An RF source

Abstract

A radiofrequency, RF, source 200 suitable for a radiotherapy device (figure 1, 100). The RF source 200 comprises an RF generator 202 and a resonant ring 204. The RF generator 202 is configured to outp

Description

The invention relates to an RF source for a radiotherapy device. Background Radiotherapy can be described as the use of ionising radiation, such as X-rays, to treat a human or animal body. Radiotherapy is commonly used to treat tumours within the body of a patient or subject. In such treatments, ionising radiation is used to irradiate, and thus destroy or damage, cells which form part of the tumour. A radiotherapy device typically comprises a gantry which supports a beam generation system, or other source of radiation, which is rotatable around a patient. For example, for a linear accelerator (linac) device, the beam generation system may comprise a source of radio frequency (RF) energy, a source of electrons, an accelerating waveguide, beam shaping apparatus, etc. A linac uses oscillating high-power RF electric fields to accelerate electrons along a waveguide. Narrowband sources of RF are typically preferable to wideband sources for effective acceleration of electrons within the linac. Typically, a narrowband high-power source of RF such as a magnetron is used. Suitable magnetron models are commercially available and are manufactured by, for example, Teledyne (registered trade mark), Thales (registered trade mark), CPI, and TMD (registered trade mark). The magnetron produces RF fields at a resonant frequency dependent upon its design, which is tailored to the linac of the radiotherapy machine. However, magnetrons constitute a very expensive component of the radiotherapy device. Furthermore, magnetrons for radiotherapy are typically operated in a pulsed mode but in many instances only a small proportion of the pulses are in fact used. For example, in some instances, even at high dose rates, the RF demand of the radiotherapy device is only one pulse per every 243 cycles. Statement of Invention An invention is set out in the claims. Figures Embodiments of the invention will now be described, by way of example, with reference to the drawings of which: Fig 1 depicts a radiotherapy device or apparatus according to the present disclosure; Fig 2 depicts an RF source according to a first implementation; Fig 3 depicts an RF source according to a second implementation. Overview In overview, the approach described herein relates to an RF source comprising a magnetron, or multiple magnetrons, which instead of feeding pulses directly to a linac, outputs to a resonant ring which can store multiple pulses and release the energy at a desired time, permitting storage of pulses that would otherwise be discarded and acting in effect as a pulse integrator. The resonant ring can for example be a microwave waveguide in the form of a ring with a length equivalent to an integral multiple of the resonant operating wavelength of the waveguide. Where multiple magnetrons feed into the resonant ring, even in the event of failure of an individual magnetron, operation can be continued. Furthermore, the arrangement can provide a simplified assembly with removal of complicated waveguide and RF systems with RF windows and similar components. Furthermore, one or more lower power RF sources may be used instead of the higher power RF source that would otherwise be required in order to operate the linac. In another example, a higher power magnetron may be operated at a lower power than would otherwise be required, thereby increasing the lifetime of the magnetron, with the resonant ring storing and increasing the lower power output of the magnetron. It will be appreciated that the disclosure herein describes magnetrons as an RF input, but other suitable types of RF input may be used in place of the magnetrons in other examples. Detailed description Fig. 1 depicts a radiotherapy device suitable for delivering, and configured to deliver, a beam of radiation to a patient during radiotherapy treatment. The device and its constituent components will be described generally for the purpose of providing useful accompanying information for the present invention. The device depicted in Fig. 1 is in accordance with the present disclosure and is suitable for use with the disclosed systems and apparatuses. While the device in Fig. 1 is an MR-linac with combined magnetic resonance (MR) and radiotherapy capabilities, the implementations of the present disclosure may be any radiotherapy device, for example a linear accelerator (linac)-based device. The device 100 depicted in Fig. 1 is an MR-linac. The device 100 comprises both MR imaging apparatus 112 and radiotherapy (RT) apparatus which may comprise a linac device. The MR imaging apparatus 112 is shown in cross-section in the diagram. In operation, the MR scanner produces MR images of the patient, and the linac device produces and shapes a beam of radiation and directs it toward a target region within a patient's body in accordance with a radiotherapy treatment plan. The depicted device does not have the usual 'housing' which would cover the MR imaging apparatus 112 and RT apparatus in a commercial setting such as a hospital. The MR-linac device depicted in Fig. 1 comprises a source of radiofrequency waves 102, a waveguide 104, a source of electrons 106, a source of radiation 106, a collimator 108 such as a multi-leaf collimator configured to collimate and shape the beam, MR imaging apparatus 112, and a patient support surface 114. In use, the device would also comprise a housing (not shown) which, together with the ring-shaped gantry, defines a bore. The moveable support surface 114 can be used to move a patient, or other subject, into the bore when an MR scan and / or when radiotherapy is to commence. The MR imaging apparatus 112, RT apparatus, and a subject support surface actuator are communicatively coupled to a controller or