Abstract:
Efficient ablation with multiple electrodes is obtained by rapidly switching electric power to the electrodes. In this way, shielding effects caused by the field around each electrode which would otherwise create cool spots, are avoided. Complex inter-electrode current flows are also avoided.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of provisional application Ser. No. 60/315,383 filed Aug. 28, 2001, entitled “A Device to Allow Simultaneous Multiple Probe Use During Application of Radio Therapy”; hereby incorporated by reference, and is further a continuation-in-part of U.S. application Ser. No. 09/873,541 filed Jun. 4, 2001 now abandoned claiming the benefit of provisional application Ser. No. 60/210,103 filed Jun. 7, 2000 entitled “Multipolar Electrode System for Radio-frequency Ablation”. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This application was made with United States government support awarded the following agency: NIH HL56143. The United States has certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to radio-frequency ablation of tumors and the like and, in particular, to a device allowing for the simultaneous use of multiple ablation electrodes. 
     Ablation of tumors, such as liver (hepatic) tumors, uses heat or cold to kill tumor cells. In cryosurgical ablation, a probe is inserted during an open laparotomy and the tumor is frozen. In radio-frequency ablation (RFA), an electrode is inserted into the tumor and current passing from the electrode into the patient (to an electrical return typically being a large area plate on the patient&#39;s skin) destroys the tumor cells through resistive heating. 
     A simple RFA electrode is a conductive needle having an uninsulated tip placed within the tumor. The needle is energized with respect to a large area contact plate on the patient&#39;s skin by an oscillating electrical signal of approximately 460 kHz. Current flowing radially from the tip of the needle produces a spherical or ellipsoidal zone of heating (depending on the length of the exposed needle tip) and ultimately a lesion within a portion of the zone having sufficient temperature to kill the tumor cells. The size of the lesion is limited by fall-off in current density away from the electrode (causing reduced resistive heating), loss of heat to the surrounding tissue, and limits on the amount of energy transferred to the tissue from the electrode. The electrode energy is limited to avoid charring, boiling and vaporization of the tissue next to the electrode, a condition that greatly increases the resistance between the electrode and the remainder of the tumor. The tissue next to the electrode chars first because of the high current densities close to the electrode and thus creates a bottleneck in energy transfer. 
     Several approaches have been developed to increase energy delivered to tissue without causing charring. A first method places temperature sensors in the tip of the electrode to allow more accurate monitoring of temperatures near the electrode and thereby to allow a closer approach to those energies just short of charring. A second method actively cools the tip of the electrode with circulated coolant fluids within the electrode itself. A third method increases the area of the electrode using an umbrella-style electrode in which three or more electrode wires extend radially from the tip of the electrode shaft after it has been positioned in the tumor. The greater surface area of the electrode reduces maximum current densities. A fourth method injects a liquid (usually saline) into the tissue to increase conductivity. The effect of all of these methods is to increase the amount of energy deposited into the tumor and thus to increase the lesion size allowing more reliable ablation of more extensive tumors. 
     A major advantage of RFA in comparison to cryosurgical ablation is that it may be delivered percutaneously, without an incision, and thus with less trauma to the patient. In some cases, RFA is the only treatment the patient can withstand. Further, RFA can be completed while the patient is undergoing a CAT scan. 
     Nevertheless, despite the improvements described above, RFA often fails to kill all of the tumor cells and, as a result, tumor recurrence rates of as high as 50% have been reported. 
     The parent application to the present application describes a system of increasing the effective lesion size through the use of a bipolar operating mode where current flows between two locally placed umbrella electrodes rather than between an individual electrode and a large area contact plate. The bipolar current flow “focuses” the energy on the tumor volume between the two umbrella electrodes producing a lesion greater in volume with higher heating and more current density between electrodes than would be obtained by a comparable number of monopolar umbrella electrodes operating individually. In this respect, the bipolar operation allows treatment of larger tumors and more effective treatment of targeted tumors due to greater tissue heating with a single placement of electrodes, improving the speed and effectiveness of the procedure and making it easier to determine the treated volume over procedures where an individual electrode is moved multiple times. 
