Patent Publication Number: US-2022211434-A1

Title: Electrosurgical system

Description:
FIELD OF THE INVENTION 
     The invention relates to an electrosurgical system for delivering electromagnetic energy to biological tissue in order to ablate target tissue. The electrosurgical system includes an electrosurgical generator for supplying microwave energy, and an electrosurgical instrument arranged to receive the microwave energy and deliver it to target tissue. The electrosurgical instrument may be arranged to ablate tissue, such as a tumour, cyst or other lesion. The system may be particularly suited for treatment of tissue in the pancreas, the lung, or the liver. 
     BACKGROUND TO THE INVENTION 
     Electromagnetic (EM) energy, and in particular microwave and radiofrequency (RF) energy, has been found to be useful in electrosurgical operations, for its ability to cut, coagulate, and ablate body tissue. Typically, apparatus for delivering EM energy to body tissue includes a generator comprising a source of EM energy, and an electrosurgical instrument connected to the generator, for delivering the energy to tissue. Conventional electrosurgical instruments are often designed to be inserted percutaneously into the patient&#39;s body. However, it can be difficult to locate the instrument percutaneously in the body, for example if the target site is in a moving lung or a thin walled section of the gastrointestinal (GI) tract. Other electrosurgical instruments can be delivered to a target site by a surgical scoping device (e.g. an endoscope) which can be run through channels in the body such as airways or the lumen of the oesophagus or colon. This allows for minimally invasive treatments, which can reduce the mortality rate of patients and reduce intraoperative and postoperative complication rates. 
     Tissue ablation using microwave EM energy is based on the fact that biological tissue is largely composed of water. Human soft organ tissue is typically between 70% and 80% water content. Water molecules have a permanent electric dipole moment, meaning that a charge imbalance exists across the molecule. This charge imbalance causes the molecules to move in response to the forces generated by application of a time varying electric field as the molecules rotate to align their electric dipole moment with the polarity of the applied field. At microwave frequencies, rapid molecular oscillations result in frictional heating and consequential dissipation of the field energy in the form of heat. This is known as dielectric heating. 
     This principle is harnessed in microwave ablation therapies, where water molecules in target tissue are rapidly heated by application of a localised electromagnetic field at microwave frequencies, resulting in tissue coagulation and cell death. It is known to use microwave emitting probes to treat various conditions in the lungs and other organs. For example, in the lungs, microwave radiation can be used to treat asthma and ablate tumours or lesions. 
     SUMMARY OF THE INVENTION 
     At its most general, the invention provides an electrosurgical system which is arranged to deliver pulsed microwave energy to biological tissue via a small-diameter (e.g. 1.0 mm or less) radiating tip portion. A benefit of using a small-diameter radiating tip portion is that a size of an insertion hole produced when inserting the radiating tip portion into target tissue can be minimised, which may reduce bleeding and facilitate healing. However, a drawback of using such a small-diameter radiating tip portion is that transmission of microwave energy through the radiating tip portion may cause excessive heating of the radiating tip portion. Such excessive heating may cause burns and thus damage healthy tissue. The inventors have overcome this drawback by configuring the electrosurgical system to deliver the microwave energy in a pulsed manner. By delivering the microwave energy in a pulsed manner, it is possible to avoid excessive heating of the radiating tip portion. This may enable effective treatment of target biological tissue the radiating tip portion, whilst avoiding damage to surrounding healthy tissue. 
     The radiating tip portion of conventional electrosurgical instruments which may be used to treat the liver typically have an outer diameter between 2-3 mm. The inventors have found that use of such electrosurgical instruments in the liver may produce excessive bleeding which can be difficult to get under control during a surgical procedure. If a surgeon is unable to get such bleeding under control during a surgical procedure, it may be necessary to remove the electrosurgical instrument and attempt to continue the procedure with other means. 
     In contrast, the electrosurgical system of the invention may be particularly suited to treating tissue in highly vascularised regions of the body (e.g. where there may be excessive bleeding when the tissue is pierced), as the small insertion hole produced by the radiating tip portion may avoid or reduce bleeding. Thus, the combination of a small-diameter radiating tip portion and pulsed microwave energy delivery may enable highly vascularised regions of the body to be treated with microwave energy. In particular, the inventors have found that using the small-diameter radiating tip portion of the electrosurgical system of the invention may avoid excessive bleeding when used to treat target tissue in the liver. Thus, the electrosurgical system of the invention may be particularly suited to use for treatment of tissue in the liver. Additionally, the small-diameter radiating tip portion may be beneficial where scarring may be an issue. For example, the electrosurgical instrument of the invention may enable scarring to be reduced when used to ablate tumours in the breasts. 
     According to the invention, there is provided an electrosurgical system for treating biological tissue, the system comprising: an electrosurgical generator configured to supply pulsed microwave energy; and an electrosurgical instrument comprising: a flexible coaxial cable arranged to convey the pulsed microwave energy; and a radiating tip portion connected at a distal end of the coaxial cable and configured to receive the pulsed microwave energy, wherein the radiating tip portion has a maximum outer diameter that is 1.0 mm or less, and wherein the maximum outer diameter of the radiating tip portion is smaller than an outer diameter of the coaxial cable, the radiating tip portion comprising: a proximal coaxial transmission line for conveying the pulsed microwave energy; and a distal needle tip mounted at a distal end of the proximal coaxial transmission line, the distal needle tip being arranged to deliver the pulsed microwave energy into biological tissue. 
     The inventors have found that, by making a maximum outer diameter of the radiating tip portion 1.0 mm or less, bleeding may be significantly reduced or avoided when the radiating tip portion is inserted into target tissue. As discussed above, the use of pulsed microwave energy may ensure that excessive heat is not generated in the radiating tip portion when the microwave energy is delivered to the radiating tip portion. In contrast to delivering the microwave energy as a continuous wave which may cause the radiating tip portion to heat up rapidly, pulsed microwave energy may facilitate maintaining the radiating tip portion at an acceptable temperature. Pulsed microwave energy delivery may also enable the total amount of time over which microwave energy is delivered to the radiating tip portion to be reduced, e.g. by delivering short high power pulses. In this manner, the electrosurgical system may be used to effectively treat (e.g. ablate) target tissue whilst avoiding damage to nearby healthy tissue. 
     The electrosurgical generator may be any suitable generator for controllably supplying microwave energy. A suitable generator for this purpose is described in WO 2012/076844, which is incorporated herein by reference. The electrosurgical generator may generate pulsed microwave energy by modulating a microwave energy source to produce a profile (or waveform) having a series of “on” periods (corresponding to the microwave pulses) separated by a series of “off” periods. Generally speaking, pulsed microwave energy may be microwave energy having a profile comprising a plurality of pulses (or bursts) of microwave energy that are separated by periods with no microwave energy. The pulsed microwave energy may be periodic, e.g. it may have periodic cycles with “on” and “off” periods. 
