Radiotherapy couch top

Abstract

A radiotherapy couch top comprised of a treatment zone to receive a part of a subject receiving radiotherapy, an attachment zone 1002 to couple the couch top to a support base and a force transition z

Description

This disclosure relates to a radiotherapy couch top, and in particular to a radiotherapy couch top comprising a treatment zone, an attachment zone and a force transition zone. This disclosure also relates to a patient support apparatus and a radiotherapy device including the radiotherapy couch top. 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 cancer, for example 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. Typically, a subject is disposed on a patient support apparatus and a source of radiation coupled to a rotatable gantry directs a beam of radiation through the subject from various different angles in order to build up the applied dose at the location of the tumour and limit the applied dose at other anatomical locations within the subject. A patient support apparatus can comprise a support base and a couch top which is supported by the support base and which the subject is disposed on. The couch top is generally a rectangular planar structure taking the form of a board that the subject lies on. Imaging can be performed to determine a position of a subject and of anatomical features within the subject. Such imaging can be performed before radiotherapy treatment begins with the subject in a treatment position, and / or can be performed while radiotherapy treatment is occurring. This helps ensure that the radiation is directed at the intended location, i.e. the location of the tumour, and not directed at other locations, e.g. locations of organs at risk. Radiotherapy treatment can be paused or adjusted if it is determined that the tumour is receiving or will receive a lower dose than would be desirable and / or if it is determined that healthy tissue is receiving or will receive a higher dose than would be desirable. By way of example, kV imaging (or MV imaging) can be used to provide images for these purposes. It is desirable for the images produced by the kV imaging to be of high quality / high accuracy and to have as few artefacts as possible. This helps ensure that determinations made regarding locations of anatomical features of the subject, and the doses these anatomical features receive, are as accurate as possible. However, a subject is typically disposed on the couch top during such imaging and during such radiotherapy treatment. In order to image the subject, the imaging beam typically passes through the couch top as well as through the subject. Properties of the couch top can therefore introduce unwanted features into the images or otherwise distort the images, which limits the accuracy with which the subject themselves can be imaged. In addition, the treatment beam can also pass through the couch top, which attenuates the dose received by the subject. Properties of the couch top can therefore also introduce unwanted distortion of the radiation applied to the subject, for example if different regions of the couch top attenuate the treatment beam to a greater degree, resulting in the subject receiving a lower dose than expected, or to a lesser degree, resulting in the subject receiving a higher dose than expected. This can lead to dosimetric errors in the irradiation of the subject. Moreover, if the couch top bends, i.e. deflects downwards, due to the weight of the subject, the position of anatomical features of the subject may not be as expected, which can lead to further inaccuracies in the imaging and treatment of the subject. Attempts to structurally reinforce the couch top can lead to undesirable attenuation of the imaging and treatment beams and can lead to high stress concentrations within the couch top when it is loaded with the weight of the subject. It will therefore be appreciated that a radiotherapy couch top is required to meet various different beam attenuation and structural requirements, which can be challenging to meet in combination. In view of the above, it would be desirable to increase the accuracy of imaging supporting radiotherapy treatment and of such radiotherapy treatment itself while simultaneously providing adequate support and rigidity such that any significant bending of the couch top and any excessive stress concentrations within the couch top are avoided. It would be advantageous to improve the structural reliability of a radiotherapy couch top. The present invention seeks to address these and other disadvantages encountered in the prior art. Summary An invention is defined in the independent claims. Optional features are set out in the dependent claims. Figures Specific embodiments are now described, by way of example only, with reference to the drawings, in which: Figure 1 depicts a radiotherapy device or apparatus according to the present disclosure; Figure 2 depicts a patient support apparatus according to the present disclosure; Figure 3 depicts a top-down view of an example couch top according to the present disclosure; Figure 4 depicts a cross-section of an example couch top according to the present disclosure; Figure 5 depicts a stack of prepregs of a couch top according to the present disclosure; Figure 6 depicts a layer of prepregs of a couch top according to the present disclosure; Figures 7a and 7b depict a patient support apparatus and a couch top according to the present disclosure; Figure 8 depicts a couch top according to the present disclosure; Figure 9 depicts a couch top according to the present disclosure; Figures lOa-lOc depict beams of a couch top according to the present disclosure. Detailed Description In overview, and without limitation, the application relates to a radiotherapy couch top comprising a treatment zone, an attachment zone and a force transition zone. The radiotherapy couch top can be implemented as part of a patient support apparatus and can be used to support the subject within a radiotherapy device. The treatment zone is configured to support a part of a subject that is to receive radiotherapy treatment. The attachment zone is configured to couple to a support base. The force transition zone is provided longitudinally between the treatment zone and the attachment zone along a longitudinal axis of the couch top. In other words, the treatment zone is at one longitudinal end of the couch top, the attachment zone is at the other longitudinal end of the couch top, and the force transition zone couples the treatment zone to the attachment zone. The force transition zone comprises a structure which varies in thickness along the longitudinal axis. It is desirable for the couch top to provide adequate support to the subject disposed thereon without significant bending, and also for the attenuation of imaging and treatment beam radiation by the couch top to be minimised, to be consistent and to avoid artefacts. The couch top can be supported on the support base at one end, with the imaging and treatment zones being located at the opposite end below which no support base is disposed. This keeps the support base away from the imaging and treatment beams and the associated hardware components. However, this means that the couch top projects from the support base in a longitudinal direction such that the couch top is not supported from below along a large portion of its length. This means that the couch top must be able to transfer the downward force applied at the end in the imaging and treatment zones to the end with the support base therebelow, and to do this without significant bending. Moreover, the structural materials that are used to transfer this force are limited in that they must not lead to high or spatially variable attenuation of the imaging or treatment beams and must not lead to excessive stress concentrations at particular points within the couch top. Therefore, according to the present disclosure, the couch top comprises the force transition zone between the treatment zone, at which treatment and imaging occurs, and the attachment zone, at which the couch top is supported from below. The force transition zone enables the gradual or smooth transfer of the load corresponding to the weight of the subject in the treatment zone to the attachment zone. In particular, the structure provided in the force transition zone which varies in thickness along the longitudinal axis enables this to be achieved without any local weak areas where the force is concentrated, as may be the case for sharp transitions between different structural arrangements. Therefore, the force transition zone enables a smooth transition between the subset of the couch top optimised for treatment and imaging and the subset of the couch top optimised for rigidity / support, thereby enabling the combination of requirements of the couch top to be met in an integrated manner that is less susceptible to structural faults or failure. Accordingly, disclosed herein is a radiotherapy couch top comprising: a treatment zone configured to support a part of a subject to receive radiotherapy treatment; an attachment zone configured to couple to a support base; and a force transition zone longitudinally between the treatment zone and the attachment zone along a longitudinal axis of the couch top, the force transition zone comprising a structure which varies in thickness along the longitudinal axis. As discussed herein, these features enable the accuracy of imaging supporting radiotherapy treatment and of such radiotherapy treatment itself to be improved while simultaneously providing adequate support and rigidity such that any significant bending of the couch top and any excessive stress concentrations within the couch top can be avoided. Therefore, these features enable the combination of requirements of a radiotherapy couch top to be met in an integrated manner via which the structural reliability of a radiotherapy couch top is improved. Optionally, the structure decreases from a first thickness at a first end of the force transition zone adjacent to the attachment zone to a second thickness at a second end of the force transition zone adjacent to the treatment zone. Optionally, thickness decreases at a constant gradient in the force transition zone. Optionally, the thickness decreases such that the structure has a curved profile in the force transition zone. Optionally, the structure comprises at least one beam parallel to the longitudinal axis, the at least one beam being comprised in the attachment zone and the force transition zone. Optionally, a vertical height of the at least one beam in the force transition zone decreases along the longitudinal axis as distance from