     The bipolar technique has some disadvantages. First, it is sensitive to the relative orientation of the two probes. Portions of the probes that are closer to each other will get hotter. Another disadvantage is that for two probe, bipolar systems, all the current exiting the first probe must enter the second probe depositing equal energy near both probes. This can be a problem when one probe is at a location, for example, near a cooling blood vessel, that requires additional deposition energy or independent control of that probe. Generally, too, a single set of bipolar probes can&#39;t treat multiple separated tumors. 
     One alternative is the simultaneous use of multiple probes in monopolar configuration. Here, as with the bipolar technique, the probes may be inserted at one time improving the speed of the procedure and eliminating ambiguity in the treatment volume that may come from repositioning probes. Current flows from each probe to the contact plate on the surface of the patient&#39;s skin. 
     A drawback to this multiple monopolar mode is that the monopolar probes may electrically shield each other causing insufficient heating between the probes. To the extent that the probes are operated at different voltages to accommodate local cooling of one probe, complex current flows can be created both between probes, and between probes and the contact plate making prediction of the ultimate effect of the probes difficult. 
     BRIEF SUMMARY OF THE INVENTION 
     The present inventors have developed a technique that combines the benefits of the bipolar probe operation in promoting large and uniform lesion sizes and the benefits of multiple, monopolar probe operation in providing individual control of the heating in the vicinity of each probe. The technique uses multiple monopolar probes operated in interleaved fashion, with a circuit switching rapidly between individual probes so that on an instantaneous basis, each probe is operating in isolation. Yet, for the purpose of heating, each probe can be considered to be operating simultaneously. Electrical shielding is reduced between probes while treatment speed is increased, the treated volume is more certain, and individual temperature, impedance, and/or time control of the probes is obtained. 
     Specifically then, the present invention provides a radio-frequency ablation system having at least three electrodes (possibly including a grounding pad) positionable in contact with a patient. A radio-frequency power source is connected through a switch system with the electrodes to sequentially connect at least one pair of the electrodes to the power source to provide for ablative current flow between the connected electrodes while inhibiting current flow between at least one unconnected pair. In the case of three electrodes, one electrode may be a surface contact, “grounding” electrode of broad area, and the other two internal electrodes positioned in or near the tumor volume. The switch system may operate to connect one of the percutaneous electrodes and the skin contact electrode together across the power supply and then the other of the percutaneous electrodes and the skin contact electrode together across the power supply. The switch system may be realized in electronic, electromechanical or other fashion. 
     Thus, it is one object of the invention to provide simultaneous multiple probe treatment of the tumor volume with more uniform lesion size by eliminating shielding effects caused by simultaneous operation of two adjacent probes. 
     The two probes may be umbrella electrodes having at least electrode wires extending from a common shaft. 
     Thus, it is another object of the invention to provide for larger lesions promoted by umbrella-type probes. The two electrodes may also be needle electrodes, with or without internal cooling. 
     The electronic switch may control the relative duration of connection of the pairs of electrodes to the power supply according to a controlled parameter of impedance, temperature, power, absolute time, or the difference between the impedance, temperature, or power of one or more electrodes. 
     Thus, it is another object of the invention to provide a simple means of independently controlling the power delivered to each of the electrodes using the switching means, which also provides for sequencing through the independent electrodes. 
     The electronic switch may include a proportional/integrator other controller controlling the switch according to the parameter of impedance, time, electrode power, or electrode temperature. 
     Thus it is another object of the invention to provide a simple method of adding more sophisticated control to the operation of the electrodes than may be provided simply through the RF power supply itself. 
     The switch system may control the voltage, or current or power that is applied to each electrode independently for the duration of application of electricity to each electrode. 
     Thus, it is another object of the invention to control the applied power, current or voltage, independently, according to the parameter of impedance, electrode power, or electrode temperature or the like. 
     The invention provides for a method in which there may be placed at least three electrodes in simultaneous contact with a patient where the electrodes may be sequentially connected to a radio-frequency power source to provide ablative current flow between a connected pair of the electrodes while inhibiting current flow between an unconnected pair. 
     Thus it is another object of the invention to allow a single step of insertion of multiple electrodes to speed treatment while obtaining the benefits in lesion control that are provided by moving a single electrode about in the patient. It is another object of the invention to eliminate ambiguities in treatment by a simultaneous insertion of the necessary electrodes before treatment. 