     Different pulsed microwave energy profiles may be used. For example, all of the microwave pulses may have a same duration, or they may have different durations. Similarly, the periods between pulses may all be the same, or they may vary over time. The pulses may have a predetermined power profile (i.e. power vs. time). In some cases, different pulses may have different power profiles, depending on a desired energy delivery profile. 
     The electrosurgical instrument may be dimensioned so that it is insertable into an instrument (or working) channel of a surgical scoping device. This may facilitate insertion of the electrosurgical instrument into the patient&#39;s body, to enable access to a treatment site. 
     The flexible coaxial cable may be a conventional low loss coaxial cable that is connectable at a proximal end to the electrosurgical generator, to receive the pulsed microwave energy. In some cases, the coaxial cable may be permanently connected to the electrosurgical generator. The coaxial cable may have a centre conductor separated from an outer conductor by a dielectric material. The coaxial cable may further include an outer protective sheath for insulating and protecting the cable. In some examples, the protective sheath may be made of or coated with a non-stick material to prevent tissue from sticking to it and/or facilitate insertion of the instrument into the instrument channel of a surgical scoping device. The radiating tip portion is located at the distal end of the coaxial cable, and is connected to receive the pulsed microwave energy conveyed along the coaxial cable. 
     The proximal coaxial transmission line may be electrically connected to the distal end of coaxial cable, to receive the pulsed microwave energy and convey it to the distal needle tip, where the pulsed microwave energy is delivered to target tissue. The materials used in the proximal coaxial transmission line may be the same or different to those used in the coaxial cable. The materials used in the proximal coaxial transmission line may be selected to provide a desired flexibility and/or impedance of the proximal coaxial transmission line. For example, a dielectric material of the proximal coaxial transmission line may be selected to improve impedance matching with target tissue. 
     The distal needle tip is formed at the distal end of the proximal coaxial transmission line. The distal needle tip may include an emitter structure which is arranged to receive the pulsed microwave energy from the proximal coaxial transmission line and deliver the energy into target tissue. The emitter structure may be configured to produce a desired ablation profile in target tissue. For example, the emitter structure may be a monopolar or bipolar microwave antenna for radiating microwave energy into surrounding tissue. In some cases, the emitter structure may also be capable of delivering radiofrequency energy to target tissue, separately or in combination with the pulsed microwave energy. 
     The distal needle tip may include a pointed distal tip, to facilitate insertion of the radiating tip portion into target tissue. 
     The maximum outer diameter of the radiating tip portion is 1.0 mm or less. For example, the radiating tip portion may be 19 gauge. In some examples, the maximum outer diameter may be 0.95 mm, 0.9 mm or less. The maximum outer diameter may refer to the largest outer diameter of the radiating tip portion along a length of the radiating tip portion. 
     The outer diameter of the radiating tip portion is smaller than the outer diameter of the coaxial cable. By using a smaller diameter radiating tip portion, the radiating tip portion may be more flexible than the coaxial cable. This may facilitate guiding the distal needle tip to a desired location, e.g. where it is necessary to guide the device around a tight bend. A benefit of using a coaxial cable with an outer diameter that is larger than that of the radiating tip portion is that heating in the coaxial cable may be reduced, as heating is generally related to the diameter of the coaxial cable. 
     A pulse duration of the pulsed microwave energy may be shorter than a thermal response time of the radiating tip portion. This may reduce heating of the radiating tip portion, as the radiating tip portion may not react thermally to a magnitude of the pulsed microwave energy within the time frame of the pulse duration. This may improve an efficiency with which microwave energy can be delivered to the distal needle tip, as heating effects (e.g. dissipation of microwave energy) along the length of the radiating tip portion may be reduced. This may serve to improve an overall efficiency with which microwave energy can be delivered to target tissue. 
     The pulse duration may correspond to a time duration of a pulse of microwave energy in the pulsed microwave energy supplied by the electrosurgical generator. The thermal response time may correspond to an amount of time taken for the radiating tip portion&#39;s temperature to react (e.g. to change by a given amount) when microwave energy at a given power level is delivered to the radiating tip portion. The radiating tip portion&#39;s thermal response time may depend on a heat capacity of the radiating tip portion, e.g. the larger the heat capacity, the greater the thermal response time. The thermal response time of the radiating tip portion may be measured experimentally, in order to determine a suitable pulse duration time. 
     In some embodiments, the electrosurgical generator may be configured to supply the pulsed microwave energy with a duty cycle of 25% or less. Making the duty cycle of the pulsed microwave energy 25% or less may avoid or reduce heating effects in the radiating tip portion. For example, a duty cycle of 25% or less may ensure that the microwave pulses are short enough so that the radiating tip portion does not have enough time to thermally react to the pulses. Herein, a duty cycle may refer to a fraction of a period of the pulsed microwave energy where microwave energy is supplied by the electrosurgical generator (the remainder of the period may correspond to an “off” period where no microwave energy is supplied). Thus, with a duty cycle of 25% or less, no microwave energy may be delivered for at least 75% of the period of the pulsed microwave energy. This may ensure that pauses between the pulses of microwave energy are sufficiently long, so that there is little or no accumulation of thermal effects across multiple pulses. 
     A pulse duration of the pulsed microwave energy may be between 10 ms and 200 ms. The inventors have found that by using a pulse duration between 10 ms and 200 ms, it may be possible to avoid or reduce heating effects in the radiating tip portion, so that the radiating tip portion may be maintained at an acceptable temperature. Combining a pulse duration between 10 ms and 200 ms with a duty cycle of 25% or less may further ensure that heating effects are avoided or reduced. 
     In some embodiments, the pulsed microwave energy may be delivered according to one of the following cycles: 
     a) 10 ms pulse duration, with 90 ms between pulses; 
     b) 10 ms pulse duration, with 50 ms between pulses; 
     c) 10 ms pulse duration, with 30 ms between pulses; 
     d) 100 ms pulse duration, with 900 ms between pulses; 
     e) 100 ms pulse duration, with 500 ms between pulses; 
     f) 100 ms pulse duration, with 300 ms between pulses; and 
     g) 200 ms pulse duration, with 800 ms between pulses. 
     Cycles a) and d) correspond to a duty cycle of 10%; cycles b) and e) correspond to a duty cycle of 16.67%; cycles c) and f) correspond to a duty cycle of 25%; and cycle g) corresponds to a duty cycle of 20%. These duty cycles may enable the radiating tip portion to be maintained at an acceptable temperature during treatment of target tissue, whilst enabling the target tissue to be effectively treated. 
