
poor nutrition in tumors as a consequence of decreased blood flow. This is
confirmed by the fact that recurrence of tumors in animals, after
hyperthermia, is found in the tumor margins; probably as a consequence of
better blood supply to those areas.
Conventional methods of cancer treatment are surgery, radiation (X-ray)
therapy and chemotherapy. In radiation therapy and chemotherapy, there are
important interactions with hyperthermia. Acidity, hypoxia (low oxygen
tension) and decreased nutrition all lead to increased susceptibility to
hyperthermia treatment. In contrast, these conditions lead to resistance
to radiation therapy and chemotherapy. Thus, hyperthermia has been
suggested as an adjunct treatment to enhance the other two treatments.
The differences are fundamental. Radiotherapy chiefly affects cells in
mitosis (cell division), while hyperthermia is most effective during the
DNA synthesis phase. Heat impairs recovery from sublethal radiation
damage. When heat and radiation are given together, or heat prior to
radiation, there is thermal enhancement of radiation damage to both normal
tissue and tumors. However, if radiation therapy is given prior to
hyperthermia, thermal enhancement of radiation damage for tumors is
greater than normal tissue.
Despite the ability to reasonably define radiation fields and the
availability of accurate radiation dosimetry, damage to normal structures,
which cannot be avoided, result in dose limiting factors in radiation
therapy. Thus, the avoidance of significant hyperthermia to adjacent
normal structures is critical for hyperthermia to become a useful adjunct
to radiation therapy.
Recent clinical studies support the proposition that radiation therapy and
hyperthermia can be combined effectively. In addition, both in vivo and in
vitro experiments show that the effects of chemotherapy are also enhanced
by hyperthermia. This enhancement may be due to increased membrane
permeability at higher temperatures (drugs get into cells more easily) and
inhibition of repair mechanisms for drug induced cellular damage. Since
chemotherapy is given to the entire body, precise localization of
hyperthermia is again essential in combination with chemotherapy to avoid
significant damage to normal tissues.
A practical hyperthermia applicator must comply with the following
criteria:
1. In order to treat tumors in all areas of the body, depth of penetration
is essential. The major limitation to many promising hyperthermia
techniques is the inability to achieve high temperatures in deep
structures.
2. The applicator must have the ability to focus hyperthermia and
quantitate absorbed heat in all areas of the tumor. Studies have shown
that very high temperatures (approximately 50.degree. C.) are most
effective in cases where this temperature can be achieved. Methods that
rely on temperature focusing, rather than on the ability of normal tissues
to dissipate heat, allow these temperatures to be achieved.
3. The temperature throughout the tumor should be well-defined and uniform.
The development of relative cool spots in a non-homogenous tumor may
result in failure of cell kill and selection of cells with thermal
tolerance (resistance to hyperthermia) within that area. Small differences
in temperature may produce large differences in cell kill.
The above criteria lead to the following requirements, which, if fulfilled,
will allow the accurate measurements needed to develop dose response to
therapy relationships which are necessary to provide uniform treatment for
all patients and evaluation of clinical studies.
1. The technique should enable induction of hyperthermia to a well-defined
volume. The fall-off of temperature beyond the tumor volume should be
steep.
2. The level of hyperthermia should be precisely controllable.
3. Temperature distribution within the tumor volume should be uniform at
therapeutic levels.
4. It should be possible to control the heat transferred in different
regions of the tumor volume.
5. The therapist and not the changing characteristics of the heated tumor
should control the temperature within the tumor volume, to avoid
overheating a necrotic liquefied tumor center or underheating a well
vascularized growing tumor edge.
6. In addition, one should be able to accurately monitor temperature.
Applying these criteria to existing hyperthermia devices, reveals that
while some devices have advantages in some areas, all have limitations.
Non-invasive hyperthermia applicators, such as ultrasound and
electromagnetic radiation, are easier to use than invasive techniques, but
are limited in depth of penetration. Ultrasound has poor penetration in
bone and air. External microwave beam heating is limited by a shallow
depth of penetration and the development of standing waves, creating hot
and cold spots.
Consequently, more recently, investigations have been conducted into the
feasibility of using invasive applicators in the form of small diameter
microwave antennas or probes as a means of producing local hyperthermia in
cancerous tissue. In this form of therapy, antenna probes are inserted
into the body through the esophagus or rectum, or directly into a tumor
using a hollow plastic catheter.