     The foregoing and other objects and advantages of the invention will appear from the following description. In this description, reference is made to the accompanying drawings, which form a part hereof, and in which there is shown by way of illustration, a preferred embodiment of the invention. Such embodiment and its particular objects and advantages do not define the scope of the invention, however, and reference must be made therefore to the claims for interpreting the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of two umbrella electrode assemblies providing first and second electrode wires deployed per an allied embodiment of the present invention at opposite edges of a tumor to create a lesion encompassing the tumor by a passing current between the electrodes; 
         FIG. 2  is a schematic representation of the electrodes of  FIG. 1  as connected to a voltage-controlled oscillator and showing temperature sensors on the electrode wires for feedback control of oscillator voltage; 
         FIG. 3  is a fragmentary cross-sectional view of a tip of a combined electrode assembly providing for the first and second electrode wires of  FIG. 1  extending from a unitary shaft arranging the wires of the first and second electrodes in concentric tubes and showing an insulation of the entire outer surface of the tubes and of the tips of the electrode wires; 
         FIG. 4  is a simplified elevational cross-section of a tumor showing the first and second electrode positions and comparing the lesion volume obtained from two electrodes operating per the present invention, compared to the lesion volume obtained from two electrodes operating in a monopolar fashion; 
         FIG. 5  is a figure similar to that of  FIG. 2  showing electrical connection of the electrodes of  FIG. 1  or  FIG. 3  to effect a more complex control strategy employing temperature sensing from each of the first and second electrodes and showing the use of a third skin contact plate held in voltage between the two electrodes so as to provide independent current control for each of the two electrodes; 
         FIG. 6  is a graph plotting resistivity in ohm-centimeters vs. frequency in hertz for tumorous and normal liver tissue, showing their separation in resistivity for frequencies below approximately 100 kHz; 
         FIG. 7  is a figure similar to that of  FIGS. 2 and 5  showing yet another embodiment in which wires of each of the first and second electrodes are electrically isolated so that independent voltages or currents or phases of either can be applied to each wire to precisely tailor the current flow between that wire and the other electrodes; 
         FIG. 8  is a flow chart of a program as may be executed by the controller of  FIG. 7  in utilizing its multi-electrode control; 
         FIG. 9   a  is a schematic block diagram of a second embodiment of the invention providing for multiplexed monopolar operation of multiple electrodes and showing a controller connecting a radio-frequency source to multiple, monopolar electrodes through a switch cycling between the electrodes and  FIG. 9   b  is a variation of the embodiment of  FIG. 9   a;    
         FIG. 10  is a timing diagram of the operation of the switch of  FIGS. 9   a  and  9   b  showing complimentary operation of two electrodes and control of the duty-cycle of operation for further electrode control; 
         FIG. 11  is a fragmentary view of a further embodiment of the switch of  FIGS. 9   a  and  9   b  showing its extension for operation of three electrodes; and 
         FIG. 12  is a timing diagram of the power received by the electrodes using a switch per the embodiment of  FIG. 11 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     I. Bipolar Electrode Operation 
     Referring now to  FIG. 1 , a liver  10  may include a tumor  12  about which a lesion  14  will be created by the present invention using two umbrella-type electrode assemblies  16   a  and  16   b  having a slight modification as will be disclosed below. Each electrode assembly  16   a  and  16   b  has a thin tubular metallic shaft  18   a  and  18   b  sized to be inserted percutaneously into the liver  10 . The shafts  18   a  and  18   b  terminate, respectively, at shaft tips  20   a  and  20   b  from which project trifurcated electrodes  22   a  and  22   b  are formed of wires  32 . The wires  32  are extended by means of a plunger  24  remaining outside the body once the shafts  18   a  and  18   b  are properly located within the liver  10  and when extended, project by an extension radius separated by substantially equal angles around the shaft tips  20   a  and  20   b . The exposed ends of the wires  32  are preformed into arcuate form so that when they are extended from the shafts  18   a  and  18   b  they naturally splay outward in a radial fashion. Although the shafts  18   a  and  18   b  are shown axially parallel, this is not required and other orientations may be used. 
