Patent Number: 041397770
Section: summary

This invention relates to a cyclotron and to a therapy installation. More particularly, this invention relates to a cyclotron suitable for use in neutron therapy, and to a neutron therapy installation including such a cyclotron. According to the invention there is provided a cyclotron suitable for use in neutron therapy, and comprising: (a) a pair of opposed, spaced pole shoes having their adjacent inner surfaces defining an accelerator zone, PA0 (b) an electromagnetic coil system, PA0 (c) at least one hollow accelerating dee electrode positioned in the accelerator zone, and having a radio frequency resonator associated therewith, PA0 (d) a magnet yoke shaped to substantially enclose the accelerator zone and constitute a neutron attenuation shield for neutrons produced in the cyclotron, PA0 (e) a vacuum chamber enclosing the accelerator zone and each dee electrode, PA0 (f) means for providing charged particles for acceleration within the accelerator zone, PA0 (g) a target zone for a target device, and PA0 (h) a neutron beam outlet in the magnet yoke for emission of a neutron beam produced in the cyclotron. In an embodiment of the invention, the cyclotron may be in the form of an isochronous cyclotron having an isochronous magnetic field with at least three hills and valleys defined by the adjacent inner surfaces of the pole shoes. In this embodiment of the invention, the hills and valleys may conveniently be radial hills and valleys. Further, in this embodiment of the invention, the cyclotron may conveniently have a dee electrode positioned in each valley, with each dee electrode having a radiofrequency resonator associated therewith. By having the dee electrodes positioned in the valleys, the width of the pole gap can be decreased and compactness of the cyclotron can thus be improved. In this specification, by a "hollow accelerating dee electrode" is meant a hollow accelerating electrode of conventional type, having an appropriate configuration for the number and positioning of the electrodes in a particular cyclotron application. Where a plurality of dee electrodes are employed, the dee electrodes may be adapted to be operated in phase or out of phase with each other. Where the dee electrodes are to be operated in phase with each other, they may conveniently be connected to each other at the centre of the accelerator zone. The cyclotron may have a suitably positioned transmission bore for a radio-frequency energy transmission line, for each dee electrode. In an embodiment of the invention, the cyclotron may have a transmission bore extending along the polar axis of one of the pole shoes, with the transmission bore having a radio-frequency transmission line located in the transmission bore and connected to the dee electrodes at the centre of the accelerator zone, and with the transmission line having coupling means for coupling the transmission line to a radio-frequency power source. The coupling means may be of any suitable type. Thus, for example, the coupling means may be in the form of capacitor or inductive coupling means, a swivel coupling connection, or the like. Connection of the dee electrodes to the transmission line at the centre of the accelerator zone, can assist in axially stabilising the dee electrodes to combat frequency variations under pulsed dee voltages during use. The means for providing charged particles for acceleration within the accelerator zone, may be any suitable conventional means. Thus, for example, the means may comprise injection means for the injection of charged particles into the accelerator zone or a loading passage for loading the cyclotron with a suitable ion source. By having the magnet yoke shaped to substantially enclose the accelerator zone, the distribution of the magnet steel is such as to facilitate the construction of a compact cyclotron in accordance with this invention. In addition, the magnet yoke will thus constitute a neutron attenuation shield for neutrons produced in the cyclotron. In general, for a cyclotron in accordance with this invention, having a sufficient energy to provide a fast neutron beam suitable for use in neutron therapy, once the magnet yoke has a sufficient wall thickness for the magnetic flux requirements, the wall thickness will be sufficient to attenuate neutron fluxes of low intensity coming off at wide angles to the forward neutron peak zone of a neutron beam produced in the cyclotron as well as stray neutrons arising in the cyclotron, to a patient tolerable level in a neutron therapy installation incorporating such a cyclotron. Where deuterons are to be accelerated with the cyclotron of this invention, applicant believes that the minimum size of the cyclotron which will be required to produce a neutron beam suitable for neutron therapy, would correspond to a deuteron energy of about 20 to 30 MeV, and conveniently, about 35 MeV. For a deuteron energy of this order, when the magnet yoke is sufficient for the magnetic flux requirements, the wall thickness should be sufficient for neutron attenuation other than in the forward peak zone, to a patient tolerable level in a neutron therapy installation. For higher deuteron energies, the magnetic flux requirements will increase correspondingly, so that the wall thickness of the magnet yoke should again be sufficient for the above purpose. However, for the acceleration of protons, applicant believes that the minimum size of the cyclotron which will be required to produce a neutron beam suitable for neutron therapy, would correspond to a proton energy of about 20 to 30 MeV, and conveniently about 35 MeV. For a proton energy of a particular value, the radius of the outer orbit would be reduced by about 40% in relation to a deuteron energy of the same value, leading to an approximately 40% reduction in size of the