Abstract:
A thermal therapy catheter for preferentially treating tissue adjacent to a body lumen includes a catheter shaft that is insertable into the body lumen. An energy-emitting element is carried by the catheter shaft, and is operable to radiate a generally symmetrical energy pattern. The catheter shaft includes a plurality of cooling lumens around the energy-emitting element, configured for circulation of a fluid therethrough. An attenuating element is located in at least one of the plurality of cooling lumens and is arranged to modify the generally symmetrical energy pattern radiated by the energy-emitting element to deliver an asymmetrical energy pattern to the tissue adjacent to the body lumen.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims priority from Provisional Application No. 60/281,891 filed Apr. 5, 2001 for “Thermal Treatment Catheter Having Preferential Asymmetrical Heating Pattern” by D. Just, E. Rudie, J. Flachman, S. Stockmoe, A. Hjelle, B. Ebner, J. Crabb and S. Kluge. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a thermal treatment catheter, and more particularly to a catheter having a thin outer wall and a defined fluid flow path within the outer wall to improve the effects of conductive cooling of the wall of the body conduit in which the catheter is inserted. The catheter of the present invention also incorporates a microwave energy-attenuating strip within the catheter which serves to attenuate microwave energy generated by the catheter in the direction of non-treatment tissues. 
     The prostate gland is a complex, chestnut-shaped organ which encircles the urethra immediately below the bladder. Nearly one third of the prostate tissue anterior to the urethra consists of fibromuscular tissue that is anatomically and functionally related to the urethra and the bladder. The remaining two thirds of the prostate is generally posterior to the urethra and is comprised of glandular tissue. The portion of the urethra extending through the prostate (i.e., the prostatic urethra) includes a proximal segment, which communicates with the bladder, and a distal segment, which extends at an angle relative to the proximal segment by the verumontanum. 
     Although a relatively small organ, the prostate is the most frequently diseased of all internal organs and is often the site of a common affliction among older men, benign prostatic hyperplasia (BPH), as well as a more serious affliction, cancer. BPH is a nonmalignant, bilateral expansion of prostate tissue occurring mainly in the transition zone of the prostate adjacent to the proximal segment of the prostatic urethra. As this tissue grows in volume, it encroaches on the urethra extending into the region of the bladder neck at the base of the bladder. Left untreated, BPH causes obstruction of the urethra which usually results in increased urinary frequency, urgency, incontinence, nocturia and slow or interrupted urinary stream. BPH may also result in more severe complications, such as urinary tract infection, acute urinary retention, hydronephrosis and uraemia. 
     Benign prostatic hyperplasia (BPH) may be treated using transurethral thermal therapy as described in further detail in U.S. Pat. No. 5,413,588 entitled DEVICE AND METHOD FOR ASYMMETRICAL THERMAL THERAPY WITH HELICAL DIPOLE MICROWAVE ANTENNA and in U.S. Pat. No. 5,575,811 entitled BENIGN PROSTATIC HYPERPLASIA TREATMENT CATHETER WITH URETHRAL COOLING, both of which are hereby incorporated by reference. During transurethral thermal therapy, the transition zone of the prostate is heated to necrose the tumorous tissue that encroaches on the urethra. Transurethral thermal therapy is administered by use of a microwave antenna-containing catheter which includes a multi-lumen shaft. The catheter is positioned in the urethra with the microwave antenna located adjacent to the hyperplastic prostatic tissue. Energization of the microwave antenna causes the antenna to emit electromagnetic energy which heats tissue within the prostate. A cooling fluid is circulated through the catheter to preserve tissue such as the urethral wall between the microwave antenna and the target tissue of the prostate. 
     The commercially available Targis™ system from Urologix, Inc. of Minneapolis, MN employs a thermal therapy catheter that embodies the aforementioned U.S. Pat. No. 5,413,588, and is a product capable of performing thermal therapy of the prostate with microwave energy delivered from an applicator positioned in the urethra. The Targis™ system has achieved substantial clinical and commercial success, indicating the efficacy of microwave thermal therapy for treating prostate disease. The success of the Targis™ microwave thermal therapy system has led to continuing development efforts in the technology of thermal therapy catheters to further enhance the effects of microwave treatment of the prostate. One such development is disclosed in U.S. Pat. No. 6,161,049, entitled “THERMAL THERAPY CATHETER” by E. Rudie, S. Stockmoe, A. Hjelle, B. Ebner and J. Crabb, which is hereby incorporated by reference. A further development is the subject of the present invention. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is a device and method for treating tissue adjacent to a body lumen such as a urethra. A catheter shaft having an outer surface is insertable into the body lumen, and the catheter shaft carries an energy-emitting element. The energy-emitting element is operable to radiate a generally symmetrical energy pattern. A plurality of cooling lumens in the catheter shaft around the energy-emitting element are configured for circulation of a fluid therethrough. An attenuating element is located in at least one of the plurality of cooling lumens. The attenuating element serves to attenuate energy in the direction of the non-treatment tissue, creating a radially asymmetrical thermal pattern in the tissue adjacent to the body lumen and thereby providing the capability to protect a designated region of healthy tissue from damaging amounts of thermal energy while permitting an increased depth of treatment of targeted tissues. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a vertical sectional view of a male pelvic region showing the urinary organs affected by benign prostatic hyperplasia. 
     FIG. 2 is a side view of the distal end of a thermal therapy catheter. 
     FIG. 3 is a section view of the proximal end of a thermal therapy catheter. 
     FIG. 4 is a section view of an intermediate portion of a thermal therapy catheter. 
     FIG. 5 is a section view of a thermal therapy catheter, taken along line  5 — 5  of FIG.  4 . 
     FIG. 6 is a section view of a thermal therapy catheter, taken along line  6 — 6  of FIG.  3 . 
     FIG. 7 is a diagram illustrating the flow path of cooling fluid through the multi-lobe balloon of a thermal therapy catheter. 
     FIG. 8 is a perspective view of the testing system used to measure the temperature distribution of the thermal therapy catheter of FIGS. 5-6 and of the present invention. 
     FIG. 8A is an enlarged a section view of the testing system taken along line  9 — 9  of FIG.  8 . 
     FIG. 9 is a graph showing the temperature of phantom tissue due to operation of the thermal therapy catheter of FIGS. 5-6 as a function of radial distance from the catheter, taken at 5 minutes into a testing procedure using the testing system of FIG.  8 . 