     Umbrella electrode assemblies  16   a  and  16   b  of this type are well known in the art, but may be modified in one embodiment of the invention, by providing electrical insulation to all outer surfaces of the shafts  18   a  and  18   b  and by insulating the tips of the exposed portions of the wires  32 . This is in contrast to prior art, umbrella electrode assemblies, which leave the shaft tips  20   a  and  20   b  uninsulated and which do not insulate the wires  32 . The purpose and effect of these modifications will be described further below. 
     Per the present invention, the first electrode  22   a  is positioned at one edge of the tumor  12  and the other electrode  22   b  positioned opposite the first electrode  22   a  across the tumor  12  center. The term “edge” as used herein refers generally to locations near the periphery of the tumor  12  and is not intended to be limited to positions either in or out of the tumor  12 , whose boundaries in practice may be irregular and not well known. Of significance to the invention is that a part of the tumor  12  is contained between the electrodes  22   a  and  22   b.    
     Referring now to  FIGS. 1 and 2 , electrode  22   a  may be attached to a voltage-controlled power oscillator  28  of a type well known in the art providing a settable frequency of alternating current power whose voltage amplitude (or current or power output) is controlled by an external signal. The return of the power oscillator  28  is connected to electrodes  22   b  also designated as a ground reference. When energized, the power oscillator  28  induces a voltage between electrodes  22   a  and  22   b  causing current flow therebetween. 
     Referring now to  FIG. 4 , prior art operation of each electrode  22   a  and  22   b  being referenced to a skin contract plate (not shown) would be expected to produce lesions  14   a  and  14   b , respectively, per the prior art. By connecting the electrodes as shown in  FIG. 2 , however, with current flow therebetween, a substantially larger lesion  14   c  is created. Lesion  14   c  also has improved symmetry along the axis of separation of the electrodes  22   a  and  22   b . Generally, it has been found preferable that the electrodes  22   a  and  22   b  are separated by 2.5 to 3 cm for typical umbrella electrodes or by less than four times their extension radius. 
     Referring again to  FIG. 2 , temperature sensors  30 , such as thermocouples, resistive or solid-state-type detectors, may be positioned at the distal ends of each of the exposed wires  32  of the tripartite electrodes  22   a  and  22   b . For this purpose, the wires  32  may be small tubes holding small conductors and the temperature sensors  30  as described above. Commercially available umbrella-type electrode assemblies  16   a  and  16   b  currently include such sensors and wires connecting each sensor to a connector (not shown) in the plunger  24 . 
     In a first embodiment, the temperature sensors  30  in electrode  22   a  are connected to a maximum determining circuit  34  selecting for output that signal, of the three temperature sensors  30  of electrode  22  that has the maximum value. The maximum determining circuit  34  may be discrete circuitry, such as may provide precision rectifiers joined to pass only the largest signal, or may be implemented in software by first converting the signals from the temperature sensors  30  to digital values, and determining the maximum by means of an executed program on a microcontroller or the like. 
     The maximum value of temperature from the temperature sensors  30  is passed by a comparator  36  (which also may be implemented in discrete circuitry or in software), which compares the maximum temperature to a predetermined desired temperature signal  38  such as may come from a potentiometer or the like. The desired temperature signal is typically set just below the point at which tissue boiling, vaporization, or charring will occur. 
     The output from the comparator  36  may be amplified and filtered according to well known control techniques to provide an amplitude input  39  to the power oscillator  28 . Thus, it will be understood that the current between  22   a  and  22   b  will be limited to a point where the temperature at any one of temperature sensors  30  approaches the predetermined desired temperature signal  38 . 
     While the power oscillator  28  as described provides voltage amplitude control, it will be understood that current amplitude control may instead also be used. Accordingly, henceforth the terms voltage and current control as used herein should be considered interchangeable, being related by the impedance of the tissue between the electrodes  22   b  and  22   a.    
     In an alternative embodiment, current flowing between the electrodes  22   a  and  22   b , measured as it flows from the power oscillator  28  through a current sensor  29 , may be used as part of the feedback loop to limit current from the power oscillator  28  with or without the temperature control described above. 
     In yet a further embodiment, not shown, the temperature sensors  30  of electrode  22   b  may also be provided to the maximum determining circuit  34  for more complete temperature monitoring. Other control methodologies may also be adopted including those provided for weighted averages of temperature readings or those anticipating temperature readings based on their trends according to techniques known to those of ordinary skill in the art. 