cyclotron. Thus, for the acceleration of protons, it will usually be necessary to increase the thickness of the magnet yoke over and above that dictated by the magnetic flux requirements to provide sufficient attenuation for the purpose specified above. For the acceleration of Helium-3, the cyclotron would tend to have a size intermediate that of the corresponding proton and deuteron accelerating cyclotrons, and thus increased thickness of the magnet yoke over and above that dictated by the magnetic flux requirements, would usually be necessary. For use in neutron therapy, auxiliary neutron shield means will be required in the forward peak zone of a neutron beam produced in the cyclotron, to shield the body of a patient being treated, against the intense flux of high energy neutrons in the forward peak zone of the neutron beam. In one embodiment of the invention, the cyclotron of this invention may therefore be used for neutron therapy with an external auxiliary neutron shield provided in the forward peak zone of a neutron beam produced in the cyclotron, between the cyclotron and a patient to be treated. In this embodiment, the external auxiliary neutron shield may be separate from the cyclotron, or may be mounted on the exterior of the cyclotron about the neutron beam outlet. However, by using an external auxiliary neutron shield, the distance between an affected area of a patient to be treated, and the neutron beam generation zone within the cyclotron, is increased so that higher neutron production rates will be required for effective treatment. In an alternative embodiment of the invention, the cyclotron may include auxiliary neutron shield means in the forward neutron peak zone of a neutron beam produced in the cyclotron. The auxiliary neutron shield means may, for example, be sufficient to attenuate neutron and gamma radiation in the forward peak zone to provide a combined neutron and gamma radiation dose rate of less than about 3% of that in a neutron beam emitted from the neutron beam outlet during use. In a specific example of the invention, the auxiliary neutron shield means may be sufficient to attenuate neutron and gamma radiation in the forward peak zone to provide a combined neutron and gamma radiation dose rate of less than about 2%, and conveniently less than about 1%, of that in a neutron beam emitted from the neutron beam outlet during use. Gamma rays arise on a target during neutron beam production on the target, and also result from neutron attenuation in the magnet yoke. In an example of the invention, the auxiliary neutron shield means may comprise a neutron attenuation shield and a neutron moderating shield. The neutron attenuation shield may be of any conventional non-magnetic material such as, for example, copper, and may be mounted on the outer periphery of the magnet yoke and/or within the inner periphery of the magnet yoke in the forward peak zone. The neutron moderating shield may be of any conventional type. It may thus, for example, comprise any suitable hydrogen containing moderating material, which may contain suitable known elements such as boron, cadmium, or the like, to capture thermal neutrons. The neutron moderating shield may be provided on the outer and/or inner periphery of the magnet yoke and/or in a cavity within the magnet yoke, in the forward peak zone. The auxiliary neutron shield means may further be provided by the magnet yoke having an increased thickness in the forward peak zone. Where the auxiliary neutron shield means comprises part of the cyclotron, this provides the advantage that shorter distances can be employed between the neutron beam source within the cyclotron, and an affected area of a patient to be treated thereby allowing for lower neutron production rates. It is a further advantage that by having the auxiliary neutron shield means in close proximity to the neutron beam source, the mass of shielding required can be reduced, thereby allowing for a reduction of the mass of the cyclotron and for improvement of the compactness thereof. It is a further advantage that the auxiliary neutron shield means would tend to be the most radioactive part of the cyclotron and can therefore be replaced when necessary or can be removed to facilitate servicing. The neutron beam outlet may conveniently be shaped to removably receive different size collimators to control the radiation field size of a neutron beam emitted from the neutron beam outlet. The collimators may be made of any conventional material. Thus, for example, the collimators may be made of water extended polyester with our without metal loading, borated teak, or borated pressed wood. In an embodiment of the invention, the vacuum chamber may conveniently be defined by the adjacent inner surfaces of the pole shoes and by an annular generally channel-shape member surrounding the accelerator zone and having its free edges sealingly secured to the opposed pole shoes. The cyclotron may include internal vacuum pump means and/or may be adapted to be connected to external vacuum pump means, to maintain the required vacuum within the vacuum chamber. Where the cyclotron includes internal vacuum pump means, the internal vacuum pump means may be provided at suitable intervals along the inner periphery of the magnet yoke, and may be of any suitable conventional type. Thus, for example, the internal vacuum pump means may comprise one or more cryogenic pumps or a combination of titanium sublimation and ion pumps. Where the cyclotron is adapted for use with external vacuum pump means, the cyclotron may have a radial vacuum conduit extending radially through the magnet yoke and/or may have an axial vacuum conduit extending along the polar axis of one of the pole pieces, for connection to an external vacuum pump means. Where the cyclotron includes a radial vacuum conduit, the conduit