Typically, these probes comprise a quarter-wavelength monopole antenna with
frequencies in the 500 MHz to 3 GHz range. These antennae are referred to
by workers in the hyperthermia field as a dipole, or a sleeve dipole. A
folded back quarter wave choke forms one-half of the antenna length (S.
Silver, "Microwave Antenna Theory and Design", Dover Publication, Chapter
8, p 241 and Electromagnetics, Vol. 1, No. 1, January-March 1981, p 58).
The latter more nearly approximates a dipole antenna pattern.
These prior art dipole antennae suffer from a number of shortcomings, such
as, poor impedance matching; high sensitivity to changes in the length of
penetration of the probe into the body; poor uniformity in electric field
and heating patterns produced; and lack of beam steering and heat sensing
capabilities. J. W. Strohbehn, et al., "An Invasive Microwave Antenna for
Locally-Induced Hyperthermia for Cancer Therapy", Journal of Microwave
Power, 14 (4), 1979, pp 339-350; D. C. deSieyes, et al., "Optimization of
an Invasive Microwave Antenna for Local Hyperthermia Treatment of Cancer",
Thayer School of Engineering, Dartmouth College, July 7, 1980; J. W.
Strohbehn, et al., "Evaluation of an Invasive Microwave Antenna System for
Heating Deep-Seated Tumors", presented at the Third International
Symposium: Cancer Therapy by Hyperthermia, Drugs and Radiation, Fort
Collins, Colo., June 22-26, 1980.
DISCLOSURE OF THE INVENTION
The invention comprises an invasive hyperthermia applicator for generating
a well-defined uniform pattern of electromagnetic radiation, preferably in
the microwave frequency spectrum of 500 megahertz to 5 gigahertz, to
produce well-defined temperatures throughout a tumor. The applicator is in
the form of an elongate tubular member forming a collinear array of
antennae fabricated from a common coaxial transmission line. In the
preferred embodiment of the invention, a plurality of antennae, i.e.,
three, are formed from the coaxial transmission line by forming
circumferential gaps in the outer conductor of the transmission line.
These gaps serve as antenna feeds for subsequent antenna elements. The
applicator has a proximal section and a distal section. The three
collinear antennas are located in the distal section and a coaxial
impedance matching transformer is provided in the proximal section in the
form of a circumferential volume of dielectric material with a conductor
external to the outer coaxial line.
Progressing from the proximal section to the distal section, a first three
half wavelength collinear antenna is provided. Then, two half wavelength
antennas are provided, which are harmonically related to the three half
wavelength antenna. The combined near field radiation pattern of the three
antenna array produces a uniform temperature pattern along the distal
section of the applicator at a frequency which produces a minimum in the
reflected power inside the coaxial transmission line.
The circumferential gaps in the outer conductor of the transmission line
result in a voltage being generated across the gap. The gap voltage
excites antenna currents that flow on the outer conductor of the
transmission line, while the power flow inside the coaxial transmission
line through the center conductor, continues down the line to the next
feed gap. The gaps thereby result in a radiation aperture. Preferably, the
gap spacings from the distal end of the array are multiples of one-half of
the antenna current wavelength. A standing wave of antenna current
develops along the entire distal end of the array.
An outer plastic sheet or coating of dielectric material is provided around
the collinear array, which may optionally include embedded therein, fiber
optic sensor bundles, for measuring the temperature at the delivery site.
Optionally, a sleeve of lossy, i.e., electromagnetic energy absorbing
material, such as ferrite, may be disposed around the outer plastic sheet
or coating at a predetermined location or locations along the distal
length of the antenna array. Some of the electromagnetic energy radiated
by the antenna is strongly absorbed by the ferrite sleeve which becomes
hot and provides a source of localized heat for heating a tumor site. This
heat source is independent of the electromagnetic energy absorbing
properties of the tumor and produces hyperthermia range temperatures with
significantly less power input than purely electromagnetic energy
radiation permits. This sleeve tends to diminish the severity of the
temperature gradient close to the antenna and thus permits more uniform
temperatures near the antenna.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view of the applicator of the present invention
showing in solid lines a cross-section of one-half of the far field
antenna array pattern resulting from each of the three antenna elements
and in dotted lines the near field heating pattern resulting from the
super position of the electromagnetic energy pattern generated by the
three antenna elements.