the attachment zone increases. Optionally, the at least one beam comprises two laterally spaced beams. Optionally, the at least one beam is a carbon fibre beam. Optionally, the at least one beam is not comprised in the treatment zone. Optionally, the radiotherapy couch top comprises a core, an upper skin covering an upper surface of the core and a lower skin covering a lower surface of the core, the upper skin and the lower skin each comprising a respective plurality of prepregs. Optionally, a thickness of the lower skin and / or a thickness of the upper skin decreases in the force transition zone along the longitudinal axis as distance from the attachment zone increases. Optionally, a number of vertically stacked layers of the prepregs comprised in the lower skin and / or comprised in the upper skin decreases in the force transition zone along the longitudinal axis as distance from the attachment zone increases. Optionally, each of the plurality of prepregs comprises respective unidirectional carbon fibres. Optionally, a majority of the unidirectional carbon fibres are oriented to align with the longitudinal axis of the radiotherapy couch top. Optionally, the prepregs are arranged in a plurality of different orientations, and a majority of the prepregs are arranged in a common orientation which aligns the majority of the unidirectional carbon fibres with the longitudinal axis of the radiotherapy couch top. Optionally, the attachment zone comprises a plurality of attachment blocks for coupling to the support base. Optionally, the plurality of attachment blocks are respectively integrally formed with the at least one beam. Optionally, the attachment zone comprises a metal material and the treatment zone does not comprise any metal material. Optionally, the treatment zone comprises: a first longitudinal portion with a first lateral width; and a second longitudinal portion with a second lateral width, the second lateral width being smaller than the first lateral width. Optionally, the first longitudinal portion of the radiotherapy couch top is configured to support a torso of the subject. Optionally, the second longitudinal portion of the radiotherapy couch top is configured to support a head of the subject. Optionally, the treatment zone is integrally formed with the force transition zone and the force transition zone is integrally formed with the attachment zone. Optionally, the treatment zone is not separable from the force transition zone and the force transition zone is not separable from the attachment zone. Also disclosed herein is a patient support apparatus comprising: the radiotherapy couch top as described above; and a patient support base configured to support the radiotherapy couch top. Also disclosed herein is a radiotherapy device comprising: a rotatable gantry; a radiation source; an imaging apparatus; and the patient support apparatus as described above. Figure 1 depicts a radiotherapy device 100. The radiotherapy device 100 is suitable for delivering, and configured to deliver, a beam of radiation 110 to a patient during radiotherapy treatment. The device 100 and its constituent components will be described generally for the purpose of providing useful accompanying information for the present application. The device 100 depicted in Figure 1 is in accordance with the present disclosure and is suitable for use with the disclosed systems and apparatuses. While the device 100 in Figure 1 is an MR-linac, in implementations of the present disclosure the device 100 may be another type of radiotherapy device, for example a linac device with a different imaging capability. In overview, the radiotherapy device 100 comprises a source of radiation 105 and an imaging apparatus 112. The source of radiation 105 is coupled to a rotatable gantry 116. The device 100 comprises a patient positioning apparatus which comprises a patient positioning surface 114 on which a patient may be positioned. Before treatment, the patient is positioned on the surface 114, and the patient positioning apparatus may be used to position the patient in an appropriate position for the treatment, for example according to a reference image on which the patient's treatment plan is based. During treatment, the source of radiation 105 delivers radiation to the patient according to the patient's treatment plan. The source of radiation 105 is configured to generate a beam of radiation 110, and in particular a beam of therapeutic radiation. The radiation source 105 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 beam 110 can be applied from different angles around the gantry 116. The source of radiation 105 may comprise a beam generation system comprising a linear accelerator (linac). For such a linac device, the beam generation system may comprise a source of RF energy 102, an electron gun 106, and a waveguide 104. 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, for example via a circulator, 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 106 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 104 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. 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 106. Electrons can be accelerated to speeds approaching the speed of light in the evacuated waveguide 104. Once the electrons have been accelerated, they travel toward a heavy metal target which, when impacted by the electrons, generates a beam of high energy photons, forming a radiation beam 110. When the electrons strike the target, X-rays are produced in a variety of directions. The device 100 comprises collimation apparatus 108. The collimation apparatus 108 may comprise a primary collimator. The primary collimator is configured to 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 collimation apparatus 108 may additionally comprise beam shaping apparatus such as a multi-leaf collimator (MLC). The beam can be shaped in various ways by the beam-shaping apparatus. The source of radiation is configured to direct the beam 110 of therapeutic radiation, having been appropriately filtered and shaped by the collimation apparatus 108, toward a patient positioned on the patient support surface 114. The device 100 comprises an imaging apparatus 112 or 'image acquisition apparatus'. The depicted imaging apparatus 112 is an MR imaging apparatus, though the imaging apparatus may take other forms, for example a cone beam computed tomography (CBCT) apparatus. The MR imaging apparatus 112 is shown in cross-section in the diagram. The imaging apparatus 112 is configured to generate imaging data. The imaging data may comprise images of the patient. The imaging apparatus 112 is therefore configured to obtain images of a patient positioned on the patient support surface 114. The imaging data generated by the imaging apparatus 112 may be used to generate a reference image to enable treatment planning, and / or may be used during the delivery of therapeutic radiation to help guide the beam of radiation 110 or to provide an input into motion management and real-time adaptive radiotherapy techniques. As described herein, the radiotherapy device 100 may comprise an imaging apparatus 112 which is a kV imaging apparatus. While the skilled person will be familiar with kV imaging apparatuses, the following brief description is provided. The kV imaging apparatus may comprise a kV beam source and a kV detector, each of which may be fixed to the rotatable gantry opposite (at 180° to) each other. Each of the kV beam source and the kV detector may be offset by 90° relative to the source of radiation 105. The kV beam source may be configured to generate a kV X-ray beam directed through the subject (and in general through the patient support surface 114), to be incident on the kV detector. The kV detector may be configured to generate an image of the subject based on the kV X- ray beam it receives, as attenuated by the subject (and in general the patient support surface 114) which the kV X-ray beam passes through. Figure 1 and its accompanying description herein is provided to give context to the application and to facilitate understanding of the present invention. In addition to the components described in overview above, a radiotherapy device also comprises many other functions, components and subsystems as will be understood by the skilled person. Figure 2 depicts a schematic of a patient support apparatus 200 according to the present disclosure. The patient support apparatus 200 comprises a couch top 202, i.e. a radiotherapy couch top 202. The patient support apparatus 200, or the couch top 202, may correspond to the patient support apparatus 114 described in relation to Figure 1. The couch top 202 is configured to support a subject, i.e. a patient, thereon. In other words, in use, a subject may be disposed on and in contact with an upper surface of the couch top 202. The couch top 202 may be stiff / rigid enough such that it does not bend or sag significantly when a subject is disposed thereon. As further described herein, the couch top may comprise a foam core with a fibre skin. The fibre skin can comprise, by way of example, carbon fibres or aramid fibres or hemp fibres. Such fibres may exhibit adequate strength for supporting the subject while not causing excessive attenuation of the imaging beam. For example, the use of carbon minimises the attenuation of the imaging and treatment beams because its low atomic number means that each carbon atom has relatively few electrons. While the couch top 202 is depicted schematically as being generally flat / linear in the side-on view of Figure 2 for ease of illustration, the couch top 202 may have first and second longitudinal portions with first and second lateral widths respectively. The patient support apparatus 200 also comprises a patient support base 204 (also referred to herein as a support base). The patient support base 204 is configured to support the couch top 202. The patient support base 204 may be disposed on a floor 206 of a room housing the patient support apparatus 200 / radiotherapy device 100. The patient support base 204 may comprise one or more drivers, actuators and / or other mechanical / electrical components configured to cause movement of the couch top 202. This movement may comprise linear movement along three perpendicular axes, as well as rotational movement around each of these three perpendicular axes. In other words, the patient support apparatus 200 may be configured for 6D movement, though in some examples the patient support apparatus 200 may not be configured to provide movement along / around one or more of these directions, e.g. may be configured for 3D, 4D or 5D movement. The couch top 202 may be fixed to the patient support