     FIG. 10 is a graph showing the temperature of phantom tissue due to operation of the thermal therapy catheter of FIGS. 5-6 as a function of radial distance from the catheter, taken at 10 minutes into a testing procedure using the testing system of FIG.  8 . 
     FIG. 11 is a graph showing an axial distribution of temperature in phantom tissue relative to the energy-emitting element of the thermal therapy catheter of FIGS. 5-6 during operation of the catheter, taken at 5 minutes into a testing procedure using the testing system of FIG.  8 . 
     FIG. 12 is a graph showing an axial distribution of temperature in phantom tissue relative to the energy-emitting element of the thermal therapy catheter of FIGS. 5-6 during operation of the catheter, taken at 10 minutes into a testing procedure using the testing system of FIG.  8 . 
     FIG. 13 is a longitudinal sectional view of the thermal therapy catheter of the present invention. 
     FIG. 14 is a cross-sectional view of the thermal therapy catheter of the present invention, taken along line  10 — 10  of FIG.  13 . 
     FIG. 15 is a graph showing the temperature of phantom tissue due to operation of the thermal therapy catheter of FIGS. 13-14 as a function of radial distance from the catheter, taken at 5 minutes into a testing procedure using the testing system of FIG.  8 . 
     FIG. 16 is a graph showing the temperature of phantom tissue due to operation of the thermal therapy catheter of FIGS. 13-14 as a function of radial distance from the catheter, taken at 10 minutes into a testing procedure using the testing system of FIG.  8 . 
     FIG. 17 is a graph showing an axial distribution of temperature in phantom tissue relative to the energy-emitting element of the thermal therapy catheter of FIGS. 13-14 during operation of the catheter, taken at 5 minutes into a testing procedure using the testing system of FIG.  8 . 
     FIG. 18 is a graph showing an axial distribution of temperature in phantom tissue relative to the energy-emitting element of the thermal therapy catheter of FIGS. 13-14 during operation of the catheter, taken at 10 minutes into a testing procedure using the testing system of FIG.  8 . 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a vertical sectional view of a male pelvic region showing the effect benign prostatic hyperplasia (BPH) has on the urinary organs. Urethra  10  is a duct leading from neck  22  of bladder  12 , through prostate  14  and out orifice  16  of penis end  18 . Benign tumorous tissue growth within prostate  14  around urethra  10  causes constriction  20  of urethra  10 , which interrupts the flow of urine from bladder  12  to orifice  16 . The tumorous tissue of prostate  14  which encroaches urethra  10  and causes constriction  20  can be effectively removed by heating and necrosing the encroaching tumorous tissue. Ideally, a selected volume of tissue of prostate  14  should be necrosed without harming adjacent healthy tissues such as urethra  10 , bladder  12 , ejaculatory duct  24  and rectum  26 . The realization of this objective is enhanced by the microwave antenna-containing catheter of the present invention, which is shown in FIGS. 5-6. 
     FIGS. 2-6 relate to a thermal therapy catheter of U.S. Pat. No. 6,161,049, which is hereby incorporated by reference. FIG. 2 shows a side view of a distal end of catheter  28 . Catheter  28  generally includes multi-port handle  30 , multi-lumen shaft  32 , shaft position retention balloon  34  (FIG.  3 ), connection manifold  35 , cooling system  36 , microwave generating source  38  and thermometry unit  39 . Multi-port handle  30  includes inflation port  40 , urine drainage port  42 , microwave antenna port  44 , cooling fluid intake port  46  and cooling fluid exit port  48 . Ports  40 - 48  communicate with corresponding lumens within shaft  32 . Handle  30  is preferably constructed as a two-piece snap-fit shell, composed of a thermoplastic elastomer or a similar material, with appropriate ports and channels being formed therein for communication with the lumens utilized by the thermal therapy catheter of the present invention. 
     Shaft  32  is connected to handle  30  at shaft distal end  50 . Shaft  32  is a multi-lumen, Foley-type urethral catheter shaft. Shaft  32 , which has an outer diameter of about  18  French, includes outer surface  52 , which is generally circular in cross-section as shown in FIG.  5 . Shaft  32  is both long enough and flexible enough to permit insertion of proximal shaft end  54  through urethra  10  into bladder  12  (FIG.  1 ). In a preferred embodiment, catheter shaft  32  is extruded from a thermoplastic elastomer. Thermoplastic materials are less expensive than medical-grade silicone, and are capable of being thermally processed, thereby obviating the need for adhesive bonding to the silicone, and the relatively long curing times associated therewith. 
     FIG. 3 is a section view of catheter shaft  32  adjacent proximal end  54  of shaft  32 , and FIG. 4 is a section view of an intermediate portion of catheter shaft  32 . Both FIG.  3  and FIG. 4 illustrate multi-lobe balloon  71  in its deflated state, for insertion of catheter  28  into urethra  10 . FIG. 5 is a section view of catheter shaft  32  taken along line  5 — 5  of FIG. 4, and FIG. 6 is a section view of catheter shaft  32  taken along line  6 — 6  of FIG.  3 . Both FIG.  5  and FIG. 6 illustrate multi-lobe balloon  71  in its inflated state, for operating to cool the wall of urethra  10  when microwave antenna  74  is energized. 
     As shown in FIGS. 3-6, shaft  32  generally includes temperature sensing fiber lumen  56 , microwave antenna lumen  58 , urine drainage lumen  60 , balloon inflation lumen  62 , cooling fluid intake lumen  64  and cooling fluid exhaust lumens  66  and  67 . Lumens  56 ,  58 ,  60 ,  62 ,  64 ,  66  and  67  generally extend from distal shaft end  50  to proximal shaft end  54 , and are located within catheter shaft  32  so as to form a catheter wall having uniform thickness throughout the cross-section of shaft  32 , the catheter wall thickness being about 0.008 inches in an exemplary embodiment. Along the length of shaft  32 , temperature sensing fiber lumen  56  communicates through the wall of shaft  32  through a channel formed in the catheter wall to temperature sensing fiber tube  81  attached to outer surface  52  of shaft  32 . Temperature sensing fiber lumen  56 , temperature sensing fiber tube  81  and the channel therebetween are sized to permit insertion of temperature sensing fiber  69  to monitor the temperature of tissue surrounding shaft  32  when it is inserted into urethra  10 . Temperature sensing fiber  69  exits handle  30  through port  44  and is connected through manifold  35  to thermometry unit  39 , which calculates temperature based on the optical information provided by temperature sensing fiber  69 . Temperature sensing fiber lumen  56  has a generally trapezoidal cross-section, and together with the catheter walls on either side between cooling lumens  64  and  67  has an included angle of about 30.5 degrees. Multi-lobe balloon  71  is attached to outer surface  52  of shaft  32 , preferably by thermal welding or a comparable attachment technique such as adhesive bonding, at one or more points on outer surface  52 . Multi-lobe balloon  71  is preferably formed of a thermoplastic film wrapped around shaft  32 , such as a polyurethane blown film in an exemplary embodiment. The construction and operation of multi-lobe balloon  71  is described in more detail with respect to FIG.  5 . 