FIG. 2 is a cross-sectional view through the applicator 10 of FIG. 1 along
the lines 2--2.
FIG. 3 is a cross-sectional view of the applicator of FIG. 1 along the
lines 3--3.
FIG. 4 is a cross-sectional view of the applicator of FIG. 1 along the
lines 4--4.
FIG. 5 is a cross-sectional view of the applicator of FIG. 1 taken along
the lines 5--5. FIG. 6 is an illustration of an insulated dipole in an
ambient medium used to depict the algebraic parameters needed for
calculating the optimum transformer parameters. FIG. 7 is a plot of
frequency versus power ratio in decibels for the applicator of the
invention. FIG. 8 is a side view of an optional embodiment of the
invention wherein a lossy sleeve 80 is provided over a selected portion of
the applicator.
FIG. 9 is a cross-sectional view of an alternate embodiment of the
outermost end of the applicator 10 of FIG. 1.
FIG. 10 is an enlarged fragmented view of FIG. 9.
FIG. 11 is a cross-sectional exploded view of a flexible coaxial connector
adaptor system 60 for use with the applicator of FIG. 1 shown in the
process of being assembled.
BEST MODE OF CARRYING OUT THE INVENTION
I. Applicator
Referring to FIGS. 1-4, the invention will now be described in detail in
connection therewith. An applicator 10, for uniform heating of a tumor
with well-defined temperatures throughout the tumor and without hot spots
either within or outside the tumor volume, is shown in the form of a
collinear array of three antennae fabricated from a coaxial transmission
line comprising inner conductor 20 and outer coaxial conductor 16 with an
impedance matching element 26.
The three antennae are formed by providing circumferential gaps 5 in the
outer conductor 16 to expose the dielectric core 18 of the transmission
line structure. Preferably, the widths of the gaps 5 are about the same
size as the distance between center conductor 20 and outer conductor 16.
Core 18 may comprise a suitable solid dielectric insulator, such as PTF
(polytetrafluorethylene). The gaps 5 provide excitation feeds for more
remote, i.e., more distal end, antenna sections and result in the
equivalent of more than one antenna pattern being generated from the
length of the center conductor. The electrical lengths of these antenna
sections are harmonically related to each other.
A dielectric outer envelope 14, containing fiber optic sensors 24, extends
over the outer surface of the applicator 10. For antenna beam steering
purposes, a resistor 22 is provided at the longitudinal axis of the
applicator. In accordance with the theoretical and experimental teaching
of Altschuler ("The Traveling-Wave Linear Antenna", E. E. Altschuler,
Cruft Laboratory, Harvard University, Cambridge, Massachusetts Scientific
Report No. 7, May 5, 1960), an essentially traveling-wave distribution of
current can be produced on a linear antenna by inserting a resistance of
suitable magnitude one-quarter wavelength from the end of the antenna. As
shown in FIG. 21 from the above-cited reference, the effect of such
resistance is to significantly change the radiation pattern of the antenna
and therefore, in the present application, its heating pattern for
hyperthermia. The collinear array applicator 10 of the present invention
is therefore provided with the appropriate value of resistance about
one-quarter wavelength from the end of the distal section. By changing the
applied frequency, or the location of the resistor, the distribution of
heat around the applicator may therefore be changed or "steered" in many
directions.
At the proximal end of the antenna array 10, a coaxial impedance matching
transformer is provided in the form of a dielectric cylinder 26 concentric
with and external to the outer conductor 16. The dielectric cylinder 26 is
covered with a metallic cylinder 27 which is electrically shorted to outer
conductor 16 at proximal end A. A dielectric outer envelope 14 extends
over the full length of cylinder 27 and distal section B-E. The
transformer minimizes the reflected power within the feed transmission
line and also prevents leakage of antenna currents along the outside of
the array applicator 10. By judicious selection of operating parameters,
both functions (minimizing reflected power and leakage prevention) occur
at approximately the same operating frequency. The operating parameters of
the coaxial impedance matching transformer are based on the theoretical
equations developed by R. W. P. King, "The Electromagnetic Field of an
Insulated Antenna in a Conducting or Dielectric Medium", R. W. P. King et
al., IEEE Transactions on Microwave Theory and Techniques, Vol. MIT-31,
No. 7, July 1983.