base 204 such that tilting or translation of all or part of the patient support base 204 causes tilting or translation of the couch top 202. This may be used to position the couch top 202, and the subject thereon, into a desired position for imaging and / or radiotherapy. The couch top 202 may be slidably coupled to the patient support base 204, enabling linear movement of the couch top 202 relative to the patient support base 204 (e.g. along the Z-axis as depicted in Figure 2). In some examples, some of the driving electronics or mechanical features may be coupled to or integrated into the couch top 202, in a region of the couch top 202 (an 'attachment zone') which is not irradiated by treatment or imaging beams. One or more attachment blocks may be provided (in the attachment zone) between the couch top 202 and the patient support base 204. For example, four such attachment blocks may be provided. The one or more attachment blocks may be configured to couple the couch top 202 to the patient support base 204. The patient support base 204 may at least be configured to cause the couch top to translate along a left-right direction as depicted in Figure 2 (parallel to the Z-axis). Other components of the radiotherapy apparatus 100 of Figure 1, such as the source of radiation 105 and the imaging apparatus 112, may generally be located to the right of the patient support apparatus 200 as depicted in Figure 2. In a 'setup position' which enables a subject to easily mount the couch top 202, the couch top 202 may be translated / disposed towards the left of Figure 2. The couch top 202 may be translated / disposed to the right as depicted in Figure 2, i.e. towards the other components of the radiotherapy apparatus 100, to place the subject in a 'treatment position' in which they can be imaged and / or treated. The couch top 202 may for example be translated in this manner using a linear guide or linear rail of the patient support base 204 coupled to the couch top 202. It will be appreciated that the couch top 202 is disposed in an overhang or cantilevered arrangement in which it is not directly supported from below in the imaging and treatment zones to the right of Figure 2. This is particularly the case when the subject is disposed in the treatment position in which the couch top 202 is translated relatively far to the right in Figure 2. Therefore, there is a need for the load of the subject on the couch top 202 in the imaging and treatment zones to be transferred in a longitudinal direction (along the Z-axis) such that this load can be supported by the support base 204. Moreover, there is a need for this to be achieved without high or irregular attenuation of imaging and treatment beams in the imaging and treatment zones respectively. The present application focuses on the form of the couch top 202 as described herein. It will be appreciated that this couch top 202 is generally applicable to various different forms of patient support base 204 and patient support apparatus 200 without limitation, and that the depiction and discussion of Figure 2 is provided merely as an example for understanding the context of the current application. The couch top 202 is the part of the patient support apparatus 200 disposed to contact the subject and configured for support of the subject, in contrast to the parts of the patient support apparatus 200 configured to control / drive movement of the couch top 202 with the subject thereon. Figure 3 depicts a top-down view of an example couch top 300 according to the present disclosure. The couch top 300 may correspond to the couch top 202 described in relation to Figure 2. The couch top 300 may comprise a first longitudinal portion 302 and a second longitudinal portion 304. In other words, the couch top 300 may comprise a first portion in a first longitudinal region 302 of the couch top 300 and a second portion in a second longitudinal region 304 of the couch top 300. The couch top 300 may also comprise a third longitudinal portion 306, i.e. a third portion in a third longitudinal region 306 of the couch top, between the first longitudinal portion 302 and the second longitudinal portion 304. The couch top 300 may comprise a first lateral side 308 and a second lateral side 310. The couch top 300 may comprise a first longitudinal end 312 and a second longitudinal end 314. The couch top 300 may extend between the first lateral side 308 and the second lateral side 310, each of which may be described as being disposed at different ends of a lateral axis 316 of the couch top 300. The couch top 300 may extend between the first longitudinal end 312 and the second longitudinal end 314, each of which may be described as being disposed at different ends of a longitudinal axis 318 of the couch top 300. The longitudinal axis 318 may be perpendicular to the lateral axis 306. The first and second lateral sides 308, 310 may be parallel to the longitudinal axis 318. The first and second longitudinal ends 312, 314 may be parallel to the lateral axis 316. The first longitudinal portion 302 of the couch top 300 may have a first lateral width 320. This lateral width is a width of the couch top 300 along the lateral axis 316 of the couch top 300 between the first lateral side 308 and the second lateral side 310. The couch top 300 may have the first lateral width 320 throughout or in most of the first longitudinal portion 302. The second longitudinal portion 304 of the couch top 300 may have a second lateral width 322. This lateral width is a width of the couch top 300 along the lateral axis 316 of the couch top 300 between the first lateral side 308 and the second lateral side 310. The couch top 300 may have the second lateral width 322 throughout or in most of the second longitudinal portion 304. The second lateral width 322 of the second longitudinal portion 304 may be smaller than the first lateral width 320 of the first longitudinal portion 302. In other words, the first longitudinal portion 302 may be wider than the second longitudinal portion 304. The second longitudinal portion 304 may be narrower than the first longitudinal portion 302. The couch top 300 may not be of uniform width along its length (along the longitudinal axis 318 thereof), but instead may have a wider portion along a first part of its length and a narrower portion along a second part of its length. The first longitudinal portion 302 is at or adjacent to the first longitudinal end 312. The first longitudinal portion 302 is disposed closer to the first longitudinal end 312 than the second longitudinal portion 304 is. The first longitudinal portion 302 may comprise an attachment zone configured to couple the couch top 300 to the patient support base 204 of the patient support apparatus 200 (see Figure 2). The attachment zone may be disposed adjacent to the first longitudinal end 312, i.e. at the bottom of Figure 3. The first longitudinal portion 302 may comprise a body treatment zone configured to support a torso of a subject disposed on the couch top 300. The body treatment zone may be disposed closer to the second longitudinal portion 304 than the attachment zone is. The body treatment zone may be disposed between the attachment zone and the second longitudinal portion 304 or between the attachment zone and the third longitudinal portion 306. In use, the couch top 300 may be positioned in relation to other components of the radiotherapy device 100 such that imaging and / or therapeutic radiation passes through the body treatment zone but not through the attachment zone. The second longitudinal portion 304 is at or adjacent to the second longitudinal end 314. The second longitudinal portion 304 may be comprise or correspond to a head treatment zone configured to support a head of a subject disposed on the couch top 300. In use, imaging and / or therapeutic radiation passes through the head treatment zone. In view of the relative sizes of human torsos and heads, the second longitudinal portion 304 may be shorter (along the longitudinal axis 318) than the first longitudinal portion 302 and is narrower (along the lateral axis 316) than the first longitudinal portion 302. Since the second longitudinal portion 304 may have the narrower second lateral width 322, the head of the subject is more exposed in that a radiotherapy beam can be directed through the head of the subject, without also having to pass through the couch top 300, from a greater range of positions / angles of the source of radiation 105. This may increase the accuracy, efficiency and / or versatility of head treatments. Typically, radiotherapy is applied with the source of radiation 105 located at various angles around the subject using the rotatable gantry 116. It will be appreciated that, if the couch top 300 had the same, first longitudinal width along its whole length, the radiotherapy beam would have to pass through the couch top 300 for irradiation with the source of radiation 105 positioned in at least the lower 180° of the circle described by the rotatable gantry 116. However, since the second longitudinal region 304 may be provided with the smaller, second lateral width 322, depending on the particular anatomical target location within the head, this enables direct irradiation of the target without the radiotherapy beam also passing through the couch top 300 with the source of radiation 105 at a greater range of angles. For example, this may be enabled with the source of radiation 105 substantially to the left or right of the couch top 300 (along the X-axis), but below the height of the couch top 300 to some extent (along the Y-axis). This increases the scope for generating treatment plans and corresponding treatments which more accurately target particular locations and shapes of tumours. Moreover, the narrower second lateral width 322 may prevent the kV imaging detector / panel from colliding with the couch top when it rotates. The above discussion has focused on the subject being reclined with their head disposed in the second longitudinal portion 304 and their torso disposed in the first longitudinal portion 302. The subject may be disposed on their back with their back in contact with an upper surface of the couch top 300. Alternatively, the subject may be disposed on their front with their front in contact with the upper surface of the couch top 300. Moreover, in some examples the subject may be disposed on the couch top in other ways. For example, in 'feet first' treatments, the subject may be disposed with their head adjacent to the first longitudinal end 312 and their feet adjacent to the second longitudinal end 314. An extension or cover board configured to couple to the second longitudinal portion 304 of the couch