     In an exemplary embodiment, microwave antenna lumen  58  is located eccentric to the longitudinal axis of shaft  32 , nearer first side  68  of shaft  32  than second side  72  of shaft  32 . In the exemplary embodiment shown in FIGS. 5-6, the center of antenna lumen  58  is offset from the center of shaft  32  towards first side  68  of shaft  32  by 0.007 inches. Antenna lumen  58  is sealed at a proximal end of shaft  32  by plug  70 A. At its distal end, microwave antenna lumen  58  communicates with microwave antenna port  44  (FIG.  2 ). Microwave antenna  74  is permanently positioned within antenna lumen  58  at the proximal end of shaft  32  near balloon  34 . Antenna  74  is positioned within antenna lumen  58  so as to be generally situated adjacent the diseased tissue of prostate  14  when shaft  32  is properly positioned in urethra  10  with retention balloon  34  anchored at bladder neck  22 . Antenna  74  includes wound coils carried at the proximal end of coaxial cable  76 . The distal end of coaxial cable  76  is connected to manifold  35  by a conventional quick-coupling fitting  73 . Coaxial cable  76  communicates with microwave generating source  38  by connection cable  76 A, which is connected between microwave generating source  38  and manifold  35 . In an exemplary embodiment, microwave antenna  74  is an impedance-matched antenna implemented in the manner generally disclosed in U.S. Pat. No. 5,413,588, which has been incorporated herein by reference. It is also preferable for antenna lumen  58  and antenna  74  to have a relatively large radial dimension, about 0.131 inches in an exemplary embodiment, since a larger antenna radius results in lower transmission line losses and also provides greater column stiffness to facilitate insertion of shaft  32  into urethra  10 . More specifically, because microwave antenna lumen  58  is located nearer first side  68  of shaft  32  than second side  70  of shaft  72 , the orientation of shaft  32  in urethra  10  must be controlled to maximize the amount of energy delivered to tumorous tissue and minimize the amount of energy delivered to healthy tissue, such as the rectum, for example. Thus, microwave antenna  74  is designed to effectively transmit 100% of the torque applied to handle  30  on to the tip of shaft  32  at proximal end  54 ; that is, if handle  30  is rotated  20  degrees, the tip of shaft  32  at proximal end  54  also rotates  20  degrees. Microwave generating source  38  produces up to  100  watts of electrical power in an exemplary embodiment, in a frequency range of 902-928 MHZ, within the FCC-ISM standard range of frequencies. When antenna  74  is energized by microwave generating source  38 , antenna  74  emits electromagnetic energy which causes heating of tissue within prostate  14 . 
     In one preferred embodiment of the thermal therapy catheter of the present invention, a tip design may be used at proximal end  54  of catheter shaft  32  as described in U.S. Pat. No. 5,628,770 entitled DEVICES FOR TRANSURETHRAL THERMAL THERAPY, which is hereby incorporated by reference. 
     As shown in FIGS. 5-6, urine drainage lumen  60  is positioned adjacent antenna lumen  58 , between antenna lumen  58  and lobe  71 A of multi-lobe balloon  71 . Urine drainage lumen  60  has a generally trapezoidal cross-section, and together with the catheter walls on either side between cooling lumens  64  and  66  has an included angle of about 30.5 degrees. Urine drainage lumen  60  communicates with urine drainage port  42  of handle  30  at distal shaft end  50  and defines a drainage path for urine when proximal end  54  of shaft  32  is inserted through urethra  10  into bladder  12 . Urine drains from bladder  12  through urine drainage lumen  60  and out urine drainage port  42  when proximal shaft end  54  is inserted within bladder  12 . Drainage of urine from bladder  12  is necessary due to frequent bladder spasms which occur during transurethral thermal therapy. Again, as mentioned above, in one preferred embodiment the tip design disclosed in U.S. Pat. No. 5,628,770, which has been incorporated by reference, may be used with catheter  28  of the present invention. 
     Retention balloon inflation lumen  62  is positioned adjacent antenna lumen  58 , between antenna lumen  58  and lobe  71 B of multi-lobe balloon  71 . Balloon inflation lumen  62  has a generally trapezoidal cross-section, and together with the catheter walls on either size between cooling lumens  66  and  67  has an included angle of about 29 degrees. Balloon inflation lumen  62  communicates with inflation port  40  of handle  30  to allow inflation fluid to flow in and out of balloon inflation lumen  62 , and communicates through aperture  88  to inflate retention balloon  34 . 
     Cooling fluid intake lumen  64  is positioned adjacent to antenna lumen  58 , between antenna lumen  58  and temperature sensing fiber tube  81  between lobes  71 A and  71 C of multi-lobe balloon  71 . Cooling fluid intake lumen  64  has a generally arcuate cross-section, and extends from distal end  50  to proximal end  54  of shaft  32 . Cooling fluid intake lumen  64  receives fluid from cooling system  36  to absorb a portion of the microwave energy emitted by the microwave antenna and thereby control the volume of prostatic tissue that is exposed to necrosing levels of heat. Fluid within cooling fluid intake lumen  64  also absorbs a portion of the heat energy generated by microwave energy from adjacent tissues via thermal conduction to avoid thermal damage to those tissues. In an exemplary embodiment, cooling fluid intake lumen  64  has an included angle of about 90 degrees. 
     Cooling fluid exhaust lumens  66  and  67  are positioned circumjacent to antenna lumen  58 , with cooling fluid exhaust lumen  66  being located generally between antenna lumen  58  and lobes  71 A and  71 B of multi-lobe balloon  71  and cooling fluid exhaust lumen  67  being located generally between antenna lumen  58  and lobes  71 B and  71 C of multi-lobe balloon  71 . Cooling fluid exhaust lumens  66  and  67  have a generally arcuate cross-section, and extend from distal end  50  to proximal end  54  of shaft  32 . Exhaust lumens  66  and  67  provide a return path to cooling system  36  for fluid circulated through intake lumen  64  and multi-lobe balloon  71 . Fluid within exhaust lumens  66  and  67  absorbs a portion of the microwave energy emitted by the microwave antenna and also absorbs a portion of the heat energy generated by microwave energy from adjacent tissues via thermal conduction, in the manner described above. In an exemplary embodiment, cooling fluid exhaust lumens  66  and  67  each have an included angle of about 90 degrees. 