     In some embodiments, a length of the radiating tip portion may be equal to or greater than 140 mm. Coaxial cables which are typically used in electrosurgical instruments (e.g. the Sucoform 86 coaxial cable) often have a heavily tinned outer jacket to enable longitudinal actuation of the cable. However, this may result in the coaxial cable being relatively stiff, such that it may require a large force to bend the coaxial cable. This may cause a large amount of friction when the device is moved through a bend, e.g. in an instrument channel of a surgical scoping device. This may impede accurate control of a position of the radiating tip portion. The inventors have realised that having a long radiating tip portion may facilitate bending of the instrument near its distal end, as the radiating tip portion may have a greater flexibility compared to the coaxial cable. By making the radiating tip portion 140 mm or longer, it may be possible to avoid having to move the coaxial cable through a bent distal portion of the surgical scoping device. This may, for example, facilitate deploying the radiating tip portion where a distal portion of the surgical scoping device is in retroflex. This configuration may be particularly beneficial for use in the pancreas, where it may be necessary to have a distal portion of the instrument in retroflex. 
     In some embodiments, the proximal coaxial transmission line may comprise: an inner conductor that extends from a distal end of the flexible coaxial cable, the inner conductor being electrically connected to a centre conductor of the flexible coaxial cable; a proximal dielectric sleeve mounted around the inner conductor; and an outer conductor mounted around the proximal dielectric, wherein the distal needle tip comprises a distal dielectric sleeve mounted around the inner conductor, and wherein a distal portion of the outer conductor overlays a proximal portion of the distal dielectric sleeve. The distal dielectric sleeve may be secured by crimping or the like. 
     The outer conductor may be a conductive tube, e.g. formed from nitinol, a material that exhibits longitudinal rigidity sufficient to transmit a force capable of penetrating target tissue. Preferably the conductive tube also exhibits lateral flex suitable to enable the instrument to travel through the instrument channel of a surgical scoping device. Advantageously, nitinol may provide sufficient longitudinal rigidity for piercing the duodenum wall, to enable treatment of tissue in the pancreas, whilst still providing a high degree of lateral flexibility. The distal needle tip may be substantially rigid, to facilitate insertion into biological tissue. 
     The inner conductor may be formed from a material with high conductivity, e.g. silver. The inner conductor may have a diameter that is less than the diameter of the centre conductor of the flexible coaxial cable. This may facilitate bending of the radiating tip portion. For example, the diameter of the inner conductor may be 0.25 mm. A preferred diameter may take into account that a dominant parameter that determines loss (and heating) along the radiating tip portion is the conductor loss, which is a function of the diameter of the inner conductor. Other relevant parameters are the dielectric constants of the distal and proximal dielectric sleeves, and the diameter and material used for the outer conductor. The dimensions of the components of the proximal coaxial transmission line may be chosen to provide it with an impedance that is identical or close to the impedance of the flexible coaxial cable (e.g. around 50Ω). 
     The radiating tip portion may be secured to the flexible coaxial cable by a collar mounted over a junction therebetween. The collar may be electrically conductive, e.g. formed from brass. It may electrically connect the outer conductor with an outer conductor of the flexible coaxial cable. 
     A distal end of the distal dielectric sleeve may be sharpened, e.g. may taper to a point. Alternatively, a separate pointed tip element may be mounted at a distal end of the distal dielectric sleeve. This may facilitate insertion of the instrument into target tissue, e.g. through the duodenal or gastric wall into the pancreas. 
     The distal dielectric sleeve may be made from a different material to the proximal dielectric sleeve. The proximal dielectric sleeve may be made from the same material as a dielectric material of the flexible coaxial cable, e.g. PTFE or the like. In contrast, the distal dielectric sleeve may be made from any of ceramic, polyether ether ketone (PEEK), glass-filled PEEK. These materials may exhibit desirable rigidity and are capable of being sharpened. It also allows for controlling (e.g. reducing or optimising) the physical length of the radiating tip portion whilst maintaining its electric length. Thus, the distal dielectric sleeve may have a higher rigidity than the proximal dielectric sleeve. The higher rigidity of the distal dielectric sleeve may facilitate insertion of the radiating tip portion into target tissue, whilst the greater flexibility of the proximal dielectric sleeve may facilitate maneuvering of the radiating tip portion, e.g. around bends. 
     In some embodiments, a proximal end of the distal dielectric sleeve may include a protrusion disposed around the inner conductor, and the protrusion may be received in a complementarily shaped cavity at a distal end of the proximal dielectric sleeve. Such a configuration may improve a mechanical connection between the proximal and distal dielectric sleeves. Moreover, the protrusion may serve to increase a breakdown voltage of the radiating tip portion at a junction between the proximal dielectric sleeve and the distal dielectric sleeve, which may improve an electrical safety of the radiating tip portion. 
     The distal needle tip may be configured to operate as a half wavelength transformer to deliver the microwave energy from the distal needle tip. An advantage of configuring the distal needle tip as a half wavelength transformer may be to minimise reflections at the interface between components, e.g. between the coaxial cable and proximal coaxial transmission line, and between the proximal coaxial transmission line and the distal needle tip. A reflection coefficient at the latter interface is typically larger due to a larger variation in impedance. The half wavelength configuration may minimise these reflections so that the dominant reflection coefficient becomes that of the interface between the proximal coaxial transmission line and the tissue. The impedance of the proximal coaxial transmission line may be selected to be identical or close to the expected tissue impedance to provides a good match at the frequency of the microwave energy. 
     In some embodiments, the electrosurgical system may further include a surgical scoping device having a flexible insertion cord for insertion into a patient&#39;s body, the flexible insertion cord having an instrument channel running along its length, and wherein the electrosurgical instrument is dimensioned to be received within the instrument channel. 
     The electrosurgical system discussed herein may also be suitable for treating tissue in treatment sites that are awkward to reach, e.g. the pancreas or lungs, due to the small-diameter radiating tip portion. 
     The term “surgical scoping device” may be used herein to mean any surgical device provided with an insertion cord that is a rigid or flexible (e.g. steerable) conduit that is introduced into a patient&#39;s body during an invasive procedure. The insertion cord may include the instrument channel and an optical channel (e.g. for transmitting light to illuminate and/or capture images of a treatment site at the distal end of the insertion tube. The instrument channel may have a diameter suitable for receiving invasive surgical tools. The diameter of the instrument channel may be 5 mm or less. In embodiments of the invention, the surgical scoping device may be an ultrasound-enabled endoscope. For example, the surgical scoping device may be an ultrasound-enabled bronchoscope, where the insertion cord is adapted for insertion through a patient&#39;s airway into the bronchial tree. The bronchoscope may comprise one or more ultrasound transducers at a distal end of the insertion cord. The ultrasound transducers may be operable to assist insertion and position of the electrosurgical instrument. In particular, they may be arranged to generate ultrasound images of the radiating tip as it extends from the distal end of the instrument channel (and beyond the catheter) to penetrate tissue on its way to the treatment site. 