top 300 may be provided. The extension may have a lateral width equal or similar to the first lateral width 320 of the first longitudinal portion 302 of the couch top 300. The extension may be used for better supporting the feet / legs of the subject in feet first treatments and may be used for better supporting the arms of the subject in some treatments in which the subject is lying on their front. The third longitudinal portion 306 may couple the first longitudinal portion 302 to the second longitudinal portion 304. The third longitudinal portion 306 may have a third lateral width which varies along its length. The third lateral width may be equal to the first lateral width 320 of the first longitudinal portion 302 at an end of the third longitudinal portion 306 that is adjacent to the first longitudinal portion 302. The third lateral width may be equal to the second lateral width 322 of the second longitudinal portion 304 at an end of the third longitudinal portion 306 that is adjacent to the second longitudinal portion 304. In other words, the third lateral width may decrease along the longitudinal axis 318 of the couch top 300 when moving towards the second longitudinal end 314 and away from the first longitudinal end 312. The rate of this decrease in width may be substantially constant. In other words, the first and second lateral sides 308, 310 of the couch top in the third longitudinal portion 306 may be straight lines. As further described herein, the first and second lateral sides 308 of the couch top in the third longitudinal portion 306 may be (diagonally) angled with respect to the longitudinal axis 318, e.g. may form an approximately 45° angle with the longitudinal axis 318. The third longitudinal portion 306 may provide a gradual transition between the first longitudinal region 302 and the second longitudinal region 304. Considering the dose applied to a subject in a radiotherapy treatment, whether or not the radiotherapy beam passes through the couch top 300 or not can significantly affect the dose received by the subject. As such, when there is a sharp cut off between presence and absence of the couch top 300, small positional errors bridging the cut off can lead to large dosimetric errors. As further described herein, the gradual transition provided by the third longitudinal portion 306 can reduce the magnitude of such dosimetric errors resulting from small positional errors. The first longitudinal portion 302 may be integrally formed with the second longitudinal portion 304. In examples in which the third longitudinal portion 306 is present, this may be achieved by the first longitudinal portion 302 being integrally formed with the third longitudinal portion 306 and the third longitudinal portion 306 being integrally formed with the second longitudinal portion 304, such that the first and second longitudinal portions 302, 304 are ultimately integrally formed. The different portions of the couch top 300 may be described as being integrally formed, being formed as one structure, being monolithic, and / or being formed without joints therebetween. As described herein, this can help prevent such joints distorting the images produced of the subject and / or can help prevent dosimetric discrepancies. In addition, various parts of the couch top 300 can be optimised to further increase the reliability and accuracy of imaging and radiotherapy treatment. In particular, various angles of the couch top 300 can be optimised to further improve the imaging and radiotherapy treatment that is achievable. These angles may be optimised to provide a gradual transition between thicker and thinner portions of the couch top 300 so as to reduce the effects of sharp discontinuities on imaging and treatment beams. While the couch top 300 of Figure 3 has been provided by way of example, it will be appreciated that various different forms of couch top may be used according to the present disclosure. In some examples, the couch top may have a constant width along its length, i.e. along the whole longitudinal axis 318 thereof. In other words, the couch top may be rectangular / substantially rectangular in the X-Z plane. The material properties of the couch top discussed herein (including the fibres and fibre arrangements as discussed herein) may be present in any or all portions of the couch top. For example, these may be present in the first, second and / or third longitudinal portions 302, 304, 306 discussed in relation to Figure 3. For a couch top with a constant width, these may be present along the whole length of the couch top, or along only part of the length of the couch top (for example only in a longitudinal portion of the couch top not directly supported therebelow by the support base 204 in use or only in a longitudinal portion of the couch top coinciding with the imaging or treatment zones of the radiotherapy device). Figure 4 depicts a cross-section of an example couch top 400 according to the present disclosure. The couch top 400 may correspond to the couch top 300 or the couch top 202 described in relation to Figures 2 and 3 respectively. Figure 4 depicts a partial cross-section of the couch top 400, i.e. does not depict its full longitudinal extent along the Z-direction, as indicated by the dotted lines to the left and right of Figure 4. Figure 4 may depict in particular at least a treatment zone of the couch top 400. The couch top 400 may comprise an upper skin 410, a lower skin 420, and a core 422 between the upper skin 410 and the lower skin 420 (along the Y-direction). The upper skin 410 may cover an upper surface of the core 422 and the lower skin 420 may cover a lower surface of the core 422. The upper and lower skins 410, 412 may also be referred to herein as upper and lower layers or upper and lower plates. The upper skin 410 may comprise a plurality of vertically stacked layers of prepregs, for example 5-10 of such vertically stacked layers. While four of such layers 402, 404, 406, 408 are shown in Figure 4 for ease of illustration, it will be understood that more or fewer of such layers may be present. The lower skin 420 may comprise a plurality of vertically stacked layers of prepregs, for example 5-10 of such vertically stacked layers. While four of such layers 412, 414, 416, 418 are shown in Figure 4 for ease of illustration, it will be understood that more or fewer of such layers may be present. Each of the layers 402, 404, 406, 408, 412, 414, 416, 418 may comprise a thickness (in the Z-direction) in the range of approximately 100 pm to 300 pm. In some examples, each of the layers 402, 404, 406, 408, 412, 414, 416, 418 comprises multiple respective prepregs arranged adjacent to each other along the Z- and X-axes to make up the full length and width of the couch top 400. In some examples, each of the layers 402, 404, 406, 408, 412, 414, 416, 418 comprises a single respective prepreg along the whole length and across the whole width of the couch top 400. As the skilled person understands, a prepreg is a composite material comprising fibres and a matrix or resin. The fibres are pre-impregnated with the resin. The resin may be an epoxy resin. As described herein, the prepregs of the present disclosure may comprise unidirectional fibres, i.e. fibres which are aligned in the same direction. In other words, each individual prepreg making up the couch top 400 may comprise unidirectional fibres. Within each individual prepreg, the fibres may be aligned in the same direction. This leads to relatively low attenuation and relatively low spatial variation in attenuation of imaging and treatment beams passing through the couch top 400. This contrasts with bidirectional woven prepregs in which fibres are arranged in a grid, forming regions where zero, one or two fibres are present along a thickness direction of the prepreg and which may thereby lead to variable attenuation of imaging and treatment beams. The fibres may each have a uniform size, shape and / or aspect ratio, which may improve the consistency of attenuation of beams passing through the couch top 400. The length and / or thickness of the individual fibres, and the production tolerance of these parameters, may be optimised according to the loads the couch top 400 is intended to be subject to. The fibres may for example be made of carbon, aramid or hemp. In some examples, a combination of these fibres can be used within the same prepreg or in different respective prepregs of the couch top. In some examples, each prepreg may comprise respective weft fibres having an orientation different to the unidirectional fibres thereof, the weft fibres being configured to maintain the structure of the unidirectional fibres. The weft fibres may not be load bearing fibres, but instead may be configured to reinforce the shape of the load bearing (unidirectional) fibres and keep them in good order. The weft fibres may be perpendicular to the load bearing (unidirectional) fibres. The weft fibres may also be referred to as holding fibres or non-load bearing fibres herein. In some examples, one, multiple or all of the prepregs may comprise a woven layer of fibres at the upper surface, and / or lower surface, thereof. In other words, the woven layer(s) of each prepreg may cover or enclose the unidirectional fibres. This may also help keep the unidirectional layers in good order, as opposed to being provided for load-bearing purposes. For example, at least 80%, and optionally at least 90%, of the respective fibres of each of the prepregs are the unidirectional fibres, with the remaining fibres being the weft fibres and / or the woven layer of fibres. Substantially all of the respective fibres of each prepreg that are load-bearing fibres may be unidirectional fibres. In other words, the fibres of the prepregs may be unidirectional in that any fibres which are not oriented in the same direction are non-load bearing fibres. In some examples, a woven layer of fibres may be disposed on an upper surface of the uppermost prepreg layer 402. In some examples, a woven layer of fibres may be disposed on a lower surface of the lowermost prepreg layer 418. Such woven layers may comprise fibres disposed in two different directions, which may be perpendicular to each other (in the X-Z plane). Such woven layers may cover the prepreg layers formed of the unidirectional fibres in order to keep them in good order / provide a desired exterior visual appearance. A small proportion of the fibres being woven as opposed to unidirectional may be acceptable within the imaging tolerances / accuracy required for the couch top. It will be appreciated that, for a practical implementation of a prepreg with unidirectional fibres, there may be slight