     FIG. 6 is a section view of catheter shaft  32  taken along line  6 — 6  of FIG.  3 . As shown in FIG. 6, at proximal end  54  of shaft  32  cooling lumens  64 ,  66  and  67  communicate with the interior of multi-lobe balloon  71  so as to provide cooling fluid to inflate multi-lobe balloon  71 . Specifically, cooling fluid intake lumen  64  communicates with the interior of lobe  71 A through aperture  64 A and communicates with the interior of lobe  71 C through aperture  64 B. Cooling fluid exhaust lumen  66  communicates with the interior of lobe  71 B through aperture  66 A, and cooling fluid exhaust lumen  67  communicates with the interior of lobe  71 B through aperture  67 A. Cooling fluid intake lumen  64  and exhaust lumens  66  and  67  cooperate with cooling system  36  via ports  46  and  48  of handle  30  to provide a selectively controlled flow of fluid through cooling lumens  64 ,  66  and  67  during a treatment session. In operation, cooling fluid flows from cooling system  36  to cooling fluid feed line  94 B and on through port  46  of handle  30  into cooling fluid intake lumen  64 . The cooling fluid continues to flow under dynamic fluid pressure through apertures  64 A and  64 B to inflate lobes  71 A and  71 C of multi-lobe balloon  71 . Cooling fluid flows from lobe  71 B through apertures  66 A and  67 A into cooling fluid exhaust lumens  66  and  67 , and exits shaft  32  at distal end  50  thereof through port  48  of handle  30 , and on through cooling fluid return line  96 B and manifold  35  to cooling system  36  for re-chilling and recirculation. Cooling fluid feed line  96 B and return line  96 B are each provided with a conventional quick-coupling fitting  65 A and  65 B, respectively, which permits catheter  28  to be easily disconnected from cooling system  36 . In an exemplary embodiment, the cooling fluid is deionized water provided by cooling system  36 , chilled to an appropriate temperature so as to maintain the temperature of tissue immediately surrounding catheter shaft  32  at a predetermined value while power is applied from microwave antenna  74  to heat diseased prostate tissue. A method of controlling coolant temperature and microwave power to maintain a predetermined tissue temperature is disclosed in U.S. Pat. No. 6,122,551, entitled “METHOD OF CONTROLLING THERMAL THERAPY,” which is hereby incorporated by reference. The water is pumped at a rate sufficient to provide dynamic pressure to inflate multi-lobe balloon  71  to create an outer balloon diameter of about 24 French, thereby ensuring excellent wall contact with the urethra and enhancing the efficiency of the conductive cooling performed by the circulating cooling fluid flowing in multi-lobe balloon  71 . 
     FIG. 7 illustrates the pattern of fluid flow through multi-lobe balloon  71  according to an embodiment of the present invention. For the purpose of illustration, multi-lobe balloon  71  is shown in FIG. 7 as “flattened out” in two dimensions; it should be understood that multi-lobe balloon  71  is wrapped around catheter shaft  32  in a final assembly of the present invention, as shown in the cross-sectional views of FIGS. 5 and 6. The cross-hashed regions of balloon  71  indicate where balloon  71  is thermally welded (or otherwise attached) to the catheter shaft, with the patterns of multi-lobe balloon  71  being formed by heat stamping or an alternative processing method. Cooling fluid is circulated into lobe  71 A of multi-lobe balloon  71  through fluid flow aperture  64 A and into lobe  71 C of multi-lobe balloon  71  through fluid flow aperture  64 B. The cooling fluid flows under dynamic pressure in the direction indicated by the arrows, through narrow channels  71 D and  71 E into lobe  71 B of multi-lobe balloon  71 , where the fluid exits through fluid flow apertures  66 A and  67 A into exhaust lumens  66  and  67  of shaft  32 . The fluid flow path provided by the present invention ensures that the cooling fluid circulates under sufficient dynamic pressure to inflate multi-lobe balloon  71  to a sufficient diameter to provide consistent wall contact with the urethra, such as about 24 French in an exemplary embodiment. More complex flow patterns in the lobes of balloon  71  are also contemplated by the present invention, which may be realized by heat stamping and thermal welding processes, or alternatively by adhesive bonding processes, to form the appropriate flow pattern. In addition, multi-lobe balloon  71  may be formed with more than the three lobes  71 A,  71 B and  71 C illustrated in FIGS. 5 and 6, thereby forming modifying the fluid flow pattern and inflation characteristics of balloon  71 . The actual amount of dynamic fluid flow pressure may be controlled by adjusting a number of parameters, such as the rate at which cooling fluid is pumped from the cooling system, the width of channels  71 D and  71 E, the size of fluid flow apertures  64 A,  64 B,  66 A and  67 A, the width of restricted flow areas elsewhere in the fluid flow path, and other parameters that will be apparent to one skilled in the art. In an exemplary embodiment, dynamic fluid pressure is controlled by an adjustable restrictor located in the fluid flow path proximate to cooling system  36 . 
     A thermal therapy catheter as described above is designed to enhance the efficiency of treatment of diseased tissue from an adjacent body lumen, particularly for treatment of diseased prostate tissue from a urethrally inserted applicator. A multi-lobe balloon is attached around the catheter shaft, with interiors of the balloon lobes in communication with cooling lumens of the catheter, so that circulation of fluid in the cooling lumens dynamically inflates the balloon lobes. The radial spacing and shaping of lobes  71 A,  71 B and  71 C are designed to define a fluid chamber that corresponds to the generally triangular cross-sectional geometry of the urethra. As a result, the balloon lobes more readily come into intimate contact with the wall of the urethra, and the cooling fluid circulating in the balloon lobes is thereby able to efficiently conduct heat away from the urethral wall tissue to preserve the urethra while delivering microwave energy to heat prostate tissue to high temperatures (above about 45° C.) for a sufficient time to necrose the targeted prostate tissue. In one embodiment, the balloon wall thickness is about 0.002 inches. In addition, the inflatable nature of the multi-lobe cooling balloon allows the catheter to be easily inserted when the balloon is not inflated (with the catheter shaft having a relatively small diameter of about 18 French) while providing the ability to firmly contact the urethral wall to enhance cooling when the balloon is inflated, up to a diameter of about 24 French in one embodiment. 