     Referring now to  FIG. 3 , the difficulty of positioning two separate electrode assemblies  16   a  and  16   b  per  FIG. 1  may be reduced through the use of a unitary electrode  40  having a center tubular shaft  18   c  holding within its lumen, the wires  32  of first electrode  22   a  and a second concentric tubular shaft  42  positioned about shaft  18   c  and holding between its walls and shaft  18   c  wires  44  of the second electrode  22   b . Wires  44  may be tempered and formed into a shape similar to that of wires  32  described above. Shafts  18   c  and  42  are typically metallic and thus are coated with insulating coatings  45  and  46 , respectively, to ensure that any current flow is between the exposed wires  32  rather than the shafts  18   c  and  42 . 
     As mentioned above, this insulating coating  46  is also applied to the tips of the shafts  18   a  and  18   b  of the electrode assemblies  16   a  and  16   b  of  FIG. 1  to likewise ensure that current does not concentrate in a short circuit between the shafts  18   a  and  18   b  but, in fact, flows from the wires  32  of the wires of electrodes  22   a  and  22   b.    
     Other similar shaft configurations for a unitary electrode  40  may be obtained including those having side-by-side shafts  18   a  and  18   b  attached by welding or the like. 
     Kits of unitary electrode  40  each having different separations between first electrode  22   a  and second electrode  22   a  may be offered suitable for different tumor sizes and different tissue types. 
     As mentioned briefly above, in either of the embodiments of  FIGS. 1 and 3 , the wires  32  may include insulating coating  46  on their distal ends removed from shafts  18   c  and  42  to reduce high current densities associated with the ends of the wires  32 . 
     In a preferred embodiment, the wires of the first and second electrodes  22   a  and  22   b  are angularly staggered (unlike as shown in  FIG. 2 ) so that an axial view of the electrode assembly reveals equally spaced non-overlapping wires  32 . Such a configuration is also desired in the embodiment of  FIG. 2 , although harder to maintain with two electrode assemblies  16   a  and  16   b.    
     The frequency of the power oscillator  28  may be preferentially set to a value much below the 450 kHz value used in the prior art. Referring to  FIG. 6 , at less than 100 kHz and being most pronounced and frequencies below 10 kHz, the impedance of normal tissue increases to significantly greater than the impedance of tumor tissue. This difference in impedance is believed to be the result of differences in interstitial material between tumor and regular cell tissues although the present inventors do not wish to be bound by a particular theory. In any case, it is currently believed that the lower impedance of the tumorous tissue may be exploited to preferentially deposit energy in that tissue by setting the frequency of the power oscillator  28  at values near 10 kHz. Nevertheless, this frequency setting is not required in all embodiments of the present invention. 
     Importantly, although such frequencies may excite nerve tissue, such as the heart, such excitation is limited by the present bipolar design. 
     Referring now to  FIG. 5 , the local environment of the electrodes  22   a  and  22   b  may differ by the presence of a blood vessel or the like in the vicinity of one electrode such as substantially reduces the heating of the lesion  14  in that area. Accordingly, it may be desired to increase the current density around one electrode  22   a  and  22   b  without changing the current density around the other electrode  22   a  and  22   b . This may be accomplished by use of a skin contact plate  50  of a type used in the prior art yet employed in a different manner in the present invention. As used herein, the term contact plate  50  may refer generally to any large area conductor intended but not necessarily limited to contact over a broad area at the patient&#39;s skin. 
     In the embodiment of  FIG. 5 , the contact plate  50  may be referenced through a variable resistance  52  to either the output of power oscillator  28  or ground per switch  53  depending on the temperature of the electrodes  22   a  and  22   b . Generally, switch  53  will connect the free end of variable resistance  52  to the output of the power oscillator  28  when the temperature sensors  30  indicate a higher temperature on electrode  22   b  than electrode  22   a . Conversely, switch  53  will connect the free end of variable resistance  52  to ground when the temperature sensors  30  indicate a lower temperature on electrode  22   b  than electrode  22   a . The comparison of the temperatures of the electrodes  22   a  and  22   b  may be done via maximum determining circuits  34   a  and  34   b , similar to that described above with respect to  FIG. 2 . The switch  53  may be a comparator-driven, solid-state switch of a type well known in the art. 
