Radiotherapy couch top

Abstract

A radiotherapy couch top comprised from prepregs 500 in layers 502a-g with unidirectional fibres 504a-g such as carbon fibres which may be arranged in a plurality of directions, where the majority of

Description

This disclosure relates to a radiotherapy couch top, and in particular to a radiotherapy couch top comprising unidirectional fibres. 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. In view of the above, it would be desirable to increase the accuracy of imaging supporting radiotherapy treatment, as well as to increase the accuracy of such radiotherapy treatment itself. It would be desirable to provide a couch top which can enable this increased accuracy while simultaneously providing adequate support and rigidity such that any significant bending of the couch top is avoided. 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 a 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. Detailed Description In overview, and without limitation, the application relates to a radiotherapy couch top for supporting a subject. 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 radiotherapy couch top includes a plurality of prepregs. Each of the prepregs includes, i.e. is formed of, respective unidirectional fibres. In other words, the fibres of a prepreg are aligned in the same direction. The orientation of the unidirectional fibres is such that a majority of them align with a longitudinal axis of the radiotherapy couch top. 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 and consistent. The couch top is typically supported on a 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. Therefore, according to the present disclosure, the couch top comprises a plurality of prepregs each comprising unidirectional fibres, and a majority of these unidirectional fibres are oriented to align with a longitudinal axis of the couch top. The unidirectional fibres may, by way of non-limiting example, be carbon fibres, aramid fibres or hemp fibres. These fibre materials may be suitable for avoiding 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. The use of the unidirectional fibres reduces the spatial variation of the attenuation of the imaging and treatment beams. This is because the fibres present a more homogeneous arrangement with a similar amount of fibre material being present at different points across the couch top, as opposed, for example, to woven arrangements in which there are various areas with one fibre, two overlapping fibres or no fibres. The orientation of the fibres such that they align with a longitudinal axis of the radiotherapy couch top helps transfer the force applied to the couch top in a longitudinal direction. This enables the overhanging portion of the couch top in the imaging and treatment zones to be indirectly supported by the support base at the opposite end of the couch top. Accordingly, disclosed herein is a radiotherapy couch top configured to support a subject thereon and comprising: a plurality of prepregs each comprising respective unidirectional fibres, wherein a majority of the unidirectional fibres are oriented to align with a longitudinal axis of the radiotherapy couch top. As discussed herein, these features enable more accurate imaging and more accurate radiotherapy treatment of a subject through reducing the absolute amount of and variation in the attenuation of the imaging and treatment beams by the couch top. Moreover, these features further enable the weight of the subject to be supported reliably without bending of the couch top, which could itself reduce the accuracy of the imaging / treatment performed. Therefore, this combination of features enables the dual provision of improved imaging and treatment of subjects through the couch top while also improving the material properties of the couch top. Optionally, the prepregs are arranged in a plurality of different orientations. Optionally, a majority of the prepregs are arranged in a common orientation. Optionally, the common orientation aligns the majority of the unidirectional fibres with the longitudinal axis of the radiotherapy couch top. Optionally, the plurality of prepregs are arranged in a plurality of vertically stacked layers. Optionally, he plurality of prepregs are arranged in 5-10 vertically stacked layers. Optionally, each of the vertically stacked layers comprises a respective subset of the prepregs. Optionally, the respective subsets of the prepregs each comprise multiple of the prepregs arranged such that they do not overlap. Optionally, the respective subsets of the prepregs each comprise multiple of the prepregs arranged such that there are no gaps therebetween. Optionally, each of the plurality of prepreg layers comprises 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. Optionally, at least 80%, and optionally at least 90%, of the respective fibres of each of the prepregs are the unidirectional fibres. Optionally, the radiotherapy couch top further comprises a core, wherein the plurality of prepregs are arranged to cover upper and lower surfaces of the core. Optionally, the core comprises a foam material. Optionally, a variation in an attenuation of a kV imaging beam by the radiation couch top is less than 2%, and preferably less than 1%. Optionally, a variation in an attenuation of an MV treatment beam is less than 0.5%. Optionally, the unidirectional carbon fibres are unidirectional carbon fibres. Optionally, the radiotherapy couch top 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 second longitudinal portion is integrally formed with the first longitudinal portion. 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, one or more of the unidirectional fibres extends between the first and second longitudinal portions. Optionally, the radiotherapy couch top comprises a third longitudinal portion of the radiotherapy couch top longitudinally between the first longitudinal portion and the second longitudinal portion, wherein the first longitudinal portion, the second longitudinal portion and the third longitudinal portion are all integrally formed with each other. Optionally, one or more of the unidirectional fibres extends between the first and third longitudinal portions, and / or wherein one or more of the unidirectional fibres extends between the third and second longitudinal portions. Optionally, the first longitudinal portion is not separable from the second longitudinal portion. 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 having low atomic numbers, which limits the effect of the fibres on image quality when a kV imaging beam passes through the couch top 202. 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. 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 couped 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 a 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. 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 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 comprises unidirectional fibres. Within each individual prepreg, the fibres are 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 themselves comprise a respective woven layer of fibres at the upper surface, and / or lower surface, of that individual prepreg. In other words, a woven layer(s) of each prepreg may be provided to cover or enclose the unidirectional fibres of that prepreg. 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 (and disposed 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 are 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 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. 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 may 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. In addition, this prevents there from being structural weak spots in the couch top. 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%. 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 configured to support a subject thereon and comprising:a plurality of prepregs each comprising respective unidirectional fibres,wherein a majority of the unidirectional fibres are oriented to align with a longitudinal axis of the radiotherapy couch top.