     Herein, the term “inner” means radially closer to the centre (e.g. axis) of the instrument channel and/or coaxial cable. The term “outer” means radially further from the centre (axis) of the instrument channel and/or coaxial cable. 
     The term “conductive” is used herein to mean electrically conductive, unless the context dictates otherwise. 
     Herein, the terms “proximal” and “distal” refer to the ends of the elongate instrument. In use, the proximal end is closer to a generator for providing the RF and/or microwave energy, whereas the distal end is further from the generator. 
     In this specification “microwave” may be used broadly to indicate a frequency range of 400 MHz to 100 GHz, but preferably the range 1 GHz to 60 GHz. Preferred spot frequencies for microwave EM energy include: 915 MHz, 2.45 GHz, 3.3 GHz, 5.8 GHz, 10 GHz, 14.5 GHz and 24 GHz. 5.8 GHz may be preferred. The device may deliver energy at more than one of these microwave frequencies. 
     The term “radiofrequency” or “RF” may be used to indicate a frequency between 300 kHz and 400 MHz. The term “low frequency” or “LF” may mean a frequency in the range 30 kHz to 300 kHz. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention are discussed below with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic diagram of an electrosurgical system for tissue ablation that is an embodiment of the invention; 
         FIG. 2  is a schematic sectional view through an instrument cord of an endoscope that can be used with the present invention; 
         FIG. 3  is a schematic side view of an electrosurgical instrument that may be used in an electrosurgical system of the invention; 
         FIG. 4  is a cross-sectional diagram of the electrosurgical instrument of  FIG. 3 , where an outer conductor has been omitted for illustration purposes; 
         FIG. 5  is a cross-sectional diagram of a distal section of the electrosurgical instrument of  FIG. 3 ; 
         FIG. 6  is a graph showing power delivery profile of pulsed microwave energy supplied by an electrosurgical generator that is part of an electrosurgical system of the invention; 
         FIG. 7  is a schematic cross-sectional diagram of a radiating tip portion that may be used in an electrosurgical system of the invention; 
         FIG. 8 a    is a schematic cross-sectional diagram of a radiating tip portion that may be used in an electrosurgical system of the invention; 
         FIG. 8 b    is a perspective view of a distal tip of the radiating tip portion of  FIG. 8   a;    
         FIG. 9 a    is a schematic cross-sectional diagram of a radiating tip portion that may be used in an electrosurgical system of the invention; 
         FIG. 9 b    is a schematic cross-sectional diagram of a distal portion of the radiating tip portion of  FIG. 9   a;    
         FIG. 10  is a schematic cross-sectional diagram of a radiating tip portion that may be used in an electrosurgical system of the invention; 
         FIG. 11  is a schematic cross-sectional diagram of a radiating tip portion that may be used in an electrosurgical system of the invention; 
         FIG. 12  shows plots of simulated return loss for electrosurgical instruments having the radiating tip portions in  FIGS. 8, 8   a ,  9   a ,  10  and  11 . 
     
    
    
     DETAILED DESCRIPTION; FURTHER OPTIONS AND PREFERENCES 
       FIG. 1  is a schematic diagram of an electrosurgical system  100  that is an embodiment of the invention. The electrosurgical system  100  is capable of supplying microwave energy to a distal end of an invasive electrosurgical instrument to perform tissue ablation. The electrosurgical system is also capable of supplying a fluid, e.g. a liquid medicament or a cooling fluid, to a distal end of the invasive electrosurgical instrument. The system  100  comprises an electrosurgical generator  102  for controllably supplying microwave energy. The electrosurgical generator is configured to supply pulsed microwave energy, as discussed in more detail below. A suitable generator for this purpose is described in WO 2012/076844, which is incorporated herein by reference. The electrosurgical generator  102  may be arranged to monitor reflected signals received back from the instrument in order to determine an appropriate power level for delivery. For example, the generator  102  may be arranged to calculate an impedance seen at the distal end of the instrument in order to determine an optimal delivery power level. 
     The electrosurgical system  100  further includes an interface joint  106  that is connected to the electrosurgical generator  102  via an interface cable  104 . The interface joint  106  is also connected via a fluid flow line  107  to a fluid delivery device  108 , such as a syringe. In some examples, the system may be arranged, additionally or alternatively, to aspirate fluid from a treatment site. In this scenario, the fluid flow line  107  may convey fluid away from the interface joint  106  to a suitable collector (not shown). The aspiration mechanism may be connected at a proximal end of the fluid flow line  107 . 
     The interface joint  106  may house an instrument control mechanism for controlling a position of the electrosurgical instrument. The control mechanism may be used to control a longitudinal position of the electrosurgical instrument, and/or bending of a distal end of the electrosurgical instrument. The Control mechanism is operable by sliding a trigger, to control a longitudinal (back and forth) movement of one or more control wires or push rods (not shown). If there is a plurality of control wires, there may be multiple sliding triggers on the interface joint to provide full control. A function of the interface joint  106  is to combine the inputs from the generator  102 , fluid delivery device  108  and instrument control mechanism into a single flexible shaft (or electrosurgical instrument)  112 , which extends from the distal end of the interface joint  106 . 
     The electrosurgical system further includes a surgical scoping device  114 , which in embodiment of the present invention may comprise an endoscopic ultrasound device. The flexible shaft  112  is insertable through an entire length of an instrument (working) channel of the surgical scoping device  114 . 
     The surgical scoping device  114  comprises a body  116  having a number of input ports and an output port from which an instrument cord  120  extends. The instrument cord  120 , which is illustrated in more detail in  FIG. 2 , comprises an outer jacket which surrounds a plurality of lumens. The plurality of lumens convey various things from the body  116  to a distal end of the instrument cord  120 . One of the plurality of lumens is the instrument channel discussed above. Other lumens may include a channel for conveying optical radiation, e.g. to provide illumination at the distal end or to gather images from the distal end. The body  116  may include an eye piece  122  for viewing the distal end. 