may conveniently be provided on the opposed side of the cyclotron to the neutron beam outlet and the vacuum conduit may be in the form of a maze to afford protection for a patient against stray neutrons escaping from the vacuum conduit. The magnet yoke may conveniently comprise a plurality of separate yoke sections which together constitute the magnet yoke. Thus the yoke sections can be separately removed for replacement or to allow access to the interior of the cyclotron for maintenance purposes. Each radio-frequency resonator may be of any suitable construction, and may be positioned to extend radially or circumferentially of the polar axis of the accelerator zone. In an embodiment of the invention, each radio-frequency resonator may extend radially and may be enclosed within the magnet yoke. In this embodiment, each radio-frequency resonator may conveniently be enclosed within the vacuum chamber. In this embodiment of the invention, each radio-frequency resonator may comprise a flat, narrow inner conductor plate mounted on the dee, and two flat, wider outer conductor plates on opposed sides of and laterally spaced from the inner conductor plate, with the outer conductor plates connected to the inner conductor plate by means of short circuit plates. In this embodiment, each outer conductor plate may include a flexible hinge, each short circuit plate may be flexible, and frequency adjustment means may extend from each outer conductor plate sealingly through the walls defining the vacuum chamber, to allow for adjustment of the characteristic impedance of each radio-frequency resonator. The electro-magnetic coil system may be of any conventional type and should include cooling means of any conventional type. In an embodiment of the invention, the coil system may comprise two annular coils surrounding the pole shoes, with the coils located outside the vacuum chamber to permit the use of radiation resistant insulation material which is not necessarily vacuum compatible. The cyclotron may, if necessary, include cooling zones associated with the pole shoes. In an embodiment of the invention, each cooling zone may comprise walls arranged in a spiral to ensure a substantially even distribution of a cooling medium over the pole shoes irrespective of the inclination of the accelerator zone. The adjacent inner surfaces of the pole shoes which define the accelerator zone and which are exposed to the dee electrode or electrodes, may be lined or coated with any conventional vacuum compatible corrosion-resistant non-magnetic layer. Thus, for example, the layer may comprise thin water cooled copper plates or copper electrolytically deposited on the adjacent inner surfaces of the pole shoes. The target zone may comprise or include a target admission zone to allow admission of a suitable target device into the accelerator zone, and may include automatic sealing means of any conventional type. The target admission zone may conveniently extend radially through the magnet yoke. The dee electrodes of the cyclotron may be arranged in relation to the neutron beam outlet and the target zone, so that a neutron beam produced in the cyclotron can be emitted from the neutron beam outlet without interfering unduly with the dee electrode or electrodes and particularly, so that the dee electrode or electrodes will not be directly in the forward peak zone of the neutron beam. The cyclotron may have a target device positioned in the target zone. Alternatively the cyclotron may include a target device for admission through the target admission zone, and a removable shielding plug for shielding the admission zone. The target device may comprise a target stem with a suitable target provided thereon. The shielding plug may be of a suitable neutron shielding material, and may have one or more enlarged step formations towards its trailing end for shielding location in a target admission zone of a complementary configuration. Each target device may have adjustment means to permit limited adjustment of the circumferential position of the target and of the extent to which a target projects into the accelerator zone of the cyclotron, thereby allowing for interception of an orbiting beam at differing circumferential positions and radial distances from its starting zone. Thus by appropriate adjustment, the peak of the neutron flux beam produced can be made to coincide with the axis of the neutron beam outlet. The target device may be in the form of a stationary target device having conventional liquid cooling tubes leading through the target stem to the target. Alternatively, the target device may be in the form of a rotatable target device which can be liquid cooled by conventional means or, if the speed of proposed rotation is sufficiently high, may be radiation cooled. The target of the target device may be of any suitable material. Thus, for example, it may be a beryllium target. The cyclotron of this invention may, if desired, be provided with an outer neutron moderating layer of any suitable type, to moderate slow neutrons escaping from the magnet yoke. In an embodiment of the invention, the outer neutron moderating layer may be provided by borated wood secured to the outer periphery of the cyclotron. In an alternative embodiment, the neutron moderating layer may comprise boron containing paraffin wax located within an outer shell about the cyclotron. The cyclotron of this invention may be provided with a plurality of neutron beam outlets, and a target zone operatively positioned relatively to each neutron beam outlet. Thus, for example, the cyclotron may be provided with two, three, or more neutron beam outlets and associated target zones. Thus, during use, a neutron beam can be produced for emission out of any desired neutron beam outlet by appropriate movement of the target devices into or out of the