     The arrangement and shape of the lumens in the catheter shaft is also designed for efficient operation of the thermal therapy catheter system. As shown in FIGS. 5 and 6, temperature sensing fiber lumen  56 , urine drainage lumen  60  and balloon inflation lumen  62  are all formed with generally trapezoidal cross-sections, so as to minimize the included angle of each of these lumens. As a result, the included angle of cooling lumens  64 ,  66  and  67  is maximized, improving the efficiency of urethral cooling. In addition, the seams which define lobes  71 A,  71 B and  71 C of multi-lobe balloon  71  correspond with cooling lumens  64 ,  66  and  67 , which ensures that sufficient cooling of the urethral wall occurs at the seams of multi-lobe balloon  71  in addition to the inflated lobes of the balloon. Cooling lumens  64 ,  66  and  67  also extend along the entire length of the microwave antenna to provide internal cooling of the catheter and thereby ensure that the thermoplastic material of the catheter shaft is not melted by the resistive heating produced by the antenna and the heating produced by absorption of microwave energy by the catheter walls. 
     Temperature sensing fiber  69  within temperature sensing fiber tube  81  is also strategically placed in the catheter design. Temperature sensing fiber tube  81  is located in the seam between lobes  71 A and  71 C of multi-lobe balloon  71 , so as to minimize its effect on the outer perimeter shape of the catheter. In addition, the location of temperature sensing fiber tube  81  also ensures that cooling lumen  64  is positioned directly between temperature sensing fiber  69  and the microwave antenna positioned in antenna lumen  58 . As a result, the resistive heating produced by the microwave antenna has no appreciable effect on the temperature reading obtained by temperature sensing fiber  69 ; the only variables that affect the temperature reading are the actual temperature of tissue immediately adjacent temperature sensing fiber tube  81  and the temperature of the cooling fluid circulating through cooling lumen  64 . The cooling fluid temperature may be compensated for by the thermometry unit to yield an accurate value for the actual tissue temperature, which is useful information for controlling the thermal therapy procedure. 
     As a result of the above-described catheter design and the efficient cooling of the body lumen wall provided by the multi-lobe balloon, a substantial depth of tissue may be heated above about 45° C. for a time sufficient to necrose the tissue, while protecting the body lumen wall from thermal damage. Under the regulation of an effective control algorithm, such as is disclosed in the aforementioned U.S. Pat. No. 6,122,551, which has been incorporated herein by reference, the catheter design of the present invention is able to necrose a substantial portion of the prostate while controlling temperatures to protect healthy tissues such as the urethral wall and the rectum, with a treatment time of approximately 30 minutes or less and no need for anesthesia. The system therefore offers an attractive therapy option for treating tissue disease such as BPH, with excellent long-term results and a low risk of morbidity or other side effects. 
     FIG. 8 is a perspective view of testing system  98  in which basic performance characteristics concerning energy and temperature distribution patterns capable of being achieved by the catheter of FIGS. 5-6 are demonstrated. As shown diagrammatically in FIG. 8, proximal end  54  of catheter  28  is inserted into a block of gelatinous phantom tissue medium  100  composed primarily of distilled water, ethylene glycol and sodium chloride. Tissue medium  100  is held in place by a cubicle container having six transparent, plastic walls. Catheter  28  is positioned in the tissue medium  100  by vertically inserting catheter  28  through a pre-positioned channel (not shown) in tissue medium  100  so that catheter  28  is located in the center of tissue medium  100 . Four radially movable temperature sensors R 1 , R 2 , R 3  and R 4  are located within four pre-positioned channels (not shown) in tissue medium  100  in a common plane, which is transverse to a vertical plane defined by catheter  28 . Catheter  28  is positioned relative to temperature sensors R 1 -R 4  such that a midpoint of microwave antenna  74  is adjacent to temperature sensors R 1 -R 4 . Temperature sensors R 1 -R 4  are capable of radial movement in unison away from catheter  28 , with a starting position of each sensor being located about 0.5 cm from catheter  28 . Temperature sensors R 2  and R 3  are located on opposite sides of catheter  28  and move radially away from each other. Temperature sensors R 1  and R 4  are also located on opposite sides of catheter  28  and move radially away from each other. Through this configuration, temperature sensors R 1 -R 4  are equally radially spaced from each other around catheter  28 . 
     Four axially movable temperature sensors A 1 , A 2 , A 3  and A 4  are located within four pre-positioned channels (not shown) in tissue medium  100 . Temperature sensors A 1 -A 4  are located adjacent to a longitudinal axis of catheter  28 , and are capable of axial movement in unison along the length of catheter  28 . Each temperature sensor A 1 -A 4  is located an axial distance of 0.5 cm from catheter  28 . Temperature sensors A 1 -A 4  are equally radially spaced around catheter  28  in order to capture temperature readings on discrete sides of catheter  28 . At the start of a testing procedure, temperature sensors A 1 -A 4  are aligned in a common horizontal plane located below proximal end  54  of catheter  28 . During a testing procedure, temperature sensors A 1 -A 4  move axially upwards along the length of antenna  74  of catheter  28 . 
     FIG. 8A shows a cross-sectional view of the testing system of FIG. 8 taken along line  9 — 9 . FIG. 8A shows the orientation of the temperature sensors R 1 -R 4  and A 1 -A 4  relative to catheter  28  and microwave antenna  74 . Temperature sensors R 1 -R 4  are shown in their starting position for a test phase. The directions of movement for temperature sensors R 1 -R 4  are illustrated by the arrows in FIG.  8 A. 
     An exemplary testing procedure using testing system  98  is composed of multiple testing phases, each testing phase lasting approximately 1.2 minutes. Testing phases may be performed at 5 and 10 minutes during a testing procedure, for example. During a testing phase, the movement of the temperature sensors R 1 -R 4  and A 1 -A 4 , and the recording of temperatures in tissue medium  100  are computer controlled by testing system  98 . Before a testing procedure begins, tissue medium  100  typically has a temperature of about 25° C. and catheter  28  typically has a temperature of about 25.3° C. Microwave antenna  74  of catheter  28  is energized to a power level of 30 watts at a frequency of 927 Hertz at the start of a testing procedure. 