     An endoscopic ultrasound device typically provides an ultrasound transducer on a distal tip of the instrument cord, beyond an exit aperture of the instrument channel. Signals from the ultrasound transducer may be conveyed by a suitable cable  126  back along the instrument cord to a processor  124 , which can generate images in a known manner. The instrument channel may be shaped within the instrument cord to direct an instrument exiting the instrument channel through the field of view of the ultrasound system, to provide information about the location of the instrument at the target site. 
     The flexible shaft  112  has a distal assembly  118  (not drawn to scale in  FIG. 1 ) that is shaped to pass through the instrument channel of the surgical scoping device  114  and protrude (e.g. inside the patient) at the distal end of the instrument cord. 
     The structure of the distal assembly  118  discussed below may be particularly designed for use with an endoscopic ultrasound (EUS) device. The maximum outer diameter of the distal assembly  118  is equal to or less than 1.0 mm, e.g. less than 0.95 mm or 0.90 mm. The length of the flexible shaft can be equal to or greater than 1.2 m. 
     The body  116  includes an input port  128  for connecting to the flexible shaft  112 . As explained below, a proximal portion of the flexible shaft may comprise a conventional coaxial cable capable of conveying the pulsed microwave energy from the electrosurgical generator  102  to the distal assembly  118 . Example coaxial cables that are physically capable of fitting down the instrument channel of an EUS device are available with the following outer diameters: 1.19 mm (0.047″), 1.35 mm (0.053″), 1.40 mm (0.055″), 1.60 mm (0.063″), 1.78 mm (0.070″). Custom-sized coaxial cables (i.e. made to order) may also be used. 
     In order to control a position of a distal end of the instrument cord  120 , the body  116  may further include a control actuator that is mechanically coupled to the distal end of the instrument cord  120  by one or more control wires (not shown), which extend through the instrument cord  120 . The control wires may travel within the instrument channel or within their own dedicated channels. The control actuator may be a lever or rotatable knob, or any other known catheter manipulation device. The manipulation of the instrument cord  120  may be software-assisted, e.g. using a virtual three-dimensional map assembled from computer tomography (CT) images. 
     The invention may be particularly suited for treatment of the pancreas. In order to reach a target site in the pancreas, the instrument cord  120  may need to be guided through the mouth, stomach and duodenum. The electrosurgical instrument is arranged to access the pancreas by passing through the wall of the duodenum. The invention may also be particularly suited to treatment of tissue in the liver. 
       FIG. 2  is a view down the axis of the instrument cord  120 . In this embodiment there are four lumens within the instrument cord  120 . The largest lumen is the instrument channel  132  in which the flexible shaft  112  is received. The other lumens comprise an ultrasound signal channel  134 , an illumination channel  136 , and a camera channel  138  but the invention is not limited to this configuration. For example, there may be other lumens, e.g. for control wires or fluid delivery or suction. 
     We will now describe an electrosurgical instrument  300  that may be part of an electrosurgical system of the invention, with reference to  FIGS. 3 and 4 .  FIGS. 3 and 4  show side views of a distal portion of the electrosurgical instrument  300 , which may correspond to the distal assembly  118  referred to above. The electrosurgical instrument  300  includes a flexible coaxial cable  302 , and a radiating tip portion  304  which is connected at a distal end of the coaxial cable  302 . The coaxial cable  302  may be a conventional flexible 50Ω coaxial cable suitable for conveying microwave energy. The coaxial cable includes a centre conductor and an outer conductor that are separated by a dielectric material. The coaxial cable  302  is connectable at a proximal end to a generator, e.g. to generator  102 , to receive the microwave energy. 
     The radiating tip portion  304  includes a proximal coaxial transmission line  306  and a distal needle tip  308  formed at a distal end of the proximal coaxial transmission line  306 . The proximal coaxial transmission line  306  is electrically connected to the distal end of the coaxial cable  302  to receive the electromagnetic energy from the coaxial cable  302  and convey it to the distal needle tip  308 . The distal needle tip  308  is configured to deliver the received electromagnetic energy into target biological tissue. In the present example, the distal needle tip  308  is configured as a half wavelength transformer to deliver microwave energy into target biological tissue, to ablate the target tissue. In other words, an electrical length of the distal needle tip  308  corresponds to a half wavelength of the microwave energy (e.g. at 5.8 GHz). When microwave energy is delivered to the distal needle tip  308  it may radiate the microwave energy along its length into surrounding biological tissue. 
     An inner conductor  310  of the proximal coaxial transmission line  306  is electrically connected to the centre conductor of the coaxial cable  302 . The radiating tip portion  304  is secured to the coaxial cable  302  via a collar  312  mounted over a junction between the coaxial cable  302  and the radiating tip portion  304 . The collar  312  is made of a conductive material (e.g. brass), and electrically connects the outer conductor of the coaxial cable  302  to an outer conductor  314  of the proximal coaxial transmission line  306 . The outer conductor  314  is formed of a tube of nitinol, which is flexible and provides a sufficient longitudinal rigidity to pierce tissue (e.g. the duodenum wall). For illustration purposes, the outer conductor  314  is omitted from  FIG. 4  to reveal an inner structure of the radiating tip portion  304 . Also for illustration purposes, a length of the proximal coaxial transmission line  306  has been omitted in  FIGS. 3 and 4 , as indicated by broken lines  307 . 
     The proximal coaxial transmission line  306  includes a proximal dielectric sleeve  320  which is disposed around the inner conductor  310  and which spaces the inner conductor  310  from the outer conductor  314 . The outer conductor  314  is formed on an outer surface of the proximal dielectric sleeve  320 . A distal dielectric sleeve  322  is disposed around a distal portion of the inner conductor  310  to form the distal needle tip  308 . The distal needle tip  308  further includes a pointed tip  324  at its distal end, to facilitate insertion of the radiating tip portion into target tissue. The distal dielectric sleeve  322  may be made of a different dielectric material compared to the proximal dielectric sleeve  504 . In one example, the proximal dielectric sleeve  504  may be made of PTFE (e.g. it may be a PTFE tube) and the distal dielectric sleeve may be made of PEEK. Specific examples of materials that may be used in the radiating tip portion  304  are discussed below in relation to  FIGS. 7-11 . 
     A distal portion of the outer conductor  314  overlays a proximal portion of the distal dielectric sleeve  322 . In this manner, a distal portion of the proximal coaxial transmission line  306  includes the proximal portion of the distal dielectric sleeve  322 . The materials of the proximal and distal dielectric sleeves and the length of the overlap between the outer conductor  314  and the distal dielectric sleeve  322  may be selected in order to adjust an electrical length of the radiating tip portion  308  and impedance matching with target tissue. 