differences in the orientations of fibres. For example, the orientations of the respective fibres within each prepreg may vary by up to + / - 5° or up to + / -10°. Such real-world variability may still be considered to fall within the definition of unidirectional as used herein and the definition of aligning with the longitudinal axis of the couch top as used herein. Moreover, such slight deviations may help ensure that the couch top has adequate strength in a lateral direction, for example if a load is placed close to a lateral edge of the couch top. Moreover, as described herein, a majority of the unidirectional fibres may be oriented to align with the longitudinal axis 318 of the couch top 400, i.e. to align with the Z-axis. In other words, where the couch top 400 comprises multiple prepregs each with a respective alignment direction of its respective unidirectional fibres, the majority of these alignment directions may be oriented to align with the longitudinal axis 318. This enables transfer of the downward force of the subject lying on the couch top 400 to the support base 204 without significant bending of the couch top 400. In some examples, all of the unidirectional fibres may be oriented to align with the longitudinal axis of the couch top 400. In some examples, a majority but not all of the unidirectional fibres may align with the longitudinal axis of the couch top 400. In some examples, approximately 50% to 100% of the unidirectional fibres may be oriented to align with the longitudinal axis of the couch top 400. In some examples, approximately 50%, approximately 60%, approximately 70%, approximately 80%, approximately 90% or approximately 100% of the unidirectional fibres may be oriented to align with the longitudinal axis of the couch top 400. The core 422 may comprise a foam, for example a plastic / polymer foam, which may not lead to significant attenuation of imaging or treatment beams and which may have a low density. The core 422 may provide structure to the couch top 400 and may increase the overall rigidity of the couch top 400 through separating the upper and lower skins 410, 420. As depicted in Figure 4, the upper surface of the couch top may be flat or planar and the lower surface of the couch top may be flat or planar. The lower surface may be parallel to the upper surface. In other examples, the lower surface and / or the upper surface may be curved (such that a lateral centre of these surfaces is at a lower vertical position than the lateral sides of these surfaces). Figure 5 depicts a stack 500 of prepregs of a couch top according to the present disclosure. The stack 500 of prepregs may be implemented in any of the couch tops described herein. For example, the upper skin 410 may comprise a respective instance of the stack 500 of prepregs, and / or the lower skin 420 may comprise a respective instance of the stack 500 of prepregs. In other words, Figure 5 depicts an example of how the prepregs may be vertically stacked as described herein. In particular, Figure 5 may depict an example of how the prepregs may be vertically stacked in at least the treatment zone. Figure 5 may depict an example of how the prepregs may be vertically stacked in the treatment zone and the force transition zone described herein, and optionally also in the attachment zone. The stack 500 of prepregs may comprise any number, for example 5-10 prepregs. Seven prepregs are depicted in Figure 5 by way of non-limiting example. In some examples, the prepregs may have the square shape depicted in Figure 5, but the present disclosure is not limited thereto. The prepregs may have a rectangular shape or have the shape of the couch top, i.e. that depicted in Figure 3. The stack 500 may comprise prepregs 502a, 502b, 502c, 502d, 502e, 502f, 502g, which may be vertically stacked on top of each other. Each of these prepregs 502a, 502b, 502c, 502d, 502e, 502f, 502g may comprise respective unidirectional fibres 504a, 504b, 504c, 504d, 504e, 504f, 504g. These are depicted schematically in Figure 5 with the four lines within each prepreg 502a, 502b, 502c, 502d, 502e, 502f, 502g, though it will be appreciated that in practice the number of fibres within each of the prepregs 502a, 502b, 502c, 502d, 502e, 502f, 502g will be far greater than this. Each prepreg 502a, 502b, 502c, 502d, 502e, 502f, 502g comprises respective unidirectional fibres 504a, 504b, 504c, 504d, 504e, 504f, 504g with a respective alignment direction. Having different alignment directions of the respective fibres in each prepreg, for example rotating by 45° or 30° as you move through the stack 500, may provide strength in different directions. However, according to the present disclosure, a majority of the prepregs (collectively referred to as 502) may be arranged in a common orientation which may align the majority of the unidirectional fibres (collectively referred to as 504) with the longitudinal axis 318 of the couch top. As depicted in Figure 5, by way of non-limiting example, the fibres 504a of the prepreg 502a may be aligned with the Z-axis, the fibres 504b of the prepreg 502b may be aligned at 45° to the Z- and X-axes, the fibres 504c of the prepreg 502c may be aligned with the Z-axis, the fibres 504d of the prepreg 502d may be aligned with the X-axis, the fibres 504e of the prepreg 502e may be aligned with the Z-axis, the fibres 504f of the prepreg 502f may be aligned at 45° to the Z- and X-axes, and the fibres 504g of the prepreg 502g may be aligned with the Z-axis. In other words, in the example of Figure 5, the fibres 504a, 504c, 504e and 504g each have a common orientation or alignment direction parallel to the Z-axis. In other words, a majority of the prepregs 502 may comprise unidirectional fibres 504 which align with the Z-axis. This may improve the ability of the couch top to transfer the load from the subject to the support structure 204 down the length of the couch top. The minority of prepregs 502 with unidirectional fibres 504 with different alignment directions may enable the couch top to provide support along different directions, i.e. in the lateral direction. It will be appreciated that the arrangement of the layers in Figure 5 is provided by way of nonlimiting example. In some examples, the prepregs 504 aligning the fibres 502 with the longitudinal axis 318 may not be interspersed with prepregs 504 aligning the fibres 502 with different orientations, but rather may be arranged in consecutive / adjacent layers. In some examples, all of the prepregs 504 may align the fibres 502 with longitudinal axis 318. In some examples, the stack 500 of prepregs may comprise an uppermost layer comprising woven fibres and a plurality of prepregs therebelow comprising unidirectional fibres. Each of the prepregs of this plurality of prepregs may have a common orientation aligned with the longitudinal axis of the couch top, i.e. the unidirectional fibres thereof may have this common orientation. In other examples, when moving along the Y-axis of the couch top, this plurality of prepregs may alternate between orientations at +30° relative to the longitudinal axis, orientations parallel to the longitudinal axis, and orientations at -30° relative to the longitudinal axis. In other examples, when moving along the Y-axis of the couch top, this plurality of prepregs may alternate between orientations at +30° relative to the longitudinal axis and orientations at -30° relative to the longitudinal axis. The angle of deviation from the longitudinal axis may provide a minimum level of strength required in the lateral direction. In some examples, the angle of deviation may not be 30° as described above, but instead may be 10°, 20°, 40° or 45°, for example. Figure 6 depicts a layer 600 of prepregs of a couch top according to the present disclosure. The layer 600 of prepregs may be implemented in any of the couch tops described herein. For example, the layer 600 may be or be part of one of the layers 402, 404, 406, 408, 412, 414, 416, 418 described in relation to Figure 4. In other words, Figure 6 depicts an example of how the prepregs may be arranged in the lateral and longitudinal directions as described herein. The layer 600 of prepregs may comprise any number of prepregs arranged so as to make up the desired shape of the couch top (in the X-Z plane). Four prepregs are depicted in Figure 6 by way of non-limiting example, showing a partial layer of the prepregs for ease of illustration. In some examples, the prepregs may have the square shape depicted in Figure 6, but the present disclosure is not limited thereto. The prepregs may have a rectangular shape, or may have multiple different respective shapes. While in Figure 6 the prepregs are depicted as being tiled in both the X- and Z-directions, in some examples the shape and size of the prepregs may such that they are only tiled in the X-direction or only tiled in the Z-direction. The prepregs may have the shape of the couch top as depicted in Figure 3 such that there is only a single prepreg within each layer. Each layer 600 of the couch top may comprise a respective subset of the total number of prepregs making up the couch top. The particular layer 600 depicted in Figure 6 by way of non-limiting example may comprise at least four prepregs 602a, 602b, 602c, 602d. In Figure 6, these prepregs (collectively referred to as 602) may have a common orientation of the respective fibres thereof, though in some examples the respective orientations of the fibres of the prepregs may differ. As depicted in Figure 6, the prepregs 602 may be arranged in the layer 600 such that there are no gaps therebetween. Moreover, as depicted in Figure 6, the prepregs may be arranged in the layer 600 such that there is no overlap of one of the prepregs 602 with another of the prepregs 602. When considering the passage of the imaging or treatment beam through the layer 600 (in the Y-direction), this means that these beams do not encounter overlapping edges where two prepregs 602 are present or gaps where no prepregs 602 are present, which would otherwise cause undesirable variable attenuation of these beams. This may be particularly important when implemented in the treatment zone. In addition, this prevents there from being structural weak spots in the couch top, which may be particularly important in the force transition zone and the treatment zone. As depicted in Figure 6, there may be joints between the prepregs 602a-d forming the layer 600 of prepregs. As will also be appreciated from Figures 4 and 5, the couch top may comprise multiple of such layers stacked on top of each other. The layers may be arranged such that the joints in each layer do not overlap with the joints in other layers. This may improve