     The output of the maximum-determining circuits  34   a  and  34   b  each connected respectively to the temperature sensors  30  of electrodes  22   a  and  22   b  may also be used to control the setting of the variable resistance  52 . When the switch  53  connects the resistance  52  to the output of the power oscillator  28 , the maximum-determining circuits  34   a  and  34   b  serve to reduce the resistance of resistance  52  as electrode  22   b  gets relatively hotter. Conversely, when the switch  53  connects the resistance  52  to ground, the maximum-determining circuits  34   a  and  34   b  serve to reduce the resistance of resistance  52  as electrode  22   a  gets relatively hotter. The action of the switch  53  and variable resistance  52  is thus generally to try to equalize the temperature of the electrodes  22   a  and  22   b.    
     If electrode  22   a  is close to a heat sink such as a blood vessel when electrode  22   b  is not, the temperature sensors  30  of electrode  22   a  will register a smaller value and thus, the output of maximum-determining circuit  34   a  will be lower than the output of maximum-determining circuit  34   b.    
     The resistance  52  may be implemented as a solid-state device according to techniques known in the art where the relative values of the outputs of maximum-determining circuits  34   a  and  34   b  control the bias and hence resistance of a solid-state device or a duty-cycle modulation of a switching element or a current controlled voltage source providing the equalization described above. 
     Referring now to  FIG. 7 , these principles may be applied to a system in which each wire  32  of electrodes  22   a  and  22   b  is electrically isolated within the electrode assemblies  16   a  and  16   b  and driven by separate feeds by switch  53  through variable resistances  54  connected either to the power oscillator  28  or its return. Electrically isolated means, in this context, that there is not a conductive path between the electrodes  22   a  and  22   b  except through tissue prior to connection to the power supply or control electronics. As noted before, a phase difference can also be employed between separate feeds from switch  53  to further control the path of current flow between electrode wires  32 . This phase difference could be created, e.g. by complex resistances that create a phase shift or by specialized waveform generators operating according to a computer program, to produce an arbitrary switching pattern. The values of the resistances  54  are changed as will be described by a program operating on a controller  56 . For this purpose, the variable resistances  54  may be implemented using solid-state devices such as MOSFETs according to techniques known in the art. 
     Likewise, similar variable resistances  54  also controlled by a controller  56  may drive the contact plate  50 . 
     For the purpose of control, the controller  56  may receive the inputs from the temperature sensors  30  (described above) of each wire  32  as lines  58 . This separate control of the voltages on the wires  32  allows additional control of current flows throughout the tumor  12  to be responsive to heat sinking blood vessels or the like near any one wire. 
     Referring to  FIG. 8 , one possible control algorithm scans the temperature sensors  30  as shown by process block  60 . For each temperature sensor  30 , if the temperature at that wire  32  is above a “ceiling value” below a tissue charring point, then the voltage at that wire is reduced. This “hammering down” process is repeated until all temperatures of all wires are below the ceiling value. 
     Next at process block  62 , the average temperature of the wires on each electrode  22   a  and  22   b  is determined and the voltage of the contact plate  50  is adjusted to incrementally equalize these average values. The voltage of the contact plate  50  is moved toward the voltage of the electrode  22  having the higher average. 
     Next at process block  64 , the hammering down process of process block  60  is repeated to ensure that no wire has risen above its ceiling value. 
     Next at process block  66  one wire in sequence at each occurrence of process block  66  is examined and if its temperature is below a “floor value” below the ceiling value, but sufficiently high to provide the desired power to the tumor, the voltage at that wire  32  is moved incrementally away from the voltage of the wires of the other electrode  22 . Conversely, if the wire  32  is above the floor value, no action is taken. 
     Incrementally, each wire  32  will have its temperature adjusted to be within the floor and ceiling range by separate voltage control. It will be understood that this process can be applied not only to the control parameter of temperature but also to other desired control parameters including, for example, impedance. 
     As shown in  FIG. 7 , this process may be extended to an arbitrary number of electrodes  22  including a third electrode set  22   c  whose connections are not shown for clarity. 