processor. The controller is also communicatively coupled to a memory device comprising computer-executable instructions which may be executed by the controller. The RT apparatus comprises a source of radiation and a radiation detector (not shown). Typically, the radiation detector is positioned diametrically opposed to the radiation source. The radiation detector is suitable for, and configured to, produce radiation intensity data. In particular, the radiation detector is positioned and configured to detect the intensity of radiation which has passed through the subject. The radiation detector may also be described as radiation detecting means and may form part of a portal imaging system. The radiation source may comprise a beam generation system. For a linac, the beam generation system may comprise a source of RF energy 102, an electron gun 106, and a waveguide 104. The radiation source is attached to the rotatable gantry 116 so as to rotate with the gantry 116. In this way, the radiation source is rotatable around the patient so that the treatment beam 110 can be applied from different angles around the gantry 116. In a preferred implementation, the gantry is continuously rotatable. In other words, the gantry can be rotated by 360 degrees around the patient, and in fact can continue to be rotated past 360 degrees. The gantry may be ring-shaped. In other words, the gantry may be a ring-gantry. The source 102 of radiofrequency waves, such as a magnetron, is configured to produce radiofrequency waves. The source 102 of radiofrequency waves is coupled to the waveguide 104 via circulator 118 and is configured to pulse radiofrequency waves into the waveguide 104. Radiofrequency waves may pass from the source 102 of radiofrequency waves through an RF input window and into an RF input connecting pipe or tube. A source of electrons 106, such as an electron gun, is also coupled to the waveguide 104 and is configured to inject electrons into the waveguide 104. In the electron gun 106, electrons are thermionically emitted from a cathode filament as the filament is heated. The temperature of the filament controls the number of electrons injected. The injection of electrons into the waveguide 104 is synchronised with the pumping of the radiofrequency waves into the waveguide 104. The design and operation of the radiofrequency wave source 102, electron source and the waveguide 104 is such that the radiofrequency waves accelerate the electrons to very high energies as the electrons propagate through the waveguide 104. The design of the waveguide 104 depends on whether the linac accelerates the electrons using a standing wave or travelling wave, though the waveguide typically comprises a series of cells or cavities, each cavity connected by a hole or 'iris' through which the electron beam may pass. The cavities are coupled in order that a suitable electric field pattern is produced which accelerates electrons propagating through the waveguide 104. As the electrons are accelerated in the waveguide 104, the electron beam path is controlled by a suitable arrangement of steering magnets, or steering coils, which surround the waveguide 104. The arrangement of steering magnets may comprise, for example, two sets of quadrupole magnets. Once the electrons have been accelerated, they may pass into a flight tube. The flight tube may be connected to the waveguide by a connecting tube. This connecting tube or connecting structure may be called a drift tube. The electrons travel toward a heavy metal target which may comprise, for example, tungsten. Whilst the electrons travel through the flight tube, an arrangement of focusing magnets act to direct and focus the beam on the target. To ensure that propagation of the electrons is not impeded as the electron beam travels toward the target, the waveguide 104 is evacuated using a vacuum system comprising a vacuum pump or an arrangement of vacuum pumps. The pump system is capable of producing ultra-high vacuum (UHV) conditions in the waveguide 104 and in the flight tube. The vacuum system also ensures UHV conditions in the electron gun. Electrons can be accelerated to speeds approaching the speed of light in the evacuated waveguide 104. The source of radiation is configured to direct a beam 110 of therapeutic radiation toward a patient positioned on the patient support surface 114. The source of radiation may comprise a heavy metal target toward which the high energy electrons exiting the waveguide are directed. When the electrons strike the target, X-rays are produced in a variety of directions. A primary collimator may block X-rays travelling in certain directions and pass only forward travelling X-rays to produce a treatment beam 110. The X-rays may be filtered and may pass through one or more ion chambers for dose measuring. The beam can be shaped in various ways by beam-shaping apparatus, for example by using a multileaf collimator 108, before it passes into the patient as part of radiotherapy treatment. In some implementations, the source of radiation is configured to emit either an X-ray beam or an electron particle beam. Such implementations allow the device to provide electron beam therapy, i.e. a type of external beam therapy where electrons, rather than X-rays, are directed toward the target region. It is possible to 'swap' between a first mode in which X-rays are emitted and a second mode in which electrons are emitted by adjusting the components of the linac. In essence, it is possible to swap between the first and second mode by moving the heavy metal target in or out of the electron beam path and replacing it with a so-called 'electron window'. The electron window is substantially transparent to electrons and allows electrons to exit the flight tube. The subject or patient support surface 114 is configured to move between a first position substantially outside the bore, and a second position substantially inside the bore. In the first position, a