The transformer provides a load impedance at the proximal end of the
collinear arrays for R.F. power coupled from source 12 via lines 30 and 32
across the inner and outer conductors 20 and 16. This load impedance
regulates the antenna current at the feed points or gaps 5 to more nearly
match the 50 ohm impedance of the feed transmission line 30 and 32 with
the input impedances of the collinear array 10. The distal section of
applicator 10 of FIG. 1 has an overall length B-E of 10 centimeters at a
frequency of 915 megahertz. This length is a multiple of one-half of the
wavelength of the input frequency, (i.e. 5, .lambda..sub.L /2 sections)
and is physically represented by a full-wave linear antenna (C-E) series
connected to a three-halves wave linear antenna (B-C). This arrangement of
antennae provides a uniform heating pattern shown in the dotted lines
labelled B4 of FIG. 1.
Note that heating pattern B4 is one-half of a plane cut through the full
cylindrical near field heating pattern extending from array 10, which is
related to the superposition of the three individual far field antenna
patterns, B1, B2 and B3, shown in solid lines. If a shorter antenna array
is desired, the frequency may be doubled and the length halved.
Alternatively, for the same frequency, section C-D can be removed to
reduce the length to 8 cm or section B-C can be removed to reduce the
length to 4 cm.
II. Theory of Operation
In operation, as the transmitted power from source 12 flows down the
coaxial line, formed by inner and outer conductors 20 and 16 separated by
dielectric 18, voltage excites each antenna section and electromagnetic
energy is radiated from the applicator which is absorbed by the lossy
tissue. The absorbed energy reduces the amplitude of the transmitted
power. By increasing the number of elements at the distal end of the array
(and decreasing the spacing between elements), a higher sectional antenna
gain is achieved, as compared to the more proximal section B-C, which will
have a lower gain because it is a single (3.lambda./.sub.2) element.
More specifically, the square of the electric field for the
half-wavelength.sup.(1), full wavelength linear.sup.(2) and 3/2
wavelength.sup.(3), antennas in free space, shown below, provides an
indication of the radiated power distribution for the collinear array in
lossy material (J. D. Jackson, "Classical Electrodynamics", J. Wiley,
1975, Second Edition, pps 402-403):
##EQU1##
wherein .theta. is measured from the longitudinal axis of the antenna.
The full wave antenna, distribution (C-E) can be considered as resulting
from the coherent superposition of the fields of two collinearly adjacent
half-wave antennae patterns B.sub.2 and B.sub.3 excited in phase; the
power intensity at .theta.=.pi./.sub.2 is 4 times that of a half-wave
length antenna and 4 times that of a three half wave length antenna. Thus,
the extreme distal section (C-E) of two series connected half wave
antennas radiates 6 dB more power per solid angle than the three half wave
length section (B-C). Based on geometric reasoning, the total power
radiated by the three half wave length antenna is about 60% of the total
power delivered to the array (6 cm length compared with 4 cm length).
Therefore, forty percent is left over for radiation by the series
connected half wave antennae (C-E). The 6 dB gain of the 3.lambda./.sub.2
section compensates for this loss. The result is a nearly uniform heating
pattern along the entire 10 cm length of the distal section B-E of arrav
applicator 10.
III. Manufacturing Process
Preferably, the collinear array applicator 10 is fabricated using standard
AWG (American Wire Gauge) solid or stranded tin plated copper wire (AWG 26
for example) for inner conductor 20. The existing insulation of the copper
wire may be increased in diameter by means of a thin wall plastic tube of
PTF to form core 18. The outer surface of the tube or core 18 is coated
with a conductive ink or paint, such as silver, to provide the outer
conductor 16 of a two conductor 50 ohm transmission line system. Etching
of the tube may be required to insure adhesian of the silver paint. The
gap location 5 are not covered with the conductive ink because they are
masked off during the paint application process. A uniform PTF coating 14
is then applied over the entire distal section B-E. The proximal section
A-B is formed in a similar manner, except that prior to application of
coating 14, a dielectric sleeve or coating 26 of appropriate dielectric
constant and loss tangent, is placed around the conductive ink 16 located
at the proximal section. The dielectric material may preferably be
polyacrylamide (See "The Polyacrylamide as a Phantom Material for
Electromagnetic Hyperthermia Studies", M. G. Bini, et al., IEEE
Transactions of Biomedical Engineering, Vol. BMD-31, No. 3, March 1984)
from which the appropriate dielectric constant may be calculated for the
proper transformer operation using the criterion that the complex
propagation constant, k.sub.L of the transformer dielectric is the same as
the k.sub.L of the distal section. A uniform silver ink coating is then
applied over the polyacrylamide material to form a second conductive layer
27. This second conductive layer 27 is present only over the length of the
proximal section It is applied in a manner to create a short circuit to
the silver ink outer conductor 16 at proximal end A but leaves an open
circuit between it and the outer conductor 16 at point B. The outer PTF
coating 14 is then applied over the proximal section A-B or continued from
the distal section.