     While this present invention has been described with respect to umbrella probes, it will be understood that most of its principles can be exploited using standard needle probes. Further, it will be understood that the present invention is not limited to two electrode sets, but may be used with multiple electrode sets where current flow is predominantly between sets of the electrodes. The number of wires of the umbrella electrodes is likewise not limited to three and commercially available probes suitable for use with the present invention include a 10 wire version. Further, although the maximum temperatures of the electrodes were used for control in the above-described examples, it will be understood that the invention is equally amenable to control strategies that use minimum or average temperature or that measure impedance or use predetermined switching times. 
     II. Multiplexed Monopolar Electrode Operation 
     Referring now to  FIGS. 9   a  and  9   b , a multiplexed monopolar system  70  provides a power oscillator  28  having a power output  72  at which a radio-frequency signal is connected to the pole of a single pole double throw switch  74 . Switch  74  is preferably implemented as a solid-state switch according to techniques well known in the art preferably, but not limited to, switching at speeds over 20 kilohertz. 
     A first throw  76  of the switch  74  is connected to a first electrode  22   a  being an umbrella-type electrode as described above with the tines of the umbrella electrically joined. At least one tine may include a temperature sensor  30   a.    
     A second throw  78  of the switch  74  is connected to second electrode  22   b  also having a temperature sensor  30   b.    
     The electrodes  22   a  and  22   b  are placed as described above flanking the volume of a tumor or in separate tumors as may be desired. If a single tumor is being treated, the electrodes  22   a  and  22   b  will be proximate to each other typically less than three times the diameter of the extension radius of the tines of the electrodes  22   a  and  22   b . Conversely, to the bipolar embodiment, in the multiplexed monopolar electrode operation, there is no limitation on the orientation at which the probes are inserted. It is also understood that the described technique can be extended to any number of electrodes  22 . 
     In one embodiment, signals from the temperature sensors  30   a  and  30   b  are received by a controller  56 , which subtracts the temperatures to create a temperature difference signal that is received by a proportional/integral (PI) type controller  56 . PI controllers are well known in the art and produce an output signal that is a function of a first control constant K 1  times the input difference signal, plus a second control constant K 2  times the integral of the input difference signal. The PI controller  56  in this case produces a control signal  80  implemented as an electrical square wave whose further properties will be described below. 
     As an alternative to the temperature difference signal, the PI controller may accept a variety of other control inputs including impedance, temperature, power, absolute time (for a regular switching among electrodes), or the difference between the impedance, temperature, or power of one or more electrodes and other similar control inputs. 
     Alternatively to the PI controller, any other conceivable control mechanism can be implemented to distribute the power to two or more probes. 
     Referring also to  FIG. 10 , generally, the square wave of the control signal  80  controls the operation of the pole of the switch  74  to create a switching pattern  82   a  for electrode  22   a  and a switching pattern  82   b  for electrode  22   b . The switching patterns  82   a  and  82   b  describe the position of the pole of the switch  74  and thus a modulation envelope of the radio-frequency waveform of the output  72  seen at each electrode  22   a  and  22   b . During times when the pole of the switch  74  is connected to throw  76 , the wave form  82   a  is in a high state indicating that radio-frequency power is being supplied to electrode  22   a . Conversely, when the pole of the switch  74  is connected to throw  78 , wave form  82   b  is high indicating that radio-frequency energy is being supplied to electrode  22   b.    
     As is illustrated in the preferred embodiment, signals  82   a  and  82   b  are exact complements indicating that only one of electrodes  22   a  and  22   b  will be receiving electrical power at any given instant and yet the power from the power oscillator  28  is fully utilized. That is, when electrode  22   a  is energized, current flows only between electrode  22   a  and contact plate  50  (as indicated by arrow  84   a  of  FIGS. 9   a  and  9   b ). Conversely, when electrode  22   b  is energized, current flows only between electrode  22   b  and contact plate  50  (as indicated by  84   b  of  FIGS. 9   a  and  9   b ). When only one of electrodes  22   a  and  22   b  is activated at a given time, there is no shielding that would tend to distort lesion volume  90   a  about electrode  22   b  or  90   b  about electrode  22   b  and that would otherwise occur if electrodes  22   a  and  22   b  were simultaneously energized. Note, however, that some overlap of the “on” states of electrodes  22   a  and  22   b  may be tolerated if it is minor in comparison to the period of non-overlap. 