patient or subject can mount the patient support surface. The support surface 114, and patient, can then be moved inside the bore, to the second position, in order for the patient to be imaged by the MR imaging apparatus 112 and / or imaged or treated using the RT apparatus. The movement of the patient support surface is effected and controlled by a subject support surface actuator, which may be described as an actuation mechanism. The actuation mechanism is configured to move the subject support surface in a direction parallel to, and defined by, the central axis of the bore. The terms subject and patient are used interchangeably herein such that the subject support surface can also be described as a patient support surface. The subject support surface may also be referred to as a moveable or adjustable couch or table. The radiotherapy apparatus / device depicted in Fig. 1 also comprises MR imaging apparatus 112. The MR imaging apparatus 112 is configured to obtain images of a subject positioned, i.e. located, on the subject support surface 114. The MR imaging apparatus 112 may also be referred to as the MR imager. The MR imaging apparatus 112 may be a conventional MR imaging apparatus operating in a known manner to obtain MR data, for example MR images. The skilled person will appreciate that such a MR imaging apparatus 112 may comprise a primary magnet, one or more gradient coils, one or more receive coils, and an RF pulse applicator. The operation of the MR imaging apparatus is controlled by the controller. The controller is a computer, processor, or other processing apparatus. The controller may be formed by several discrete processors; for example, the controller may comprise an MR imaging apparatus processor, which controls the MR imaging apparatus 110; an RT apparatus processor, which controls the operation of the RT apparatus; and a subject support surface processor which controls the operation and actuation of the subject support surface. The controller is communicatively coupled to a memory, e.g., a computer readable medium. The linac device also comprises several other components and systems as will be understood by the skilled person. For example, in order to ensure the linac does not leak radiation, appropriate shielding is also provided. A first implementation according to the approach described herein is shown in Fig. 2 in which a magnetron 202 outputs pulses to a resonant ring 204. In typical known arrangements, the magnetron 202 would feed directly to a linac 206 which accelerates electrons from an electron gun 208 to a treatment head 210. In current systems, electrons are only required during, for example, one of every 243 magnetron / RF pulses, the remaining pulses being discarded. However, using the approach disclosed herein, because the magnetron 202 feeds to the resonant ring 204, the RF power from the magnetron can be stored for the duration of 242 pulses, rather than being discarded. This in turn means that one or more lower power magnetrons or other RF inputs can used to charge up the ring which can then pulse the combined stored RF into the linac when required. Resonant ring structures are known in RF and optical systems but principally in optical systems such as lasers or for testing high voltage components where they are not used for storage capability. Resonant rings, or travelling wave resonators, are described, for example, in "Resonant Ring for Testing of Accelerator RF Windows" E. Gerken et al. Los Alamos National Laboratory. They comprise, typically, a ring or loop of waveguide where the length of the waveguide matches an integer number of wavelengths of the frequency of operation, and within the operating range of the waveguide. In the present approach, the ring includes an input (from the magnetron), an output port to the linac (although in some examples both input and output can be part of the same physical item) and a tuner designated generally 212 which can be any appropriate structure including a tuner, impedance matcher, trombone, stub or an additional part of the physical input / output structure. The tuner 212 for the input and output of the resonant ring 204 is set to allow feed in (e.g. from the magnetron) and out (e.g. to the linac) of the structure as needed without impedance losses through e.g. reflection. The Q-value (the ratio of frequency to bandwidth of the resonator) of the cavity within the waveguide forming the resonant ring 204 is typically very high such that it may require several cycles to fill the structure at the resonant / operational frequency, amplifying apparent power through the coupling of waves at the input. This is considered an acceptable trade off against the close matching with the operational frequency that is thus obtained. It will further be noted that there is no maximum length to the resonant ring as long as it is an integral number of operational frequency wavelengths. As will be seen, because energy is stored over multiple cycles through the summation of multiple pulses, multiple lower power RF input sources such as commercially available magnetrons can be used, and these can be used in pulsed or continuous mode as appropriate. To control output of the resonant ring 204, a "kicker" 214 is provided comprising a physical mechanism such as a ferrite or solid-state mechanism connected to a control system (which can be integral with 214 or separate) that is triggered based on the power in the resonant ring 204 and, for example, causes an impedance change from a reflection mode to transmission mode. The kicker is synchronised to the electron gun 208 of the linac 206, for example using a conventional synchronisation control signal. Hence the power output by the magnetron 202 can be stored in the resonant ring 204, built up gradually, compensating for the filling time associated with a high Q-value, and released into the linac 206 when required. The magnetron 202, resonant ring 204, tuner 212, and