This coating 14 permits the probe to operate within wide limits of
variations of temperature, tissue dielectric constant and electrical
conductivity. A 10 mil thick coating of PTF permits the array to maintain
a constant heating pattern (ignoring the effects of heat loss or gain by
conduction or convection) for a change in the dielectric constant of
tissue from 30 to 80 which may occur during heat application.
Within the dielectric coating 14, fiberoptic thermometry sensors 24 may be
embedded. A sensor, such as produced by the Luxtron Corporation
("16-Channel Fiberoptic Thermometry System with Multisensor Arrays for
Thermal Mapping", Wickersheim et al.) may be appropriately modified for
application to the array 10. Several linear phosphor sensors 24 about 0.25
mm in diameter (10 mils) may be embedded in the outer dielectric 14. The
phosphor sensors 24 utilize the temperature dependence of the fluorescent
decay time of the phosphor to determine temperature.
This technique yields a simple, cost-effective multichannel system, which
can support a number of small-diameter multi-sensor arrays.
IV. Load Impedance/Transformer Length and Dielectric Constant
To determine the required value of the load impedance, the proper length of
the transformer and its dielectric constant are theoretically determined
from the complex propagation constant k.sub.L associated with the current
on the antenna, in the manner described below in connection with FIG. 6.
Consider a simple insulated dipole, FIG. 6, consisting of a central
conductor (Region 1) with the half-length "h" and radius "a" surrounded by
a cylinder of dielectric which may consist of one (Region 2) or two layers
(Region 3)*, with the outer radii "b" and "c", respectively. Outside this
insulating sheath is the infinite ambient medium (Region 4) which is a
lossy or dielectric. The central conductor is sufficiently highly
conducting to be well approximated by a perfect conductor. The wavenumbers
of the dielectric layers are:
k.sub.2 =.omega.(.mu..sub.0 .epsilon..sub.2).sup.1/2 and k.sub.3
=(.omega..sub.0 .epsilon..sub.3).sup.1/2,
where .epsilon..sub.2 and .epsilon..sub.3 are the relative dielectric
constants of regions 2 and 3, respectively, and are taken to be real since
the dielectrics actually used are highly nonconducting and .mu.=relative
permeability of free space and .omega.=the radian frequency. The
wavenumber of the lossy dielectric ambient medium is:
k.sub.4 =.beta..sub.4 +i.alpha..sub.4 =.omega.(.mu..sub.0
.epsilon..sub.4).sup.1/2, .epsilon..sub.4 =.epsilon..sub.4 +i.sigma..sub.4
/.omega.;
wherein .beta.=the phase constant in radians/meter; .alpha.=the attenuation
constant in Nepers/meter and .sigma.=the electrical conductivity in
Siemens/meter.
*For simplicity, only Region 2 is shown in FIG. 6.
The general theory of the insulated antenna applies when the wavenumber of
the ambient medium is large compared to that of the insulating sheath and
the cross-section of the antenna is electrically small. That is
.vertline.k.sub.4 /k.sub.2 .vertline..sup.2 >>1;.vertline.k.sub.4 k.sub.3
.vertline..sup.2 >>1;(k.sub.2 b).sup.2 <<1;(k.sub.3 c).sup.2 <<1. (1)
Subject to these conditions and with the time dependence e.sup.-iwt, the
current in the central conductor is
##EQU2##
where admittance is:
Y.sub.o =-(i/2Z.sub.c)tan k.sub.L h. (2b)
For a dielectric with two layers:
##EQU3##
wherein H.sub.o.sup.(1) (k.sub.4 c) and H.sub.1.sup.(1) (k.sub.4 c) are
zero and first order Hankel functions of the first king.