the strength of the couch top by avoiding a situation in which a joint extends through a substantial thickness of the couch top. In other words, the joints between the prepregs in each layer of the couch top may be offset in the Z-direction and / or in the X-direction from the joints between prepregs in one, multiple, or all of the other layers of the couch top. In some examples, prepregs of the couch top may be cut to desired shapes from a prepreg sheet. In some examples, the prepregs may be manufactured using a pultrusion technique, involving impregnating the fibres with the resin, pulling the fibres through a heated die and curing in the die. The manufacturing process may comprise pre-characterisation / measurement of the prepregs before they are formed into the layers in order to make sure they are in tolerance. The manufacturing process may also comprise determining that there are no overlaps or gaps between the prepregs once they are arranged into one or more layers, for example through measurement, visual monitoring or automated inspection / machine vision techniques. In some examples, the couch top may have the same thickness along its whole longitudinal length. In other examples, the couch top may have a smaller thickness in the second longitudinal portion 304 (and optionally in the third longitudinal portion 306). For example, the thickness of the core 422 may be smaller in the second longitudinal portion 304 than in the first longitudinal portion 302. For example, fewer layers of prepregs may be provided in the second longitudinal portion 304 than the first longitudinal portion 302. Since the head of the subject is relatively light, this may be acceptable without causing significant bending of the couch top. The couch top may gradually reduce in thickness when moving in the negative Z-direction from the first longitudinal portion 302 to the second longitudinal portion 304 (for example in the third longitudinal portion 306). As described herein, the combination of features of the couch top enable the weight of the subject to be suitably supported while minimising attenuation, and variation in attenuation, of imaging and treatment beams. The attenuation of these beams can be measured in terms of relative electron density. The lower the energy of the beam, the more sensitive it is to the material content of the material it passes through, i.e. the more it is likely to be attenuated. The imaging beam may comprise X-rays with kV energies, for example between 70 and 120 keV. According to the described features of the couch top, the variation in the attenuation of the imaging beam across the couch top may preferably be less than + / -2%, or even more preferably less than + / -1%. The treatment beam may comprise X-rays with MV energies, for example between 6 and 25 MeV. Since the treatment beam has higher energy it is less sensitive to attenuation. However, it is particularly important that the attenuation of the treatment beam is predictable and constant since it directly affects the dose applied to the subject. According to the described features of the couch top, the variation in the attenuation of the treatment beam across the couch top may preferably be less than + / -0.5%. Figure 7a depicts a patient support apparatus 700 according to the present disclosure. The patient support apparatus 700 may correspond to the patient support apparatus 200 of Figure 2 or the patient support apparatus 114 of Figure 1. The patient support apparatus 700 comprises a couch top 702, which may correspond to and comprise corresponding features to any of the couch tops described herein. Figure 7b depicts a schematic view of the couch top 702. The patient support apparatus comprises a support base 704, which may correspond to the support base 204 of Figure 2. The support base 704 is configured to support the couch top 702. The couch top 702 may not be uniform in structure, but instead may comprise or be made up of different zones which may have different structural features. The different zones may be disposed adjacent to each other along a longitudinal axis 712 of the couch top 702. The couch top comprises a treatment zone 706, an attachment zone 708 and a force transition zone 710 longitudinally between the treatment zone and the attachment zone. These different zones may be described herein as different sections or different segments of the couch top 702. The couch top 702 may comprise a first structure in the treatment zone 706, a second structure in the attachment zone 708 and a third structure in the force transition zone 710. Each of the first structure, the second structure and the third structure may be different to each other. As described herein, the couch top 702 comprises the treatment zone 706, the attachment zone 708 and the force transition zone 710, each of which comprises respective features. Equivalently, the couch top 702 may be described as comprising, in each of the treatment zone 706, the attachment zone 708 and the force transition zone 710, these respective features. The treatment zone 706 is the part of the couch top 702 which, in use, supports a part of the subject which is to receive radiotherapy. The subject may be disposed along most of the length of the couch top 702, but an anatomical part of the subject which radiotherapy treatment is applied to may be disposed in the treatment zone 706. A treatment beam and / or an imaging beam may pass through the treatment zone 706. The treatment zone 706 may be the part of the couch top 702 closest to the rotatable gantry 116 (see Figure 1). As can be seen in Figure 7a, the treatment zone 706 may be cantilevered, i.e. not directly supported from below. The treatment zone 706 may have the material properties and structure described in relation to Figures 4-6, which may enable improved imaging and treatment and transferring of the load of the subject in a longitudinal direction along the couch top 702, as described herein. The attachment zone 708 is configured to couple to the support base 704, which provides direct support to the couch top 702 from underneath. Since the attachment zone 708 of the couch top 702 is outside of the treatment or imaging beams, its material structure and properties can be optimised to provide support and rigidity to the couch top 702 without concern for any attenuation of treatment or imaging beams. For example, the attachment zone 708 may comprise a metal material, such as steel or aluminium. In some examples, the structure of the attachment zone 708 may be similar to the structure of the treatment zone 706, except that a steel, aluminium or composite structure may be integrated into the upper skin, lower skin and / or core. Moreover, the upper / lower skin thickness may be larger in the attachment zone 708 than in the treatment zone 706. In the attachment zone 708, a smaller proportion of the unidirectional carbon fibres may be oriented to align with the longitudinal axis than in the treatment zone 706. This may be achieved, for example by distributing the orientations of the prepregs more evenly than is depicted in Figure 5. These structural features may improve the rigidity and support provided by the attachment zone 708, and this support may be less confined to the longitudinal direction. The attachment zone 708 of the couch top 702 may comprise a plurality of attachment blocks configured to couple the couch top 702 to the patient support base 704. The attachment zone 708 may comprise at least one beam, for example a carbon fibre beam. The beam may be oriented parallel to the longitudinal axis 712. The attachment zone 708 may comprise two of such beams laterally spaced relative to each other. The force transition zone 710 may be considered an intermediate zone coupling the treatment zone 706 to the attachment zone 708. As will be appreciated, according to the current disclosure the treatment zone 706 and the attachment zone 708 may be structured in different ways in order to meet different requirements of the couch top 702 in an optimised manner. However, coupling different structures may lead to junctions or discontinuities at which there are large stress concentrations within the material of the couch top 702. Therefore, according to the current disclosure, the force transition zone 710 is provided in order to provide a smoother or more gradual transition between the different structures of the treatment zone 706 and the attachment zone 708. The force transition zone 710 comprises a structure which varies in thickness along the longitudinal axis 712, for example such that the structure in the attachment zone 708 gradually transitions to the structure in the treatment zone 706 when moving along the couch top 702 (in the negative Z-direction). This may enable the couch top 702 to provide the required support and rigidity along the length of the couch top 702 without leading to large local stress concentrations and while maintaining the low and relatively constant attenuation desired in the treatment zone 706. The thickness of the structure may be a thickness in the Y-direction, i.e. in a direction through / aligned with the vertical height of the couch top 702. The structure which varies in thickness along the longitudinal axis 712 may decrease from a first thickness at a first end of the force transition zone 710 adjacent to the attachment zone 708 to a second thickness at a second end of the force transition zone 710 adjacent to the treatment zone 706. The second thickness may be smaller than the first thickness. The first thickness may be equal to the thickness of the structure in the attachment zone 708. The second thickness may be equal to the thickness of the structure in the treatment zone 706. The second thickness may be zero, i.e. the structure may end at a second end of the force transition zone 710 adjacent to the treatment zone 706. The structure may comprise at least one beam, which may be longitudinally oriented and may be formed of carbon fibre. This at least one beam may correspond to / be part of / be integrally formed with the respective at least one beam that is comprised in the attachment zone 708. In other words, the at least one beam may extend from the attachment zone 708 into the force transition zone 710. The structure of the force transition zone 710 may be similar to the structure of the treatment zone 706. A steel, aluminium or composite structure may be integrated into the upper skin, lower skin and / or core in the force transition zone 710, similar to in the attachment zone 708. Moreover, the upper / lower skin thickness may be larger in the force transition zone 710 than in the treatment zone 706 (and may be smaller than in the attachment zone 708). In