     The collar  312  includes a substantially cylindrical body  316  which is mounted on the distal end of the coaxial cable  302  and which is electrically connected to the outer conductor of the coaxial cable  302 . The collar  312  further includes a distal portion  318  which extends from the body  316  of the collar  312  to a proximal end of the outer conductor  314  of the proximal coaxial transmission line  306 . The distal portion  318  of the collar  312  includes a distal surface which is rounded. This may reduce friction between the electrosurgical instrument  300  and an instrument channel of a surgical scoping device when the electrosurgical instrument  300  is moved along the channel, by avoiding sharp edges at the interface between the coaxial cable  302  and the radiating tip portion  304 . This may also facilitate moving the electrosurgical instrument along the channel when the channel is in retroflex. 
     A maximum outer diameter of the radiating tip portion  304  is indicated in  FIG. 3  by arrows  326 . In the present example, the maximum outer diameter of the radiating tip portion  304  corresponds to an outer diameter of the outer conductor  314 , as this is the component of the radiating tip portion  304  having the largest outer diameter. The maximum outer diameter of the radiating tip portion  304  is 1.0 mm or less. For example, it may be 1.0 mm, 0.95 mm or 0.90 mm. This may ensure that a size of an insertion hole produced by the radiating tip portion  304  when it is inserted into target tissue is small, which may minimise bleeding. This may make the electrosurgical instrument  300  particularly suited to use in highly vascularised regions of the body, e.g. in the liver, where excessive bleeding may be an issue. 
     An outer diameter of the coaxial cable  302  is indicated by arrows  328  in  FIG. 3 . The outer diameter of the coaxial cable  302  is larger than the maximum outer diameter of the radiating tip portion  304 . For example, the outer diameter of the coaxial cable  302  may be between 1.19 mm and 2.0 mm, or it may be greater than 2.0 mm. By providing the radiating tip portion  304  with a smaller maximum outer diameter than the coaxial cable, it is possible to increase the flexibility of the radiating tip portion  304  relative to the coaxial cable  302 . This may facilitate maneuvering the radiating tip portion  304  to a particular treatment location. At the same time, by providing the coaxial cable  302  with a larger diameter, transmission losses (e.g. due to heating) in the coaxial cable  302  may be reduced, as transmission losses are generally related to the diameter of the coaxial cable  302 . This may enable microwave energy to be conveyed more efficiently along the coaxial cable  302  to the radiating tip portion  304 . 
     In some embodiments, the electrosurgical instrument  300  may be housed in a catheter (not shown). The electrosurgical instrument  300  may be movable relative to the catheter, so that the radiating tip portion  304  can be retracted inside the catheter when not in use. This may serve to protect the radiating tip portion, and prevent it from catching on the instrument cord when it is inserted into the instrument cord of a surgical scoping device. 
     The radiating tip portion  304  may have a length equal to or greater than 30 mm, e.g. 40 mm. In this manner, the radiating tip portion  304  may be long enough for the distal needle tip  308  to reach a treatment site, without having to insert a portion of the coaxial cable  302  into tissue. In some cases the radiating tip portion  304  may have a length of 140 mm or greater. The inventors have found that this may facilitate inserting the electrosurgical instrument  300  into an instrument cord where a distal portion of the instrument cord is in retroflex, as it may avoid having to push the more rigid coaxial cable  302  through the distal portion of the instrument cord. 
       FIG. 5  illustrates an interface between the proximal dielectric sleeve  320  and the distal dielectric sleeve  322  in more detail.  FIG. 5  shows a cross-sectional view of a distal section of the radiating tip portion  304 . For illustration purposes, the outer conductor  314  is omitted from  FIG. 5 . A proximal end of the distal dielectric sleeve  322  includes a protrusion  502  which extends from the proximal end of the distal dielectric sleeve  322 . The protrusion  502  has a generally cylindrical shape, with an outer diameter smaller than that of the distal dielectric sleeve  322 , and is disposed around the inner conductor  310 . The proximal dielectric sleeve  320  includes a cavity having a shape complementary to that of the protrusion  502 , in which the protrusion  502  is received. Thus, the proximal dielectric sleeve  320  steps around the protrusion  502 . As the protrusion  502  of the distal dielectric sleeve  322  is received in the proximal dielectric sleeve  320 , this serves to provide a strong mechanical connection between the distal and proximal dielectric sleeves. Additionally, the protrusion  502  may serve to increase a breakdown voltage of the radiating tip portion  304  at the interface between the distal dielectric sleeve  322  and the proximal dielectric sleeve  320 . This may improve an electrical safety of the radiating tip portion  304 . 
     As the radiating tip portion  304  of electrosurgical instrument has a small diameter (i.e. 1.0 mm or less), it may heat up rapidly when microwave energy is delivered to it. This may result in inefficient delivery of microwave energy to the distal needle tip. Heating of the radiating tip portion  304  may also cause damage to healthy surrounding tissue. The inventors have overcome this drawback by configuring the electrosurgical generator (e.g. electrosurgical generator  102 ) of the electrosurgical system of the invention to deliver the microwave energy in pulses. The inventors have found that pulsed delivery of microwave energy may avoid or reduce heating effects in the radiating tip portion, so that the radiating tip portion may be maintained at an acceptable temperature during a surgical procedure. 
     In order to avoid heating of the radiating tip portion during application of microwave energy, a pulse duration of the microwave pulses may be set to be greater than a thermal response time of the radiating tip portion. In this manner, the radiating tip portion may not have time to react thermally to the pulsed microwave energy on the timescale of the microwave pulses. The thermal response time of the radiating tip portion may be measured experimentally, by determining an amount of time taken for a temperature of the radiating tip portion to increase by a given amount (e.g. 5° C.) when microwave energy at a given power level (e.g. a power level to be used during an electrosurgical procedure) is delivered to the radiating tip portion. The pulse duration may then be set accordingly, to ensure that the temperature of the radiating tip portion remains at an acceptable temperature over the course of an electrosurgical procedure. 
     The inventors have found that configuring the electrosurgical generator to deliver pulsed microwave energy with a duty cycle of 25% or less may avoid or reduce heating effects in the radiating tip portion so that it may be maintained at an acceptable temperature during use. The electrosurgical generator may be configured to deliver microwave energy according to one of the following example cycles: 
     a) 10 ms pulse duration, with 90 ms between pulses; 
     b) 10 ms pulse duration, with 50 ms between pulses; 
     c) 10 ms pulse duration, with 30 ms between pulses; 
     d) 100 ms pulse duration, with 900 ms between pulses; 
     e) 100 ms pulse duration, with 500 ms between pulses; 
     f) 100 ms pulse duration, with 300 ms between pulses; and 
     g) 200 ms pulse duration, with 800 ms between pulses. 
