GB2502109A

GB2502109A - X-ray radiotherapy apparatus with indirectly heated emissive cathode

Info

Publication number: GB2502109A
Application number: GB1208631.0A
Authority: GB
Inventors: Keith Albert Spanswick; George Andrew Leeder
Original assignee: ARIANE MEDICAL SYSTEMS Ltd
Current assignee: ARIANE MEDICAL SYSTEMS Ltd
Priority date: 2012-05-16
Filing date: 2012-05-16
Publication date: 2013-11-20
Also published as: EP2850634A1; WO2013171491A1; EP2850634B1; GB201208631D0; ES2644288T3

Abstract

A radiotherapy apparatus is described having a vacuum chamber 20 in which an indirectly heated cathode electron source 22 capable of emitting an electron beam is provided. A radiation generating target anode 27, capable of generating X-rays in response to the incidence of the electron beam thereon, is positioned at the distal end of a rod-shaped conduit 24 which guides the electron beam to the anode.Â

Description

RADIOTHERAPY APPARATUS

This invention relates to the field of radiotherapy apparatus, particularly for the treatment of human cancers.

BACKGROUND

When treating cancers with low energy X-rays, a technique known as contact radiation therapy, X ray brachytherapy or electronic brachytherapy, it is desirable that the source of the radiation is placed physically close to the treatment site. One means of achieving this is to use radiotherapy apparatus in the form of a rod anode X-ray tube of the known type illustrated in Figure 1.

The rod anode X-ray tube consists of a vacuum chamber 10 containing an emissive filament 11 and a rod structure 12 terminated with a high density metal target or transmission anode 13. The filament 11 is connected to a high voltage generator 15 which is grounded by grounded anode 16. The filament typically consists of a coiled tungsten wire that is heated until white hot when electrons are liberated from the surface. Electrons emitted from the filament 11 are focussed along a desired path by a focus electrode 14. The rod structure 12 is a rod-shaped electron beam conduit or drift tube through which the electron beam travels from the filament to the anode. X-rays are produced when the electrons, attracted by the strong positive charge (typically 50 kV) of the anode 13, collide with the anode's surface. Most of the electron energy is dissipated in the form of heat but a small number of photons (X-rays) with peak energy equal to the attracting potential are produced.

The transmission anode 13 needs to be very thin (typically halt a wavelength, of the order of 5pm) to allow the X-rays to exit. Alternatively, a reflective anode (rather than a transmission anode) can be used with a radiation transparent window in the otherwise radiopaque rod 12 providing an exit path for the X-ray beam. After suitable filtering of unwanted low energy components and collimation to correspond with the surface to be treated, the radiation is directed to the cancerous tissue, normally at a distance of 20-50 mm from the anode 13.

The extent to which electrons emitted by the filament 11 can be effectively focussed is limited because of the non-uniform shape of the coiled wire filament. Poor tocussing of the electrons reduces the quantity of useable X-ray radiation output by the tube and increases the risk of X-ray radiation being undesirably generated in other parts of the apparatus than the anode 13.

Furthermore, the tube can be relatively inefficient because so much of the electron energy is dissipated as heat. Either the tube has to be operated at low power (to allow the heat to dissipate) which undesirably increases treatment time, or a cooling mechanism for the anode needs to be provided such as that disclosed in US8094784 (Rapiscan Systems, Inc).

An X-ray tube needs to have very specific characteristics in order to be successfully used as radiotherapy apparatus for the treatment of human cancers. By way of example, The rod needs to be suitably sized and shaped in order to reach into the body cavity (e.g. rectum or vagina or intra-operative surgical site) where treatment is required.

* The X-ray dose rate is proportional to the tube beam current and this should be sufficient to deliver therapeutic dose as quickly as possible to avoid error due to involuntary movement, discomfort or in the case of intra-operative treatment, undesirable increases in the procedure time.

The X-ray radiation beam should be isotropic (multi-directional) so that the tumour can be accessed regardless of the orientation in relation to the rod structure. The radiation beam must be uniform in all directions (preferably having a circular profile) and its position stable. In apparatus of the type illustrated in Figure 1, the output X-ray beam has a profile dependent upon the shape of the filament. This may differ considerably from the circular profile which would be ideal to achieve uniformity.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided radiotherapy apparatus comprising: a vacuum chamber (20) in which is provided an electron source (22) capable of emitting an electron beam; a radiation-generating target anode (27) capable of generating X-rays in response to the incidence of the electron beam thereon; and a rod-shaped conduit (24) for guiding the electron beam to the anode which is located at a distal end thereof, characterised in that the electron source comprises an indirectly heated electron-emissive cathode.

According to a second aspect of the invention there is provided a radiotherapy system comprising radiotherapy apparatus as described above and a set of applicators.

Further features of the invention are defined in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be more particularly described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a schematic illustration of a prior art X-ray tube apparatus with transmission anode; Figure 2 is a schematic illustration of radiotherapy apparatus embodying aspects of the invention; and Figure 3 shows the target anode from Figure 2, drawn to a larger scale, and with the electron beam represented.