     During each testing phase, temperature sensors R 1 -R 4  begin at their start positions 0.5 cm from catheter  28  and move in radial directions shown in FIG. 8A away from catheter  28  to a distance of about 60 mm. As temperature sensors R 1 -R 4  move through tissue medium  100 , the temperature sensors measure the temperature of tissue medium  100 . A temperature recording is taken at 2.5 mm increments in tissue medium  100 . After a testing phase is completed, the temperature sensors R 1 -R 4  are repositioned in their start positions in preparation for the following testing phase. Temperature sensors R 1 -R 4  move at a constant rate of about 0.8 mm per second during each testing phase of the testing procedure. 
     Temperature sensors A 1 -A 4  begin each testing phase at a location proximal of antenna  74 . Throughout a testing phase, temperature sensors A 1 -A 4  maintain a constant radial distance of 0.5 cm away from catheter  28  as temperature sensors A 1 -A 4  move adjacent to the longitudinal axis of catheter  28 . Temperature sensors A 1 -A 4  pass along a section of catheter  28  containing microwave antenna  74  during a testing phase, and the temperature sensors end each testing phase at a location distal of microwave antenna  74 . Temperature sensors A 1 -A 4  record a temperature of tissue medium  100  every 2.5 mm during a testing phase. Once a testing phase is complete, temperature sensors A 1 -A 4  are repositioned in their starting locations in preparation for a following testing phase. Temperature sensors A 1 -A 4  move at a constant rate of about 0.8 mm per second during each testing phase of the testing procedure, in coordination with the movement of sensor R 1 -R 4 . 
     FIGS. 9-10 show temperature recordings of temperature sensors R 1 -R 4  in comparison to the distances of temperature sensors R 1 -R 4  from catheter  28  during a complete testing procedure. FIGS. 9-10 each represent temperature and distance data from a single testing phase. The vertical axes of FIGS. 9-16 correspond to the temperature of tissue medium  100 , and the horizontal axes correspond to the radial distance away from catheter  28  of temperature sensors R 1 -R 4 . In FIGS. 9-10, temperature recordings of R 1  are shown by diamond marks, temperature recordings of R 2  are shown by square marks, temperature recordings of R 3  are shown by triangle marks, and temperature recordings of R 4  are shown by “x” marks. FIGS. 9-10 show that temperature sensors R 1 -R 4  record substantially similar temperatures at similar locations in tissue medium  100  throughout each testing phase. Thus, FIGS. 9-10 illustrate that catheter  28  produces a radially symmetrical temperature distribution in tissue medium  100 . Also, FIGS. 9-10 show that temperature of tissue medium  100  steadily increases during a testing procedure, and that a similar temperature distance pattern is developed during each testing phase. 
     FIGS. 9-10 show the temperature recordings of temperature sensors R 1 -R 4  during a testing phase performed at 5 and 10 minutes, respectively, into the testing procedure. FIGS. 9-10 show an initial temperature increase as temperature sensors R 1 -R 4  move radially away from catheter  28 . After reaching peak temperatures ranging from about 40° C. to 70° C. (with the higher peak temperatures occurring after phantom tissue medium  100  has been exposed to the emitted energy for a longer period of time), temperatures sensors R 1 -R 4  show a steady decrease in temperature as temperature sensors R 1 -R 4  move further away from catheter  28 . FIGS. 9-10 illustrate that temperature sensors R 1 -R 4  are recording generally similar temperatures at similar distances away from catheter  28  for each respective test, with only slightly lower temperatures being measured in tissue adjacent to balloon lobes  71 A and  71 C, as reflected by the curves for sensors R 3  and R 4 . Thus, FIGS. 9-10 show that catheter  28  is generating a substantially uniform temperature distribution radially throughout tissue medium  100  after microwave antenna  74  has been energized. 
     FIGS. 11-12 show the temperature recordings of temperature sensors A 1 -A 4  in comparison to the positions of temperature sensors A 1 -A 4  along the length of catheter  28  during a complete testing procedure. FIGS. 11-12 each represent temperature and distance data from a single testing phase, at 5 and 10 minutes into a testing procedure, respectively. The vertical axes of FIGS. 11-12 correspond to the temperature of tissue medium  100 , and the horizontal axes correspond to the axial distance along catheter  28  of temperature sensors A 1 -A 4 . In FIGS. 11-12, temperature recordings of A 1  are shown by diamond marks, temperature recordings of A 2  are shown by square marks, temperature recordings of A 3  are shown by triangle marks, and temperature recordings of A 4  are shown by “x” marks. FIGS. 11-12 show that temperature sensors A 1 -A 4  record substantially similar temperatures at similar locations along catheter  28  in tissue medium  100  throughout each testing phase. Thus, FIGS. 11-12 illustrate that catheter  28  produces a generally radially symmetrical temperature distribution along catheter  28 , with the intensity of energy and therefore the measured temperature increasing as a function of time. 
     FIGS. 9-10 display the temperature data recorded by temperature sensors R 1 -R 4  throughout a complete testing procedure. Note that FIGS. 9-10 show a similar pattern for temperature variation (a gradual increase and then a decrease) as temperature sensors R 1 -R 4  move radially away from catheter  28 . Also note that temperature sensors R 1 -R 4  are recording similar temperatures at similar distances away from the catheter, even though temperature sensors R 1 -R 4  are moving in four separate directions relative to four different sides of catheter  28 . Thus, FIGS. 9-10 illustrate that catheter  28  generates a radially symmetrical temperature distribution in tissue medium  100  surrounding catheter  28 . 
     FIGS. 11-12 display the temperature data recorded by temperature sensors A 1 -A 4  throughout a complete testing procedure. Similar to temperature sensors R 1 -R 4 , temperature sensors A 1 -A 4  show a symmetrical heating distribution in all radial directions from catheter  28  in tissue medium  100 . Thus, FIGS. 9-10 in combination with FIGS. 11-12 confirm that catheter  28  generates a generally radially symmetrical heating pattern throughout tissue medium  100 . 
     Microwave thermal therapy such as is described above may be further enhanced with the present invention to maximize the amount of energy emitted toward targeted tissues while controlling the energy emitted toward certain healthy tissues to avoid thermal damage to those tissues. The rectum, for example, contains tissue that is susceptible to thermal damage. The radial extent of necrosis produced by a catheter emitting a symmetrical heating pattern is limited by the close proximity of the rectum relative to the urethra. While a control algorithm may be utilized to limit power to the microwave antenna so as to protect the rectum from thermal damage, modifying the thermal treatment catheter to restrict the amount of energy delivered toward the rectum enhances the ability to effectively necrose the greatest possible volume of diseased prostate tissue without thermally damaging the rectum. A catheter system achieving this objective and associated test results are disclosed in FIGS. 13-18. Clinicians using the nonuniform heat-distributing catheter system of the present invention can position the catheter within a body cavity in such a manner that tissues designated for treatment are exposed to high amounts of thermal energy while healthy tissues are exposed to lower, non-damaging amounts of thermal energy. 