the force transition zone 710, a majority of the unidirectional carbon fibres may be oriented to align with the longitudinal axis, for example as depicted in Figure 5, in order to transfer the load of the subject in the longitudinal direction. The structure which varies in thickness along the longitudinal axis 712 may comprise the upper skin and / or the lower skin. In other words, the upper skin and / or the lower skin may gradually decrease in thickness in the force transition zone 710 along the longitudinal axis 712 as distance from the attachment zone 708 increases. This may be achieved by reducing the number of prepregs which are vertically stacked to form the upper and / or lower skin as the distance from the attachment zone 708 increases. While the couch top 702 is depicted as having a constant width for ease of illustration, in some examples the couch top 702 may have the shape depicted in Figure 3. The treatment zone 706 may comprise the second longitudinal portion 304 and optionally the third longitudinal portion 306, as well as an adjacent part of the first longitudinal portion 302. The attachment zone 708 and the force transition zone 710 may be comprised in the first longitudinal portion 302. In some examples, treatment and / or imaging in the transition zone 710 may be permitted if the attenuation variation in the transition zone 710 is within acceptable limits. Generally, this would be avoided in order to simply the treatment plan and have increased confidence in the accuracy of the imaging and treatment performed. However for special cases such as a particularly tall patient or particularly long fields it may be appropriate to use the transition zone 710 for treatment and / or imaging. Figure 8 depicts a couch top 800 according to the present disclosure. The couch top 800 may correspond to and comprise corresponding features to any of the couch tops described herein. In particular, Figure 8 depicts an underside of the couch top 800. Figure 8 depicts an example in which the lower surface of the couch top 800 is curved, though the features of Figure 8 can be applied to couch tops with both curved and flat lower surfaces. The attachment zone 802 of the couch top comprises one or more attachment blocks 804 on the underside of the attachment zone 802 / couch top 800. For example, the attachment zone 802 may comprise two attachment blocks, with one on each lateral side of the attachment zone 802. As depicted in Figure 8, the attachment zone 802 comprises four attachment blocks 804, with a first pair including one attachment block 804 on each lateral side of the attachment zone 802, and a second pair longitudinally spaced from the first pair and also including one attachment block 804 on each lateral side of the attachment zone 802. The attachment blocks 804 may be configured to couple the attachment zone 802 to the support base in order to support the couch top 800 and enable movement of the couch top 800 relative to the support base. The attachment blocks 804 provide may provide the interface between the couch top 800 and the support base, e.g. a linear rail or linear guide thereof, to facilitate movement of the couch top relative to the support base parallel to the Z-axis. Each of the attachment blocks 804 may comprise a respective rail carrier configured to couple to a corresponding linear rail or linear guide of the support base so as to enable movement therebetween in the Z-direction. For example, the rail carriers may be shaped and positioned to receive respective linear rails at the upper surface of the support base so as to enable this movement to be implemented in a smooth manner without crabbing or large fractional forces. The support base may comprise a first rail configured to couple to the rail carriers of the attachment blocks 804 on a first lateral side of the couch top 800 and a second rail configured to couple to the rail carriers of the attachment blocks 804 on a second lateral side of the couch top 800. The well-defined contact interfaces provided by the attachment blocks 804 may provide the locations through which all load from the couch top 800 is transferred to the support base. Figure 9 depicts a couch top 900 according to the present disclosure. The couch top 900 may correspond to and comprise corresponding features to any of the couch tops described herein. Figure 9 focuses on the attachment zone 902 comprising attachment blocks 904. The couch top 900 may comprise at least one beam 906a, 906b in the attachment zone 902, which may also extend into the force transition zone. The at least one beam 906a, 906b may comprise a first vertically-aligned bar 906a and a second vertically-aligned bar 906b which is laterally separated from the first vertically aligned bar 906a. Each of the first vertically-aligned bar 906a and the second vertically-aligned bar 906b may be considered a respective one of the at least one beams as described herein. Alternatively, the combination of the first vertically-aligned bar 906a, the second vertically-aligned bar 906b, the lower surface of the couch top 900 and the upper surface of the couch top 900 may be considered to form the at least one beam. The at least one beam 906a, 906b may be disposed in a longitudinal direction parallel to the longitudinal axis of the couch top 900. The at least one beam 906a, 906b may comprise two beams laterally spaced apart from each other (e.g. with each disposed adjacent to a respective lateral side of the couch top 900 as depicted in Figure 9). The at least one beam 906a, 906b may by way of nonlimiting example be formed from carbon fibre. In some examples, the couch top 900 may comprise one or more further beams disposed in the lateral direction (parallel to the X-axis) in order to improve the stability of the couch top 900. Figure 9 depicts an example in which the lower surface of the couch top 900 is curved, though the features of Figure 9 can be applied to couch tops with both curved and flat lower surfaces. Where the top surface and the bottom surface of the couch top 900 are flat, or at least where they are parallel to each other, the first and second vertically-aligned bars 906a, 906b may have the same vertical height as each other. Figure 10a depicts a view of a beam 1000 according to the present disclosure. The beam 1000 may correspond to any of the beams described herein, including the at least one beam 906a, 906b described in relation to Figure 9. Figure 10a focuses on the attachment zone 1002 comprising attachment blocks 1004. The black portion of Figure 10a may depict the beam 1000 extending from the attachment zone 1002 into the force transition zone 1006. As depicted in Figure 10a, the attachment blocks 1004 may be an integrally formed feature of the beam 1000 / the attachment blocks 1004 may be integrally formed with the beam 1000. When moving along the beam 1000 in the negative Z-direction, the dividing line between the attachment zone 1002 and the force transition zone 1006 may be considered to be disposed at the end of the last present attachment block 1004. To the right of Figure 10a, dashed lines indicate a region where the beam 1000 is not present in order to indicate that the beam 1000 does not extend into the treatment zone. In some examples, the beam 1000 may have a constant thickness in the attachment zone 1002 and the force transition zone 1006, as depicted in Figure 10a. In other examples, the beam 1000 may end in the attachment zone 1002 and may not extend into the force transition zone 1006. As depicted in Figures 10b and 10c, the beam 1000 may vary in thickness in the force transition zone 1006. In particular, the beam 1000 may decrease in thickness in the force transition zone 1006 moving in the negative Z-direction away from the attachment zone 1002. The shape of the beam 1000 may be optimised in the force transition zone 1006 in order to avoid local stress concentrations. The beam 1000 may gradually become thinner when moving in the negative Z-direction, i.e. may become thinner in a thickness direction of the couch top 1000 parallel to the Y-axis. Since the beam 1000 gradually becomes thinner, it becomes progressively weaker when moving away from the attachment zone 1002 and towards the treatment zone, allowing gradually more flex. This minimises the stress levels in the couch top 1000, e.g. in the upper skin and / or the lower skin. This may provide an optimised balance between providing transfer of the load of the subject in the longitudinal direction and avoiding local stress concentrations. As depicted in Figure 10b, a lower surface 1008 of the beam 1000 in the force transition zone 1006 may be planar and may be angled diagonally upwards in the Y-Z plane such that the thickness of the beam 1000 decreases at a constant gradient. As depicted in Figure 10c, the lower surface 1008 of the beam 1000 in the force transition zone 1006 may alternatively have a curved profile in the Y-Z plane. The curve may be concave. The curve may take the form of an arc of a circle. The curved profile may be particularly effective at preventing local stress concentrations since it may provide a particularly smooth transition at the interface with the treatment zone 706 and / or the interface with the attachment zone 708. Accordingly, the present disclosure provides a couch top with a force transition zone longitudinally between a treatment zone and an attachment zone along a longitudinal axis of the couch top. The force transition zone comprises a structure which varies in thickness along the longitudinal axis, which provides support and rigidity to the couch top without leading to excessive stress concentrations within the couch top and without compromising the accuracy of imaging or treatment of a subject disposed on the couch top. Therefore, these features enable the combination of requirements of a radiotherapy couch top to be met in an integrated manner via which the structural reliability of a radiotherapy couch top is improved. It is to be understood that the above description is intended to be illustrative, and not restrictive. Many other implementations will be apparent to those of skill in the art upon reading and understanding the above description. Although the present disclosure has been described with reference to specific example implementations, it will be recognized that the disclosure is not limited to the implementations described, but can be practiced with modification and alteration within the spirit and scope of the appended claims. Accordingly, the specification and drawings are to be regarded in an illustrative sense rather than a restrictive sense. The scope of the disclosure should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.