kicker 214 together form an RF source 200 which can power a linac of a radiotherapy device. In addition to conventional uses of the arrangement, the extremely high-power RF output that can be achieved permits use in other radiotherapy treatment processes such as tumour ablation. In this case RF can be taken from the resonator ring, for example using co-axial cable and directed appropriately. This is particularly attractive as it is possible to take a direct co-axial output from the resonant ring. Yet further, the high Q-value of the ring resonator ensures that the desired frequencies are intrinsically selected. A second implementation is shown in Fig. 3 in which like elements are designated with like numerals. In particular it will be seen that a pair of magnetrons 300, 302 are combined through a combiner 304, for example a hybrid combiner such as a magic tee, 3dB coupler or other appropriate components. Using such an arrangement, the power supplied to the resonant ring 204 can be increased and / or multiple lower power magnetron inputs can be implemented. Further power can thus be added. The magnetrons 300, 302, combiner 304, resonant ring 204, tuner 212, kicker 214 thus together form an RF source 310 which can power a linac of a radiotherapy device. Frequency differences between input magnetrons will be pulled to the operational / resonant frequency of the resonant ring, and phase match can be achieved by using the inherent phase pulling operation of the combiner, or other phase locking techniques. Phase conservation between pulses in the time domain may be achieved by virtue of the resonant structure of the resonant ring 204. The individual components can be conventional elements with appropriate operating parameters. For example, the RF input source (such as the magnetron 202 of Fig. 2) may be an E2V (registered trade mark) 3.1 MW magnetron or any standard radiotherapy magnetron, for example operating at 3 GHz. Alternatively, the RF input sources can be klystrons, which may provide improved frequency and phase accuracy. Any suitable microwave waveguide can be adopted as the resonant ring, for example a WR284 (3 GHz) waveguide with a length equivalent to an integral number of wavelengths, which will provide an inherent high Q value. Additionally, it will be noted that the arrangement can be used in either a pulsed or continuous-wave configuration as appropriate; indeed this arrangement can simplify inter-pulse phase matching and the kicker can be used to provide an output pulse from the ring resonator synchronised with the linac electron injector; furthermore, even if continuous-wave magnetron sources are used, they can be pulsed because of the lower power requirements to provide acceptable operation using multiple sources. In operation, the magnetron or magnetrons are powered continuously or in pulsed mode and power is built up in the resonant ring 204 until release is triggered for example by the kicker either based on power levels attained or in conjunction with firing of other components of the device such as the electron gun 208. Synchronisation of magnetron pulses with each other, and of the kicker with the electron gun, can be achieved with conventional control electronics. It will be seen that by using the resonant ring, a lower power pulse or continuous-wave RF source can be used and that multiple magnetrons can for example be implemented such that even if there is individual component failure, the system will continue to operate, albeit at a lower power. Because of the use of multiple lower-power magnetrons, the power output is acceptable even if there is limited failure of individual components. This in turn permits extending the lifetime of the magnetrons as they can be operated at a below maximum power. Disclosed herein is a method of operating a radio frequency, RF, source for a radiotherapy device, the method comprising: generating a plurality of input pulses by an RF generator; providing the plurality of input pulses to a resonant ring; and controlling the resonant ring to provide a single output pulse for the plurality of input pulses. In some examples of the method, the RF generator comprises a magnetron. In some examples of the method, the RF source comprises a plurality of RF generators. In some examples of the method, the RF generators are arranged to each provide input pulses to the resonant ring via a combiner. In some examples of the method, the RF source further comprises a kicker, the method further comprising using the kicker to control the resonant ring to provide the single output pulse. In some examples of the method, the resonant ring comprises a waveguide of integral wavelength length of a predetermined operational frequency. In some examples of the method, the resonant ring is provided around a linac receiving the output pulse from the resonant ring. In some examples of the method, the method further comprises using the single output pulse to operate a linear accelerator of a radiotherapy device. It will be seen that various alternative approaches can be implemented without departing from the inventive concept. In an example, it is possible to put the resonant ring around the linac with the output at the magnetron RF input port permitting the entire system to be maintained at vacuum and providing an integrated, compact e.g., copper structure without the requirement for RF windows between vacuum and non-vacuum components. It will be seen that the arrangement can be used to feed any linac, for example in a radiotherapy apparatus, for either conventional beam therapy or ablation therapy. The above implementations have been described by way of example only, and the described implementations and arrangements are to be considered in all respects only as illustrative and not restrictive. It will be appreciated that variations of the described implementations and arrangements may be made without departing from the scope of the invention.