DETAILED DESCRIPTION

Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and comprises", means "including but not limited to", and is not intended to (and does not) exclude other moieties, additives, components, integers or steps.

Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith.

FILAMENT

Figure 2 is a schematic illustration of radiotherapy apparatus having a vacuum chamber containing a heating filament 21. Unlike the prior art filament of Figure 1, the heating filament 21 is used to heat an electron emissive cathode 22 comprising a generally cylindrical shape with a hemispherical end coated with an electron emissive material such as strontium. Electrons are emitted to produce an electron beam. In this way, the electrons used to produce the electron beam are emitted from an indirectly heated cathode rather than directly from the filament. Electrons emitted from the cathode 22 (the electron beam) are focussed along a desired path by a focus electrode 23. The shape of the cathode 22 can be selected to optimise the profile of the electron beam so that it preferably has a circular profile.

BEAM POSITIONING

The electron beam is attracted towards a rod-shaped anode structure or conduit 24 within the vacuum chamber 20, at the distal end of which it is focussed onto an anode (described in more detail below). The rod is typically at least 10-15 cm long and preferably 20 cm in order to reach into body cavities such as rectum and vagina. It is desirable for the electron beam to enter the rod structure 24 centrally for it to reach the anode. Centering of the electron beam can be achieved by magnetic deflection. It is known to use electromagnets for this purpose but their performance is temperature dependant, they are relatively large, require control circuits, and it can be difficult to sense when they are working correctly.

An alternative positioning apparatus for the electron beam is illustrated in Figure 2 in which an array of high field intensity permanent magnets 25 located near the proximal end of the rod 24 are position-adjustable. There are preferably three permanent magnets spaced with 120° separation around the longitudinal axis of the rod 24. The magnetic field intensity can be changed by advancing or retracting the magnets 25 radially in relation to the longitudinal axis of the rod.

A further advantage of the electron beam profile being symmetrical is that the effect of such magnetic positioning electrodes (and indeed the influence of the electrostatic focussing electrode 23) is uniform, permitting a relatively simple design thereof.

A "scraper' electrode assembly 26 comprising a non-radiation-emissive (low density) material, for example aluminium, is located at the point along the axis immediately forward (distal) of the deflecting magnets 25. The scraper electrode has the effect of absorbing any electrons which are significantly misaligned that would, if not absorbed, produce unwanted and potentially dangerous X-ray radiation at the rod entrance.

The beam positioning apparatus and scraper electrode assembly may be useful in any radiotherapy apparatus having a rod-shaped electron beam conduit, not necessarily only such apparatus having an indirectly heated electron-emissive cathode.

ANODE

The target anode 27 located at the end of the rod 24 is generally hemispherical.

Incidence of the electron beam on the anode causes the emission of X-rays therefrom.

The anode 27 is preferably of the transmission type whereby X-rays are emitted from the assembly isotropically or in a spherical field due to the anode being sufficiently thin (less than 0.5 of the radiation wavelength) to avoid self-absorption. The target anode 27 is deposited on a beryllium transmissive hemispherical window 28 to ensure good thermal conduction to a surface external to the vacuum chamber 20.

By making the target anode 27 larger in diameter than the electron beam a small amount of misalignment can be accommodated (see Figure 3).

The curvature of the window 28 and hence the anode surface, when combined with the large focal spot, eliminates lateral fall off which is normally associated with micro focus electron beams when they interact with a conventional planar perpendicular X-ray target anode.

The large diameter (preferably 4 mm) focal spot also has two further advantages: * The electron beam gives up its energy in the form of heat distributed over a larger surface area and therefore eliminates high temperature gradients.

The resulting X-ray beam is diffuse and can be considered as a large number of different rays each with their own beam path. The effect is to mask or blur any small absorbing artefacts with the beam path.

A ceramic insulator 29 is located in the rod proximally of the anode 27 so that X-rays are emitted preferably from a 3100 field rather than a 360° spherical field.

The features of the target anode described above may be useful in any radiotherapy apparatus having a rod-shaped electron beam conduit, not necessarily only such apparatus having an indirectly heated electron-emissive cathode.

COOLING

Heat is generated at the distal end of the rod 24 and the proximal end at the vacuum chamber 20 must be maintained at near ambient temperature to avoid instability within the vacuum chamber 20. This temperature gradient (which may be in excess of 200 degrees Centigrade) can cause undesirable physical distortion of the rod 24. Known cooling systems tend to be located at and for the purpose of cooling the anode only.

A cooling system is provided comprising a jacket 30 closely surrounding substantially the whole rod 24. The cooling jacket 30 contains a circulating coolant fluid (for example oil or water) which extracts heat to minimise temperature stresses on the rod.

The features of the cooling system described above may be useful in any radiotherapy apparatus having a rod-shaped electron beam conduit, not necessarily only such apparatus having an indirectly heated electron-emissive cathode.

OUTPUT MONITOR

Assuming the energy of the X-ray beam is stable, the radiation output will be proportional to the electron beam current. This only holds true if the entire beam hits the target anode 27. It is, therefore, useful to monitor the current flow at the target window 28 rather than at the electron gun (i.e. filament 21/cathode 22).