accelerator zone within the cyclotron. For each neutron beam outlet not in use at any time, a suitably shaped shielding member may be located therein, and a shielding plug may be located in its corresponding target admission zone. While the cyclotron of this invention is particularly suitable for use in neutron therapy, it will be appreciated that when heated in a suitably biologically shielded chamber, the cyclotron can be used for isotope production. The cyclotron of this invention may have pivot means for pivotally mounting the cyclotron to allow variation of the direction of a neutron beam emitted from the neutron beam outlet during use. In one embodiment of the invention, the pivot means may comprise pivot bores in opposed sides of the cyclotron, for receiving pivot shafts to pivotally support the cyclotron. In an alternative embodiment of the invention, the pivot means may comprise two pivot shafts extending outwardly from opposed sides of the cyclotron. In this embodiment of the invention, each pivot shaft may have a support roller mounted thereon for pivotally supporting the cyclotron on suitable support rails. Each support roller may conveniently be in the form of a gear wheel for supporting the cyclotron on suitable support rails in the form of linear gear rails having teeth to mesh with the gear wheel teeth. In this embodiment of the invention, the linear gear rails may be horizontal or may be inclined to the horizontal. The pivot means may conveniently have its polar axis extending normally to the plane of the accelerator zone along the polar axis of the cyclotron. In a specific embodiment of the invention, the pivot means may comprise a pivot frame in which the cyclotron is mounted, the pivot frame having a pair of opposed supporting legs, with each supporting leg having a pivot bore for pivotally mounting the frame on a pair of support axles, and the pivot frame having a pivot beam mounted thereon, with the pivot beam having guide gear wheels mounted at its opposed ends to co-operate with a pair of curved, complementarily toothed guide rails positioned concentrically with the support axles to guide pivotal displacement of the cyclotron about the pivot bores. The pivot bores may conveniently be positioned so that their axes will intersect the core of a neutron beam emitted from the neutron beam outlet during use, in the treatment zone of such a beam where the centre of an affected area of a patient to be treated, would be positioned during treatment. Thus, an isocentric therapy system would be provided. In yet a further alternative embodiment of the invention, the pivot means may comprise a turntable to support the cyclotron on a surface for pivotal displacement about a vertical axis. The invention further extends to a neutron therapy installation defined by biological shielding means, and having a cyclotron as described herein, mounted therein. Where the cyclotron has pivot means, the installation may have complementary pivot support means for engaging with the pivot means. Where the cyclotron has pivot means in the form of a turntable to support the cyclotron on the floor of a neutron therapy installation for pivotal displacement about a vertical axis, the installation may have a plurality of separate treatment rooms circumferentially spaced about the cyclotron, with each room having an access opening to register with the neutron beam outlet of the cyclotron. The same arrangement may be provided for a neutron therapy installation for use with a cyclotron having a plurality of neutron beam outlets. In this embodiment of the invention, the cyclotron may be mounted so that it is stationary, and has its polar axis extending vertically, and each separate treatment room may be in register with one of the neutron beam outlets. With these arrangements, the rate of treatment can be increased since, while a patient is being treated in one of the separate treatment rooms, a patient can be set up for treatment in another of the treatment rooms. The neutron therapy installation may include a separate servicing zone having collimator storage means for storing collimators when not in use. The collimator storage means may conveniently include a rotatably mounted collimator holder for holding a plurality of collimators at circumferentially spaced intervals. The holder may include transferring means for transferring collimators from and to the neutron beam outlet when such an outlet has been correctly positioned by pivotal displacement of the cyclotron. Thus, when a particular collimator is to be replaced because it has become too radio-active, or because a differing size is required, this can be effected automatically by remote control in the servicing zone thereby reducing the risk of radiation contamination. The therapy installation may include one or more plinths for supporting patients. The plinths may be of any conventional type, and may be vertically and/or laterally adjustable. The cyclotron of this invention may be designed for the acceleration of charged particles in the form of deuterons, protons and helium-3. Where the cyclotron of this invention is to be used in neutron therapy it may, for example, have a capacity to provide accelerated charged particles having an energy in the range of about 15 to 80 MeV deuterons, and conveniently at least about 35 MeV deuterons. In the case of protons, the cyclotron may, for example, have a capacity to provide accelerated charged particles having an energy in the range of about 15 to 100 MeV protons, and conveniently at least about 35 MeV. In the case of helium-3, the cyclotron may, for example, have a capacity to provide accelerated particles having an energy in the range of about 20 to 100 MeV helium-3 particles. Embodiments of the invention are now described by way of example with reference to the accompanying drawings.