     Cycles a) and d) correspond to a duty cycle of 10%; cycles b) and e) correspond to a duty cycle of 16.67%; cycles c) and f) correspond to a duty cycle of 25%; and cycle g) corresponds to a duty cycle of 20%. 
       FIG. 6  illustrates a power delivery profile according to cycle a) given above. The power delivery profile of  FIG. 6  shows power of microwave energy supplied by the electrosurgical generator against time. The power delivery profile includes a series of microwave pulses  600 , each having a duration of 10 ms. The microwave pulses  600  are separated by intervals  602 , each having a duration of 90 ms. The microwave pulses  600  each have a power P, as indicated in  FIG. 6 . During the intervals  602 , no microwave energy is supplied by the electrosurgical generator (i.e. the supplied power is 0 W). Each of the pulses  600  is identical, and includes a constant power level. Note that the power delivery profile of  FIG. 6  is not drawn to scale. In other examples, the power level of a microwave pulse may vary over the course of the pulse, depending on a desired energy delivery profile. In some cases, a microwave pulse cycle may include pulses having different durations and/or power levels. 
     We will now describe specific examples of radiating tip portions of electrosurgical instruments that may be used in an electrosurgical system of the invention, with reference to  FIGS. 7-11 . The radiating tip portions described below may, for example, be used instead of the radiating tip portion  304  of electrosurgical instrument  300  discussed above. Radiating tip portions  700 ,  800 ,  900 ,  1000  and  1100  discussed below each have a similar overall configuration. Similarly to radiating tip portion  304 , each of radiating tip portions  700 ,  800 ,  900 ,  1000  and  1100  has an inner conductor electrically connected to a centre conductor of a coaxial cable (not shown), and an outer conductor electrically connected to an outer conductor of the coaxial cable. The radiating tip portions  700 ,  800 ,  900 ,  1000  and  1100  each further include a proximal dielectric sleeve and a distal dielectric sleeve disposed around the inner conductor, in order to form a proximal transmission line and a distal needed tip as discussed above in relation to radiating tip portion  304 . 
       FIG. 7  shows a cross-sectional view of a distal section of a radiating tip portion  700 . A proximal dielectric sleeve  706  of radiating tip portion  700  may be made of a flexible insulating material, e.g. PTFE. A distal dielectric sleeve  708  of radiating tip portion  700  is made of a cylindrical piece of Zirconia. A distal tip  710  of the distal dielectric sleeve  708  is sharpened, to facilitate insertion of the radiating tip portion  700  into tissue. Making the distal dielectric sleeve  708  of Zirconia may provide a rigid distal needle tip to the radiating tip portion  700 , which may facilitate piercing of tissue. Use of Zirconia may also enable a physical length of the radiating tip portion to be shortened, whilst maintaining a desired electrical length. 
     Example dimensions of the radiating tip portion  700  are shown in  FIG. 7 . The dimension indicated by reference numeral  712 , which corresponds to a length of the proximal dielectric sleeve  706 , may be 37 mm. Note the total length of the proximal dielectric sleeve  706  is not shown in  FIG. 7 . The dimension indicated by reference numeral  714 , which corresponds to an overlap between an outer conductor  704  of the radiating tip portion  700  and the distal dielectric sleeve  708 , may be 3.6 mm. The dimension indicated by reference numeral  716 , which corresponds to a length of an inner conductor  702  of the radiating tip portion  700  that protrudes beyond a distal end of the outer conductor  704 , may be 1.5 mm. The dimension indicated by reference numeral  718 , which corresponds to a length of the distal tip  710 , may be 1.5 mm. A maximum outer diameter of the radiating tip portion  700 , indicated by reference numeral  720 , is 1.0 mm or less. The junction between the distal dielectric sleeve  708  and the proximal dielectric sleeve may be crimped, e.g. around the outer surface of the outer conductor  704 ,  706 , in order to strengthen the connection between the distal dielectric sleeve  708  and the remainder of the instrument. 
       FIG. 8 a    shows a cross-sectional view of a distal section of a radiating tip portion  800 . A proximal dielectric sleeve  806  of radiating tip portion  800  may be made of a flexible tube of insulating material, e.g. PTFE. A distal dielectric sleeve  808  of radiating tip portion  800  is made of a cylindrical piece of Zirconia. The distal dielectric sleeve  808  includes a bore in which the inner conductor is received. A distal tip  810  made of Zirconia is mounted at a distal end of the distal dielectric sleeve  808 . A perspective view of the distal tip  810  is shown in  FIG. 8 b   . The distal tip  810  has a conical body  812  forming a pointed tip, to facilitate insertion of the radiating tip portion  800  into tissue. The distal tip  810  includes a protrusion  814  extending from a proximal face  816  of the conical body  812 . The protrusion of the distal tip  810  is received in the bore in the distal dielectric sleeve  808 , where it is secured in placed. The tip may be secured using an adhesive. Additionally or alternatively, the tip may be secured by crimping the distal end. 
     Example dimensions of the radiating tip portion  800  are shown in  FIG. 8 a   . The dimension indicated by reference numeral  818 , which corresponds to a length of the proximal dielectric sleeve  806 , may be 37 mm. Note the total length of the proximal dielectric sleeve  806  is not shown in  FIG. 8 a   . The dimension indicated by reference numeral  820 , which corresponds to an overlap between an outer conductor  804  of the radiating tip portion  800  and the distal dielectric sleeve  808 , may be 3.6 mm. The dimension indicated by reference numeral  822 , which corresponds to a length of the distal dielectric sleeve  808  that protrudes beyond a distal end of the outer conductor  804 , may be 2.0 mm. The dimension indicated by reference numeral  824 , which corresponds to a length of the conical body  812  of the distal tip  810 , may be 1.5 mm. The dimension indicated by reference numeral  826 , which corresponds to a length of the protrusion  814 , may be 0.5 mm. A maximum outer diameter of the radiating tip portion  800 , indicated by reference numeral  828 , is 1.0 mm or less. 
       FIG. 9 a    shows a cross-sectional view of a distal section of a radiating tip portion  900 . A proximal dielectric sleeve  906  of the radiating tip portion  900  may be made of a flexible tube of insulating material, e.g. PTFE. A distal dielectric sleeve  908  of the radiating tip portion  900  is made of a cylindrical piece of Polyether ether ketone (PEEK). The distal dielectric sleeve  908  includes a cavity at a distal end thereof in which a distal tip  910  made of Zirconia is received. A “push-fit” connection is formed between the distal tip  910  and the distal dielectric sleeve  908 . 