These formulas can be simplified by the introduction of an effective
wavenumber K.sub.2e and an effective permittivity .epsilon..sub.2e for an
equivalent dielectric composed of a single layer with the outer radius c,
viz,
##EQU4##
With (5), the above formulas become
k.sub.L =k.sub.2e [ln(c/a)+F].sup.1/2 [ln(c/a)+n.sub.24.sup.2 F].sup.-1/2(
6)
Z.sub.c =(.mu..sub.o k.sub.L /2.pi.k.sub.2.sup.2)[ln(c/a)+n.sup.2.sub.4
F](7)
where n.sub.2e4.sup.2 =k.sub.2e.sup.2 /k.sub.4.sup.2.
Equation (3) is the complex wave number for current on the surface of
cylindrical structures embedded in electrically lossy media, such as
tumors.
The input impedance of the bifurcated coaxial line matching transformer is
given on page 59 of reference "Embedded Insulated Antennas for
Communication and Heating" by R. W. P. King et al., Electromagnetics, Vol.
1, Number 1, January-March 1981. The phase constant of the dielectric
inside the transformer must match with .beta..sub.L and .beta..sub.L d
.congruent..pi./2 gives the required length of the transformer.
.beta..sub.L is the real part of k.sub.L of Equation 6. The transformer
length is the length of the proximal section. Proper impedance matching of
the collinear antenna array is therefore dependent on the value of
k.sub.L. For the proper choice of dielectric inside the transformer and
length of transformer, a high value of impedance will exist at the input
(Section B). This will effectively isolate the array from the feed line
and, with the proper location of the input of the transformer from gap 5,
give a collinear array which is properly matched to the 50 ohm feed line.
FIG. 7 shows the ratio of reflected power (P.sub.r) to transmitted power
(P.sub.t) in decibels in the coaxial line for a 10 cm long, 3 gap,
collinear array of 2 millimeter diameter made in accordance with the
invention. The frequency fo is the frequency which yields the highest
value of terminating impedance for the array wherein the elements of the
array are harmonically related. For the 10 cm device in the example, the
collinear array that achieves the uniform heating pattern consists of the
elements depicted in the distal section B-E of FIG. 1, wherein the
frequency is 915 megahertz. The transformer length is about 1 centimeter
with a PTF dielectric inside the transformer, having a dielectric constant
of 40.
As shown in the optional embodiment of FIG. 8, a lossy sleeve 80 comprised
of ferrite cores or beads formed in the shape of a cylinder with an inner
bore may be disposed about the applicator 10 at the distal end thereof.
Preferably, the inner diameter of the bore in sleeve 80 forms a press fit
with the outer diameter of the applicator 10 and is held in place along
the longitudinal length of the applicator by a suitable adhesive or other
means. The sleeve may be used to modify the heat distribution near and
around the applicator 10. We have found that by placing ferrite sleeves 80
around the antenna 10, a significant increase in heat close to the antenna
and adjacent to the sleeve is produced for the same power level into the
collinear antenna array applicator 10 as compared to an applicator without
the sleeve. The electromagnetic fields generated by the antenna applicator
10 produce currents in the ferrite material of sleeve 80 and the resultant
heat is transferred by conduction to the surrounding tumor. Significantly
less input power to create hyperthermia range temperatures near the
applicator (40.degree.-44.5.degree. C.) is required (1-2 watts as compared
to 5 watts). The ferrite sleeve 80 therefore creates a source of heat for
the tumor that is not dependent on the electrical properties of the tumor.
Use of this sleeve provides a source of highly localized heat without
requiring an electromagnetic energy absorption capability of tissue. The
sleeve may also be used in conjunction with electromagnetic power
dissipation in tissue to provide complex heat distribution patterns that
conform to the tumor geometry. The Curie temperature of ferrite material
determines the upper temperatures beyond which the material becomes
non-magnetic and hence non-lossy. By selecting an appropriate Curie
temperature for the ferrite sleeve, an upper limit on the temperature
produced by the sleeve can be established.
An alternate embodiment for the extreme distal end of the applicator is
shown in FIGS. 9 and 10 wherein like items in FIG. 1 retain their numeral
reference in FIGS. 9 and 10. As may be seen more clearly in FIG. 10, in
this embodiment, the outer conductor 16 of the antenna array is terminated
by a radially inwardlv extending ring shown as section 16a and 16b. A beam
steering resistor 22 may be disposed along the longitudinal axis of the
antenna in the path of inner conductor 20, as shown. Alternatively, an
equivalent beam steering resistor 21 may be formed as a circular ring
embedded in outer insulator 14.