Claims

1. A radiotherapy couch top comprising:a treatment zone configured to support a part of a subject to receive radiotherapy treatment;an attachment zone configured to couple to a support base; anda force transition zone longitudinally between the treatment zone and the attachment zone along a longitudinal axis of the couch top, the force transition zone comprising a structure which varies in thickness along the longitudinal axis.

2. The radiotherapy couch top according to claim 1, wherein the structure decreases from a first thickness at a first end of the force transition zone adjacent to the attachment zone to a second thickness at a second end of the force transition zone adjacent to the treatment zone.

3. The radiotherapy couch top according to claim 1 or claim 2, wherein the thickness decreases at a constant gradient in the force transition zone.

4. The radiotherapy couch top according to claim 1 or claim 2, wherein the thickness decreases such that the structure has a curved profile in the force transition zone.

5. The radiotherapy couch top according to any preceding claim, wherein the structure comprises at least one beam parallel to the longitudinal axis, the at least one beam being comprised in the attachment zone and the force transition zone.

6. The radiotherapy couch top according to claim 5, wherein a vertical height of the at least one beam in the force transition zone decreases along the longitudinal axis as distance from the attachment zone increases.

7. The radiotherapy couch top according to claim 5 or claim 6, wherein the at least one beam comprises two laterally spaced beams.

8. The radiotherapy couch top according to any of claims 5-7, wherein the at least one beam is a carbon fibre beam.

9. The radiotherapy device according to any of claims 5-8, wherein the at least one beam is not comprised in the treatment zone.

10. The radiotherapy couch top according to any preceding claim, comprising a core, an upper skin covering an upper surface of the core and a lower skin covering a lower surface of the core, the upper skin and the lower skin each comprising a respective plurality of prepregs.

11. The radiotherapy couch top according to claim 10, wherein a thickness of the lower skin and / or a thickness of the upper skin decreases in the force transition zone along the longitudinal axis as distance from the attachment zone increases.

12. The radiotherapy couch top according to claim 11, wherein a number of vertically stacked layers of the prepregs comprised in the lower skin and / or comprised in the upper skin decreases in the force transition zone along the longitudinal axis as distance from the attachment zone increases.

13. The radiotherapy couch top according to any of claims 10-12, wherein each of the plurality of prepregs comprises respective unidirectional carbon fibres.

14. The radiotherapy couch top according to claim 13, wherein a majority of the unidirectional carbon fibres are oriented to align with the longitudinal axis of the radiotherapy couch top.

15. The radiotherapy couch top according to claim 14, wherein the prepregs are arranged in a plurality of different orientations, and wherein a majority of the prepregs are arranged in a common orientation which aligns the majority of the unidirectional carbon fibres with the longitudinal axis of the radiotherapy couch top.

16. The radiotherapy couch top according to any of claims 5-9, wherein the attachment zone comprises a plurality of attachment blocks for coupling to the support base.

17. The radiotherapy couch top according to claim 16, wherein the plurality of attachment blocks are respectively integrally formed with the at least one beam.

18. The radiotherapy couch top according to any preceding claim, wherein the attachment zone comprises a metal material and wherein the treatment zone does not comprise any metal material.

19. The radiotherapy couch top according to any preceding claim, wherein the treatment zone comprises:a first longitudinal portion with a first lateral width; anda second longitudinal portion with a second lateral width, the second lateral width being smaller than the first lateral width.

20. The radiotherapy couch top according to claim 19, wherein the first longitudinal portion of the radiotherapy couch top is configured to support a torso of the subject.

21. The radiotherapy couch top according to claim 19 or claim 20, wherein the second longitudinal portion of the radiotherapy couch top is configured to support a head of the subject.

22. The radiotherapy couch top according to any preceding claim, wherein the treatment zone is integrally formed with the force transition zone and the force transition zone is integrally formed with the attachment zone.

23. The radiotherapy couch top according to any preceding claim, wherein the treatment zone is not separable from the force transition zone and the force transition zone is not separable from the attachment zone.

24. A patient support apparatus comprising:the radiotherapy couch top of any preceding claim; andthe support base configured to support the radiotherapy couch top.

25. A radiotherapy device comprising:a rotatable gantry;a radiation source;an imaging apparatus; andthe patient support apparatus of claim 24.s

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