     FIG. 13 is a side view of one preferred embodiment of a catheter of the present invention. Catheter  104  incorporates all of the previously-described features of catheter  28  shown FIGS. 2-7. Additionally, catheter  104  includes metal strip  102  composed of a material such as brass, for example, located in cooling lumen  64  of catheter  104 . Metal strip  102  has a length at least corresponding to the length of microwave antenna  74 , and serves to alter the microwave energy pattern emitted from microwave antenna  74  of catheter  104  in a way that reduces heating in the direction of strip  102 . Thus, incorporating strip  102  in catheter  104  has been shown to change the temperature distribution pattern of catheter  104 , since the temperature of the tissue surrounding catheter  104  is directly related to the amount of energy delivered to the tissue. With strip  102  in place, catheter  104  has two treatment zones, a preferential heating zone and a non-preferential heating zone. The preferential heating zone experiences temperatures significantly higher than those experienced in the non-preferential heating zone. When catheter  104  is used for the treatment of BPH, the non-preferential heating zone corresponds to the rectal tissue region of the patient. 
     FIG. 14 is a sectional view of catheter  104 , taken along line  14 - 14  of FIG.  13 . As shown in FIG. 14, metal strip  102  is positioned inside cooling lumen  64  of catheter  104  so that it is adjacent to microwave antenna  74 . Metal strip  102  is approximately 2 inches long, 0.08 inches wide and 0.0053 inches in height. Metal strip  102  has a length slightly longer than microwave antenna  74  and a width slightly less than the radial width of cooling lumen  64 . Strip  102  has a concave shape, which is formed by bending strip  102  over a mandrel after cutting strip  102  to size. The concave shape of metal strip  102  allows the strip  102  to be held in place within cooling lumen  64  by friction at three points of contact on the walls of cooling lumen  64 . An end of metal strip  102  is secured to proximal end  54  of catheter  104  with an adhesive to prevent metal strip  102  from moving within cooling lumen  64 . The concave shape of metal strip  102  also permits the cooling fluid to pass through cooling lumen  64  without interference from strip  102 . Also, metal strip  102  does not interfere with fluid flow through the cooling lumen ports  64 A and  64 B. 
     To test the temperature distribution pattern generated by catheter  104 , the catheter is placed in testing system  98  as shown in FIG.  8 . The same parameters and testing procedure as described previously for catheter  28  is used to test catheter  104 , with the individual testing phases performed at 5 and 10 minutes into the testing procedure. 
     FIGS. 15-16 show temperature recordings of temperature sensors R 1 -R 4  in comparison to the distances of temperature sensors R 1 -R 4  from catheter  104  during a complete testing procedure. FIGS. 15-16 each represent temperature and distance data from a single testing phase. The vertical axes of FIGS. 15-16 correspond to the temperature of tissue medium  100 , while the horizontal axes correspond to the radial distance away from catheter  104  of temperature sensors R 1 -R 4 . In FIGS. 15-16, temperature recordings of R 1  are shown by diamond marks, temperature recordings of R 2  are shown by square marks, temperature recordings of R 3  are shown by triangle marks, and temperature recordings of R 4  are shown by “x” marks. FIGS. 15-16 show that temperature sensors R 1  and R 2  record substantially dissimilar temperatures from the temperatures recorded by temperature sensors R 3  and R 4  at similar locations in tissue medium  100  throughout each testing phase. Thus, FIGS. 15-16 illustrate that catheter  104  produces a radially asymmetrical temperature distribution throughout tissue medium  100 , and specifically, that catheter  104  generates two distinct heating zones in the tissue medium—a preferential heating zone and a non-preferential heating zone. The generation of two distinct heating zones is caused by metal strip  102 , which attenuates microwave energy in the direction of the non-preferential heating zone. FIGS. 15-16 also illustrate that the temperature distribution throughout the preferential heating zone is generally uniform, and that the temperature distribution throughout the non-preferential heating zone is also generally uniform. Over the course of the complete testing procedure, FIGS. 15-16 indicate that the temperature of tissue medium  100  is steadily increased, and that temperature sensors R 1 -R 4  record similarly shaped temperature-distance patterns during each testing phase. 
     FIG. 15 shows the temperature recordings of temperature sensors R 1 -R 4  during a testing phase performed at 5 minutes into a testing procedure. Similar to FIG. 27, FIG. 28 shows that temperature sensors R 1  and R 2  record significantly higher temperatures than temperature sensors R 3  and R 4  in regions of tissue medium  100  which experience a substantial thermal change due to the energy generated by microwave catheter  104 . The higher temperatures recorded by R 1  and R 2  are due to the fact that R 1  and R 2  are located in the preferential heating zone of catheter  104 , while R 3  and R 4  are located in the non-preferential heating zone of catheter  104 . The temperature variation between R 1 ,R 2  and R 3 ,R 4  decreases as temperature sensors R 1 -R 4  increase in radial distance from catheter  104  and the heating effects of microwave antenna  74  therefore diminish in tissue medium  100 . FIG. 15 also illustrates that temperature sensors R 1  and R 2  experience similar temperatures at similar radial distances, and temperature sensors R 3  and R 4  experience similar temperatures at similar radial distances. FIG. 15 indicates that temperatures throughout the preferential heating zone are generally uniform, and that temperatures throughout the non-preferential heating zone are generally uniform. Temperature sensors in the preferential heating zone, R 1  and R 2 , experience temperatures approximately 10° higher than temperature sensors R 3  and R 4  in the non-preferential heating zone. 
     FIG. 16 shows the temperatures recorded by temperature sensors R 1 -R 4  during a testing phase performed at 10 minutes into a testing procedure. FIG. 16 illustrates that temperature sensors R 1  and R 2  experience significantly higher temperatures than temperature sensors R 3  and R 4 . Temperature sensors in the preferential heating zone, R 1  and R 2 , once again experience temperatures about 10°  24  higher than temperature sensors R 3  and R 4  in the non-preferential heating zone, with peak temperatures being higher than the peak temperatures of the previous testing phase. 