The inner walls of ring sections 16a and 16b are insulated from resistor 22
or (in the event resistor 22 is not present) from inner conductor 20 by
dielectric disk 62. The inner conductor is extended radially from the
longitudinal axis by disk-like conductor member 18c which is integral with
coaxial conductor 18a encased in dielectric 14.
V. Flexible Coaxial Connector
The collinear applicator array 10 may be connected to a commercially
available coaxial cable, as shown in FIG. 11, by means of a flexible
coaxial connector adaptor 60. This type of connector will eliminate the
need of using expensive commercially available SMA connectors. In
addition, the size of SMA connectors may be excessive in diameter for
certain application, thereby creating the need of a special connector
whose diameter will conform to the diameter of the collinear applicator.
As shown in FIG. 11, the adaptor comprises a laminated metal conductive
ring 40 or ferrule having an inner diameter conforming to the outer
diameter of the outer conductor 16 of applicator 10 affixed around the
outer conductor. The outer conductor 16, dielectric core 18 and inner
conductor 20 of applicator 10 is allowed to extend longitudinally outward
from the proximal end of the applicator, with the core 18 extending beyond
the outer conductor 16 and the inner conductor 20 extending beyond the
core 18. An insulative sleeve 64 is affixed around the extension of core
18. An adaptor pin 42 is secured around the extension of inner conductor
20 to provide an enlarged transition from the outer diameter of inner
conductor 20 to the outer diameter of standard coaxial cable inner
conductors. For example, the O.D. of inner conductor 20 is preferably
about 0.010 inches, the O.D. of pin 42 is 0.018 inches and the O.D. of
sleeve 64 is 0.050 inches.
Pin 42 is adapted to be inserted into tapered bore 51 formed within the
inner conductor 50 of a standard SMA cable inner conductor having an O.D.
of 0.045 inches.
Dielectric insulator sleeve 64 is adapted to extend into coaxial channel 53
around inner conductor 50. The metal connector shell of the standard
coaxial line slides over sleeve 64 and abuts ring 40. Conductive plastic
elastomeric extrusion 44 is bonded at one end by conductive epoxy to shell
46 and is held to ring 40 by friction.
Clinical Utilization
The applicator of the invention can be made as described above with an
outer diameter of about 0.050 inches. With this small diameter, it can be
placed almost anywhere within a patient, with or without fiber optics,
using current techniques and equipment, such as endoscopes, CT scanners,
ultrasound imaging systems, and fluoroscopy units.
For example, in the hyperthermia treatment of urinary tract problems,
access to this anatomic system for placement of the applicator could be
obtained by any one of the following commonly practiced procedures:
1. Angiographic techniques for access to arterial or venous components
(using fluoroscopy);
2. Endoscopic techniques for access to the urethera, prostate, bladder,
ureters, and renal pelvis via retrograde cannulation (using fiber optic
endoscopy, i.e., cyrtoscopes);
3. Percutaneous techniques for direct access by way of a so-called
antegrade nonsurgical approach through the flank or back to the renal
pelvis, ureter and bladder (using CT, ultrasound, fluoroscopic or even
endo-urologic equipment).
The currently available state-of-the-art imaging equipment (particularly
ultrasound and CT) allows visualization and direct puncture of masses in
the neck, abdomen, pelvis, and extremities. Under ultrasonic or CT
guidance, long, small diameter needles (18-23 gauge) are easily introduced
through the skin and into superficial or deep lesions. In a similar
manner, the applicator probe 10 could be easily introduced into these
lesions through any number of widely available biopsy needles.
The same techniques and equipment can be used for the relatively
non-invasive (i.e., non-surgical) access and treatment of other anatomical
sites. For example, the gastrointestinal tract, specifically, the biliary
system, is routinely approached by endoscopic means (ERCP-endoscopic
retrograde cannulation of the pancreas), as well as percutaneously by
direct intercostal puncture and catherization of the liver and bile ducts
for diagnosis and treatment of malignant and benign obstructions (due to
hepatic, biliary, pancreatic, and lymph node diseases). Other lesions of
the GI tract, such as in the stomach are now approached through
gastroscopy. The relatively large size of the endoscope easily allows
passage of a probe of the present size.
The small OD size of this probe, moreover, lends itself to intraoperative
use, as is now being performed with small ultrasound probes in certain
neurosurgical procedures.
Brain tumors are a potential area for application of t | | |