2. The radiotherapy couch top according to claim 1, wherein the prepregs are arranged in a plurality of different orientations.

3. The radiotherapy device according to any preceding claim, wherein a majority of the prepregs are arranged in a common orientation.

4. The radiotherapy couch top according to claim 3, wherein the common orientation aligns the majority of the unidirectional fibres with the longitudinal axis of the radiotherapy couch top.

5. The radiotherapy couch top according to any preceding claim, wherein the plurality of prepregs are arranged in a plurality of vertically stacked layers.

6. The radiotherapy couch top according to claim 5, wherein the plurality of prepregs are arranged in 5-10 vertically stacked layers.

7. The radiotherapy device according to claim 5 or claim 6, wherein each of the vertically stacked layers comprises a respective subset of the prepregs.

8. The radiotherapy device according to claim 7, wherein the respective subsets of the prepregs each comprise multiple of the prepregs arranged such that they do not overlap.

9. The radiotherapy device according to claim 7 or claim 8, wherein the respective subsets of the prepregs each comprise multiple of the prepregs arranged such that there are no gaps therebetween.

10. The radiotherapy couch top according to any preceding claim, wherein each of the plurality of prepreg layers comprises respective weft fibres having an orientation different to theunidirectional fibres thereof, the weft fibres being configured to maintain the structure of the unidirectional fibres.

11. The radiotherapy couch top according to claim 10, wherein at least 80%, and optionally at least 90%, of the respective fibres of each of the prepregs are the unidirectional fibres.

12. The radiotherapy couch top according to any preceding claim further comprising a core, wherein the plurality of prepregs are arranged to cover upper and lower surfaces of the core.

13. The radiotherapy couch top according to claim 12, wherein the core comprises a foam material.

14. The radiotherapy couch top according to any preceding claim, configured such that a variation in an attenuation of a kV imaging beam by the radiation couch top is less than 2%, and preferably less than 1%.

15. The radiotherapy couch top according to any preceding claim, configured such that a variation in an attenuation of an MV treatment beam is less than 0.5%.

16. The radiotherapy couch top according to any preceding claim, wherein the unidirectional fibres are unidirectional carbon fibres.

17. The radiotherapy couch top according to any preceding claim, comprising: 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.

18. The radiotherapy couch top according to claim 17, wherein the second longitudinal portion is integrally formed with the first longitudinal portion.

19. The radiotherapy couch top according to claim 17 or claim 18, wherein the first longitudinal portion of the radiotherapy couch top is configured to support a torso of the subject and / or wherein the second longitudinal portion of the radiotherapy couch top is configured to support a head of the subject.

20. The radiotherapy couch top according to any of claims 17-19, wherein one or more of the unidirectional fibres extends between the first and second longitudinal portions.

21. The radiotherapy couch top according to any of claims 17-19, comprising a third longitudinal portion of the radiotherapy couch top longitudinally between the first longitudinal portion and the second longitudinal portion, wherein the first longitudinal portion, the second longitudinal portion and the third longitudinal portion are all integrally formed with each other.

22. The radiotherapy couch top according to claim 21, wherein one or more of the unidirectional fibres extends between the first and third longitudinal portions, and / or wherein one or more of the unidirectional fibres extends between the third and second longitudinal portions.

23. The radiotherapy couch top according to any of claims 17-22, wherein the first longitudinal portion is not separable from the second longitudinal portion.

24. A patient support apparatus comprising:the radiotherapy couch top of any preceding claim; anda patient 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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