     In FIGS. 15-16, the temperature variation between R 1 , R 2  and R 3 , R 4  is substantial for a distance from 0 to approximately 30 mm away from catheter  104 . Beyond 30 mm from catheter  104 , temperature sensors R 1 -R 4  experience similar temperatures due to the decreased amount of energy delivered to the phantom tissue medium at those distances. FIGS. 15-16 illustrate that catheter  104  does not generate a radially symmetrical heating pattern in tissue medium  100 , but instead, creates two distinct temperature zones—a preferential heating zone and a non-preferential heating zone, with the non-preferential heating zone being exposed to substantially lower amounts of thermal energy. The placement of metal strip  102  in catheter  104  creates the two distinct heating zones. Metal strip  102  attenuates microwave energy in the direction of the non-preferential heating zone, thereby decreasing the thermal energy delivered into the non-preferential heating zone during a treatment procedure. FIGS. 15-16 also illustrate that the temperature distribution throughout the preferential heating zone is generally uniform, and the temperature distribution throughout the non-preferential heating zone is also generally uniform. 
     FIGS. 17-18 show temperature recordings of temperature sensors A 1 -A 4  in comparison to the positions of temperature sensors A 1 -A 4  along catheter  104  during a complete testing procedure. FIGS. 17-18 each represent temperature and distance data from a single testing phase. The vertical axes of FIGS. 17-18 correspond to the temperature of tissue medium  100 , and the horizontal axes correspond to the axial position along catheter  104  of temperature sensors A 1 -A 4 . In FIGS. 17-18, temperature recordings of A 1  are shown by diamond marks, temperature recordings of A 2  are shown by square marks, temperature recordings of A 3  are shown by triangle marks, and temperature recordings of A 4  are shown by “x” marks. FIGS. 17-18 show that temperature sensor A 1  records substantially higher temperatures and temperature sensor A 3  records substantially lower temperatures than temperature sensors A 2  and A 4  at similar locations along catheter  104  in tissue medium  100  throughout each testing phase. Thus, FIGS. 17-18 illustrate that catheter  104  produces a radially asymmetrical temperature distribution along catheter  104 . Over the course of the testing procedure, FIGS. 17-18 indicate that the temperature of tissue medium  100  is steadily increased, and that temperature sensors A 1 -A 4  record similar temperature-distance patterns during each testing phase. 
     FIG. 17 illustrates the temperature recordings of temperature sensors A 1 -A 4  during a testing phase performed at 5 minutes into a testing procedure. FIG. 17 shows that the temperatures recorded by temperature sensors A 1 -A 4  gradually increase and then decrease over the course of a testing phase. This gradual increase and decrease in recorded temperatures corresponds to the movement of temperature sensors A 1 -A 4  along the length of catheter  104 . The temperature increase corresponds to temperature sensors A 1 -A 4  sensing increased temperatures in tissue medium  100  as they move along the length of the energized microwave antenna  74  of catheter  104 . The temperature decrease corresponds to the region of tissue medium  100  which is not adjacent to microwave antenna  74  and thus is not exposed to as much thermal energy as tissue directly adjacent to antenna  74 . FIG. 17 illustrates that temperature sensor A 1  experiences substantially higher temperatures and temperature sensor A 3  experiences substantially lower temperatures than temperature sensors A 2  and A 4  as the temperature of tissue medium  100  increases. Temperature sensor A 3  experiences lower temperatures because it is located in the non-preferential heating zone of catheter  104 , due to microwave attenuation by metal strip  102 . Conversely, temperature sensor A 1  is located in the preferential heating zone of catheter  104 , and thus, experiences higher temperatures. Temperature sensors A 2  and A 4  experience similar temperatures at similar axial distances, with the temperatures experienced being between the temperatures experienced by temperature sensors A 1  and A 3 . 
     FIG. 18 corresponds to the temperature recordings of temperature sensors A 1 -A 4  during a testing phase performed at 10 minutes into a testing procedure. FIG. 18, similar to FIG. 17, illustrates a temperature difference between temperature sensors A 1  and A 3  and temperature sensors A 2  and A 4 . Temperature sensor A 1  in the preferential heating zone experiences substantially higher temperatures than temperature sensor A 3 , which is located in the non-preferential heating zone. Temperature sensors A 2  and A 4  experience similar temperatures, with those temperatures being between the temperatures experienced by temperatures sensors A 1  and A 3 . FIG. 18 also illustrates that the temperature distribution is axially uniform throughout the preferential heating zone, and also that the temperature distribution is axially uniform throughout the non-preferential heating zone. The peak temperatures experienced by temperature sensors A 1 -A 4  in tissue medium  100  are greater than those shown in FIG.  17 . 
     FIGS. 15-16 display the temperature data recorded by temperature sensors R 1 -R 4  throughout a complete testing procedure. FIGS. 15-16 each show a similar pattern for temperature variation (a gradual increase and then decrease in temperatures of tissue medium  100 ) as temperature sensors R 1 -R 4  move radially away from catheter  104 . FIGS. 15-16 show that catheter  104  does not heat tissue medium  100  symmetrically in radial directions. Instead, two heating zones are generated by catheter  104 —a preferential heating zone and a non-preferential heating zone. The preferential heating zone experiences substantially higher temperatures than the non-preferential heating zone. However, the temperature distributions within the preferential heating zone and the non-preferential heating zones are generally uniform. 
     FIGS. 17-18 display the temperature data recorded by temperature sensors A 1 -A 4  throughout a complete testing procedure. Similar to temperature sensors R 1 -R 4 , temperature sensors A 1 -A 4  show a radially asymmetrical heating distribution in tissue medium  100 . Thus, FIGS. 17-18 in combination with FIGS. 15-16 con firm that catheter  104  generates two zones of thermal energy within tissue medium  100 , and that one of the heating zones experiences substantially greater temperatures than the other zone. The generation of the two distinct heating zones is a result of metal strip  102  attenuating microwave energy in the direction of the non-preferential heating zone. 
     The present invention is a simple, inexpensive modification to a microwave thermal treatment catheter system that produces an asymmetrical heating pattern, thereby enabling a clinician to utilize the catheter in such a manner as to expose tissues in a first selected region to necrosing levels of energy at a large depth and to expose tissues in a second selected region to necrosing levels of energy at a smaller depth, to protect healthy tissues adjacent the second selected region from thermal damage. The invention is particularly useful for treatment of a prostate from a urethral catheter, employed to necrose prostate tissue in a region anterior to the urethra (opposite the rectum) to a first depth and to necrose prostate tissue in a region posterior to the urethra (adjacent the rectum) to a second depth, less than the first depth, without thermally damaging the rectum. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.