This is achieved by coupling the window 28 to the rod 24 via the ceramic insulator 29. A resistor R1 (typically 1 kO) is then used to electrically connect the window 28 to the grounded rod 24. A potential proportional to the beam current is generated which is used as a radiation output monitor.

The features of the radiation output monitor described above may be useful in any radiotherapy apparatus having a rod-shaped electron beam conduit, not necessarily only such apparatus having an indirectly heated electron-emissive cathode.

APPLICATORS

The radiotherapy apparatus of the present invention is capable of being used in association with a set of applicators of the type described in the inventors' co-pending application FR1153529.

In particular, a set of applicators (not illustrated) is provided in which the choice of individual applicator to be used depends upon the nature and size of the treatment site.

Preferably, each applicator in the set comprises a spherical hollow head of different diameter having an outer surface adapted to be in contact with a cavity of living tissue and an inner surface defining an internal volume adapted to receive X-rays from the radiotherapy apparatus, wherein at least one zone of the head is capable of being traversed by said X-rays, and wherein the thickness of said zone is a function of the diameter of said head, configured so that the X-ray dose produced at the outer surface is between 18 and 22 Gray.

The features of the applicators described above may be useful in any radiotherapy apparatus having a rod-shaped electron beam conduit, not necessarily only such apparatus having an indirectly heated electron-emissive cathode.

Radiotherapy apparatus as described herein is capable of delivering a therapeutic dose in less than three minutes to avoid error due to involuntary movement, discomfort or in the case of intra-operative treatment, undesirable increases in the procedure time.

Claims

CLAIMS1. Radiotherapy apparatus comprising: a vacuum chamber (20) in which is provided an election souice (22) capable of emitting an electron beam; a radiation-generating taiget anode (27) capable of geneiating X-iays in iesponse to the incidence of the electron beam thereon; and a rod-shaped conduit (24) for guiding the electron beam to the anode which is located at a distal end thereof, characterised in that the electron source comprises an indirectly heated electron-emissive cathode.
2. The radiotherapy apparatus of claim 1 wherein the cathode is suitably shaped to emit an election beam having a substantially ciicular profile.
3. The radiotherapy apparatus of claim 1 or claim 2 further comprising a cooling system for cooling the surface of the rod-shaped conduit comprising a cooling jacket substantially surrounding said rod, the jacket having cooling fluid therein.
4. The radiotherapy apparatus of claim 3 wherein the cooling fluid comprises oil or water.
5. The radiotherapy apparatus of claim 3 or claim 4 wherein the cooling fluid is circulated within said cooling jacket.
6. The radiotherapy apparatus of any of the preceding claims wherein the anode is generally concave, preferably generally hemispherical.
7. The radiotherapy apparatus of claim 6 wherein the anode is a transmission anode capable of emitting X-rays in a substantially spherical field.
8. The radiotherapy apparatus of any of the preceding claims where in the anode is deposited on a beryllium X-ray transmissive window.
9. The radiotherapy apparatus of any of the preceding claims wherein the anode has a focal spot of larger diameter than the expected diameter of the incident electron beam, preferably wherein the focal spot has a diameter of 4mm
10. The radiotherapy apparatus of any of the preceding claims further comprising election beam positioning means located at the proximal end of said rod-shaped conduit.
11. The radiotherapy apparatus of claim 10 wherein the beam positioning means compnses permanent magnets which are selectively radially moveable toward and away from a longitudinal axis of said rod-shaped conduit.
12. The radiotherapy apparatus of claim 11 wherein said permanent magnets comprise three permanent magnets generally equispaced around the circumference of said rod-shaped conduit.
13. The radiotherapy apparatus of any of the preceding claims further comprising a scraper electrode assembly comprising a non-radiation-emissive material located at the proximal end of said rod-shaped conduit for absorbing a misaligned electron beam.
14. The radiotherapy apparatus of any of the preceding claims further comprising a radiation output monitor.
15. The radiotherapy apparatus of claim 14 wherein the target anode is electrically isolated and the radiation output monitor includes a bridging resistor to facilitate monitoring of electron beam current at the anode.
16. Radiotherapy apparatus as claimed in any of the preceding claims wherein the radiotherapy apparatus is a brachytherapy device.
17. Radiotherapy apparatus substantially as described herein with reference to and as illustrated in Figures 2 and 3.
18. Radiotherapy system comprising radiotherapy apparatus as claimed in any of the preceding claims and a set of applicators.
19. Radiotherapy system as claimed in claim 18 wherein each applicator comprises a spherical hollow head of different diameter having an outer surface adapted to be in contact with a cavity of living tissue and an inner surface defining an internal volume adapted to receive X-rays from said radiotherapy apparatus, wherein at least one zone of the head is capable of being traversed by said X-rays, and wherein the thickness of said zone is a function of the diameter of said head, configured so that the X-ray dose produced at the outer surface is between 18 and 22 Gray.