       FIG. 9 b    shows the connection between the distal tip  910  and the distal dielectric sleeve  908  in greater detail. The distal tip includes a body  912  which is received in the cavity in the distal dielectric sleeve  908 . The body  912  includes a bump  914  on its outer surface, which is arranged to press outwards against the distal dielectric sleeve  908 , in order to retain the distal tip  910  in the cavity. Thus, the distal tip  910  may be automatically retained within the cavity once it has been inserted into the cavity. The distal tip  910  may further be secured in the cavity using adhesive. The distal tip  910  further includes a conical portion  916 , which forms a pointed tip a distal end of the radiating tip portion  900 . An outer surface of the distal dielectric sleeve  908  is tapered at an angle matching a tapering angle of the conical portion  916 , so that an outer surface of the radiating tip portion  900  is smooth. Making the distal tip  910  out of Zirconia may enable a sharper distal tip to be provided, as Zirconia may have a higher rigidity than PEEK. 
     Example dimensions of the radiating tip portion  900  are shown in  FIG. 9 a   . The dimension indicated by reference numeral  918 , which corresponds to a length of the proximal dielectric sleeve  906 , may be 37 mm. Note the total length of the proximal dielectric sleeve  906  is not shown in  FIG. 9 a   . The dimension indicated by reference numeral  920 , which corresponds to an overlap between an outer conductor  904  of the radiating tip portion  900  and the distal dielectric sleeve  908 , may be 7.0 mm. The dimension indicated by reference numeral  922 , which corresponds to a length of an inner conductor  902  of the radiating tip portion  900  that protrudes beyond a distal end of the outer conductor  904 , may be 5.0 mm. The dimension indicated by reference numeral  924 , which corresponds to a length of distal tip  910 , may be 2.0 mm. A maximum outer diameter of the radiating tip portion  900 , indicated by reference numeral  926 , is 1.0 mm or less. 
       FIG. 10  shows a cross-sectional view of a distal section of a radiating tip portion  1000 . A proximal dielectric sleeve  1006  of radiating tip portion  1000  may be made of a flexible tube of insulating material, e.g. PTFE. A distal dielectric sleeve  1008  of radiating tip portion  1000  is made of a cylindrical piece of PEEK. Similarly to radiating tip portion  800 , radiating tip  1000  includes a distal tip  1010  made of Zirconia mounted at a distal end of the distal dielectric sleeve  1008 . The distal tip  1010  has a similar configuration to distal tip  810  shown in  FIG. 8 b   , i.e. it includes a conical body and a protrusion  1014  that is received in a bore in the distal dielectric sleeve  1008 . 
     Example dimensions of the radiating tip portion  1000  are shown in  FIG. 10 . The dimension indicated by reference numeral  1018 , which corresponds to a length of the proximal dielectric sleeve  1006 , may be 37 mm. Note the total length of the proximal dielectric sleeve  1006  is not shown in  FIG. 10 . The dimension indicated by reference numeral  1020 , which corresponds to an overlap between an outer conductor  1004  of the radiating tip portion  1000  and the distal dielectric sleeve  1008 , may be 6.0 mm. The dimension indicated by reference numeral  1022 , which corresponds to a length of the distal dielectric sleeve  1008  that protrudes beyond a distal end of the outer conductor  1004 , may be 5.5 mm. The dimension indicated by reference numeral  1024 , which corresponds to a length of the conical body of the distal tip  810 , may be 1.5 mm. The dimension indicated by reference numeral  1026 , which corresponds to a length of the protrusion  1014 , may be 0.5 mm. A maximum outer diameter of the radiating tip portion  1000 , indicated by reference numeral  1028 , is 1.0 mm or less. 
       FIG. 11  shows a cross-sectional view of a distal section of a radiating tip portion  1100 . A proximal dielectric sleeve  1106  of radiating tip portion  1100  may be made of a flexible tube of insulating material, e.g. PTFE. A distal dielectric sleeve  1108  of radiating tip portion  1100  is made of a cylindrical piece of PEEK. A distal tip  1110  of the distal dielectric sleeve  1108  is sharpened, to facilitate insertion of the radiating tip portion  1100  into tissue. 
     Example dimensions of the radiating tip portion  1100  are shown in  FIG. 11 . The dimension indicated by reference numeral  1118 , which corresponds to a length of the proximal dielectric sleeve  1106 , may be 37 mm. Note the total length of the proximal dielectric sleeve  1106  is not shown in  FIG. 11 . The dimension indicated by reference numeral  1120 , which corresponds to an overlap between an outer conductor  1104  of the radiating tip portion  1000  and the distal dielectric sleeve  1108 , may be 6.0 mm. The dimension indicated by reference numeral  1122 , which corresponds to a length of an inner conductor  1102  of the radiating tip portion  1100  that protrudes beyond a distal end of the outer conductor  1004 , may be 5.5 mm. The dimension indicated by reference numeral  1024 , which corresponds to a length of the distal tip  1110 , may be 1.5 mm. A maximum outer diameter of the radiating tip portion  1100 , indicated by reference numeral  1028 , is 1.0 mm or less. 
       FIG. 12  shows simulated plots of the S-parameter (also known as the “return loss”) against frequency of microwave energy for electrosurgical instruments having radiating tip portions  700 ,  800 ,  900 ,  1000  and  1100  described above. As is well known in the technical field, the S-parameter is a measure of the return loss of microwave energy due to impedance mismatch, and as such the S-parameter is indicative of the degree of impedance mismatch between the target tissue and the radiating tip portion. The S-parameter can be defined by the equation P I =SP R , where P I  is the outgoing power in the instrument towards the tissue, P R  is the power reflected back from the tissue, and S is the S-parameter. In  FIG. 12 , curve  1200  corresponds to a simulation for radiating tip portion  700 ; curve  1202  corresponds to a simulation for radiating tip portion  800 ; curve  1204  corresponds to a simulation for radiating tip portion  900 ; curve  1206  corresponds to a simulation for radiating tip portion  1000 ; curve  1208  corresponds to a simulation for radiating tip portion  1100 . Note that curves  1200  and  1202  are very close to one another and so appear to overlap. 
     As shown in  FIG. 12 , at a microwave energy frequency of 5.8 GHz, the S-parameter is −19.2 dB for radiating tip portion  700 , −19.3 dB for radiating tip portion  800 , −26.7 dB for radiating tip portion  900 , −33.5 dB for radiating tip portion  1000  and −30.4 dB for radiating tip portion  1100 . This means that for each of these radiating tip portions, very little microwave energy is reflected back from the tissue at 5.8 GHz. This indicates a good impedance match between the radiating tip portions and biological tissue at the operating frequency of 5.8 GHz, and that microwave energy may be efficiently delivered from the radiating tip into the tissue at this frequency.