     A period of time  94   a  during which electrode  22   a  is activated expressed as a ratio with a period of time  94  during which electrode  22   b  is activated, defines a “duty-cycle”. The control signal  80  forming the output of the PI controller  56  controls this duty-cycle so that power is steered preferentially to one of electrodes  22   a  and  22   b  having the lower temperature. In this way, the controller  56  may act to bring their relative temperatures of the two electrodes  22   a  and  22   b  into equilibrium. Alternatively, the duty-cycle may be controlled based on impedance between the connected pairs of electrodes or power dissipated between the connected pairs of the electrodes. The speed at which the duty-cycle is adjusted in response to temperature differences and controlled by the settings of K 1  and K 2  described above and is adjusted to reflect average temperatures at the electrodes  22   a  and  22   b  whose actual temperatures may deviate instantaneously with the switching of power. 
     The frequency of the switching of switch  74  is selected to be fast compared to the cooling time of the tissue (e.g., 2 Hz or above). Higher switching speeds above 10 kHz and near 20 kHz may be preferred to avoid low-frequency components that could excite nerves and tissue, especially cardiac tissue. Switching is performed preferentially at the zero-crossings of the signal provided by the radio-frequency power supply to avoid transient currents. 
     The PI controller may also provide a limiter reducing the average power delivered to electrodes  22   a  and  22   b  when a threshold temperature (approximately 95 degrees C.) is reached by decreasing simultaneously periods  94   a  and  94   b  while preserving their ratio. In this case, the patterns  82   a  and  82   b  are no longer complementary but still have non-overlapping high states. 
     The power output of the radio-frequency power supply may further be controlled by the temperature or impedance of electrodes  22   a  and  22   b . In this embodiment, patterns  82   a  and  82   b  are complementary. The switch is controlled in a way to bring temperatures of electrodes  22   a  and  22   b  to equilibrium. The power output of the radio-frequency power supply is adjusted to bring average temperature of electrodes  22   a  and  22   b  to a set temperature, typically below the temperature where charring and boiling would occur. 
     In an alternative embodiment shown in  FIG. 9   b , the temperature sensors  30   a  and  30   b  may be routed as indicated by dotted lines  96  to a secondary switch  98  being a single pole, double throw switch whose pole is connected to a temperature input on a standard power oscillator  28 . In this case, the power oscillator  28  may be directly controlled so as to reduce its output voltage or current as a function of the temperature received from a given temperature probe  30   a  or  30   b  such as will alternate according to the operation of the switch  74 . Thus during the time the power oscillator  28  is delivering power to electrode  22   a , it will also be receiving the temperature from temperature sensor  30   a  to control it appropriately. Then when switch  74  changes state and the power oscillator is connected to electrode  22   b , the power oscillator may receive a temperature signal from temperature  30   b.    
     Referring now to  FIG. 11 , the switch  74  may in fact accommodate any number of electrodes  22   a ,  22   b , and  22   c  here depicted as needle electrodes in multiple tumors  12  and  12 ′. Thus the present invention may provide the benefits of locating an arbitrary number of electrodes in place about a tumor at one instant and then providing essentially simultaneous treatment of the volume with combined thermal effects without the need to move electrodes. 
     As depicted, switch  74  is a single pole, triple throw switch with one throw connected to each of electrodes  22   a ,  22   b  and  22   c  to provide modulated radio-frequency energy according to patterns  82   c ,  82   d , and  82   e  as shown in  FIG. 12 . Switching patterns  82   c ,  82   d , and  82   e  are analogous to switching patterns  82   a  and  82   b  described above except for the fact that the duty-cycle of three wave forms  82   a ,  82   b  and  82   c  is independently controlled (per arrows  83 ) to proportionally move power to the lowest temperature electrode  22 , and they are no longer complementary but simply have non-overlapping on times. Ideally, when one or more electrodes  22  have temperatures below the threshold, one of the switching patterns  82   c ,  82   d , and  82   e  is on at all times. In certain control algorithms there may be cycles, where power is not steered to any of the probes. In that case on pole of the multi throw switch is not connected to any probe, or is connected to some element dissipating the power. 
     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims. For example, the switch may be implemented using multiple radio-frequency sources that are enabled and disabled appropriately. Hybrid systems in which multiple electrodes are energized simultaneously and alternating are also contemplated. While percutaneous electrodes are described, the invention is also applicable to cauterizing probes and operative or laparoscopically placed electrodes.