Claims

1. A radiofrequency, RF, source for a radiotherapy device, the RF source comprising a plurality of RF generators and a resonant ring, the RF generators configured to output a plurality of input pulses to a resonant ring, the resonant ring being controllable to provide a single output pulse from a plurality of input pulses,the RF source further comprising a kicker, the kicker configured to control the resonant ring to provide the single output pulse,wherein the resonant ring is provided around a linac of a radiotherapy device arranged to receive the output pulse from the resonant ring and the kicker is synchronised to an electron gun of the linac.

2. The RF source of claim 1, wherein the plurality of RF generators comprises a magnetron.

3. The RF source of claim 1 or claim 2, wherein the RF generators are arranged to each provide input pulses to the resonant ring via a combiner.

4. The RF source of any preceding claim, wherein the resonant ring comprises a waveguide of integer wavelength length of a predetermined operational frequency.

5. A radiotherapy device comprising an RF source as claimed in any of claims 1-4.

6. A radiotherapy ablation device comprising an RF source as claimed in any of claims 1-4.

7. A method of operating a radio frequency, RF, source for a radiotherapy device, the method comprising:generating a plurality of input pulses by an RF generator;providing the plurality of input pulses to a resonant ring; andcontrolling the resonant ring to provide a single output pulse for the plurality of input pulses, wherein the RF source comprises a plurality of RF generators and a kicker, the method further comprising using the kicker to control the resonant ring to provide the single output pulse,wherein the resonant ring is provided around a linac of a radiotherapy device arranged to receive the output pulse from the resonant ring and the kicker is synchronised to an electron gun of the linac.

8. The method of claim 7, wherein the plurality of RF generators comprises a magnetron.

9. The method of claim 7 or claim 8, wherein the RF generators are arranged to each provide input pulses to the resonant ring via a combiner.

10. The method of any of claims 7 to 9, wherein the resonant ring comprises a waveguide of integer wavelength length of a predetermined operational frequency.

11. The method of any of claims 7 to 10, the method further comprising using the single output pulse to operate a linear accelerator of a radiotherapy device.

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