Headrest

Abstract

A headrest (2) for use in radiotherapy and / or imaging is described. The headrest includes a support (3) shaped for stably seating the head of a patient. The headrest also includes a fluid actuator ass

Description

Field of the invention The present invention relates to a headrest for use in radiotherapy and / or imaging, and systems and methods using the headrest. Background Accurate positioning in the radiation treatment of head and neck cancer patients is important because multiple, critical organs and other anatomical structures often surround a target volume (such as a tumour). Setup errors can result in significant underdose to the tumour and / or overdose to one or more critical organs. Head rests are frequently used to restrict the movement of a patient and to provide a level of comfort during a head and neck radiotherapy treatment. Headrests come in different shapes that can accommodate a variety of neck and head sizes. Thermoplastic masks are then used to hold the head and neck still in the right position. There are two main objectives; firstly to immobilise during treatment delivery and secondly to reproduce the exact position for every fraction (i.e. to act as a template). The mask clamps the head in position against a cradle formed by the headrest. The forming of a customised mould dedicated to each particular patient is costly and requires a significant amount of time. Also, any changes in the patient anatomy can occur during the course of radiotherapy treatment, requiring recasting of the mould. Restraint using a mask can be uncomfortable and unpleasant for the patient. In present radiotherapy procedures, patients are positioned on a headrest, and / or extended with extra supporting wedges for the positioning of the neck. The headrest is used for that particular patient during the whole course of radiotherapy. The patients are positioned with the head tilted backwards using a five point thermoplastic mask and a knee support for stability. During therapy, patients are setup using localisation lines on the mask and skin to align the patient to the isocentre lasers. Subsequently, the patient positioning couch is shifted to align the patient to the planned treatment position and then a positioning couch shift correction of the imaging protocol is performed. Radiotherapy patients will typically receive a planning Computed Tomography (CT) scan of the (whole) cranium to the sternum (upper part). Using this imaging, a radiation oncologist delineates a Gross Tumour Volume (GTV) / Clinical Target Volume (CTV) + CTV lymph nodes, or other parameters as may be relevant for the treatment. The clinician delineates the organs at risk and performs the expansions from Clinical 137175GB1 Target Volume (CTV) to Planning Target Volume (PTV). A typical tolerance may be in the region of 5 mm. During treatment, patients typically undergo cone beam computer tomography (CBCT) guided radiotherapy. The local setup errors (compared to the planning procedure) are computed using region-of-interest registration. For example on 9 bony structures -cervical vertebrae 1 (Cl), 3 (C3), 5 (C5) and 7 (C7), lower jaw, hyoid bone, larynx, skull and jugular notch. When tumour shrinkage is observed, for example >1.0 cm, >2 cm or more, is seen on the CBCT scan, the radiation oncologist may requests a new planning CT and treatment plan if the tumour (CTV) is not in the PTV. The average of the local setup errors is used to perform the patient positioning couch shift correction. The range and precision of movements performed using a patent positioning couch are limited, and often axes of rotations are offset significantly from the head and neck, making it difficult to decouple rotations and translations. Apparatuses and methods for immobilising portions of patients' anatomies for radiotherapy are described in CN 112843504 A, US 2012 / 0305007 Al, US 9,913,689 B2, US 7,894,573 B2, WO 2017 / 089457 Al, and CN 114082112 A. Similar support and / or restraint approaches are also applied to imaging applications applied to the head and / or neck of a patient. Summary According to a first aspect of the invention there is provided a headrest for use in radiotherapy and / or imaging. The headrest includes a support shaped for stably seating the head of a patient. The headrest also includes a fluid actuator assembly operatively coupling the support to a baseplate and configured for six degree-of-freedom movement of the support relative to the baseplate. The headrest is formed from non-metallic materials. The six degrees-of-freedom may be independent over at least a portion of the range of movement. In other words, a rotation of the support relative to the baseplate may be possible without a corresponding translation. The headrest does not include metallic materials. The headrest being formed from non-metallic materials does not exclude the presence of thin metal coatings or similar structures which provide no, or negligible, mechanical structure or function to the headrest. In this way, attenuation or scattering by the headrest of radiation being used for radiotherapy or imaging may be reduced. The support may be shaped for stably seating the head of the patient if, when the patient's head is received by the support, the weight of the head under gravity and the shape of the support discourage relative movement between the head and the support. In other words, the shape of the support may be such that a movement (translation or rotation) of the head relative to the support would result in raising the centre of gravity of the head. In this way, the head may be stably supported in that a perturbation in the position or orientation of the head relative to the support will result in a net restoring force urging the head back towards its initial position. The support may be shaped for stably seating the head of the patient across a range of motion of the fluid actuator assembly. The headrest does not include any part which passes across or restrains the front of the head, face or neck of the patient. The headrest does not include a mask. The headrest does not include a headband. The headrest does not support the torso or limbs of the patient. Only the head and neck of the patient may be supported by the headrest. The headrest may be integrally formed with a patient positioning couch. The headrest may be configured to be operatively coupled to a patient positioning couch. The fluid actuator assembly may directly couple the support to the baseplate. The fluid actuator assembly may include, or take the form of, a hydraulic actuator assembly. Alternatively, the fluid actuator may be pneumatic. The fluid actuator assembly may include, or take the form of, a hexapod actuator. A hexapod actuator may sometimes be referred to as a "Stewart platform". The support may include, or take the form of, an annulus for stably seating the head of the patient. The annulus may also be referred to as a "ring-shaped" portion. The annulus may be circular. The annulus may be elliptical. The annulus may be polygonal. The annulus may be irregularly shaped. The annulus may be toroidal. A toroidal annulus may have a cross section which is circular, semi-circular, or any other regular or irregular shape. The annulus may be a full (or "complete") annulus. The annulus may be a partial annulus, i.e. the annulus may comprise a gap. The headrest may be configured such that rotational degrees-of-freedom of the support relative to the baseplate act about respective axes of rotation that intersect the support when viewed from above. The direction of above may refer to the vertical in use. In other words, the headrest may be configured so the support is not rotated about axes-of rotation which are offset outside the footprint of the support (and hence the head of the patient in use). This may allow for greater control in positioning the anatomy of a patient, because for example a rotation of the head may be substantially decoupled from the angle and / or extension of the neck. All of the rotational degrees-of-freedom of the support relative to the baseplate may act about respective axes which meet at a point. However, this is not essential. The support may include a channel for stably seating the neck of the patient. The channel (and / or a structure defining the channel) may have a U-shaped cross-section. The channel (and / or a structure defining the channel) may have a saddle shape. The channel (and / or a structure defining the channel) may be connected to the annulus. The channel (and / or a structure defining the channel) may be integrally formed with the annulus. The support may include, or take the form of, the union of the annulus and the channel (and / or a structure defining the channel). The headrest may have a transmission which is 98% or greater for 6 MeV photons having a 10 cm by 10 cm field size. The headrest may have a transmission of 98% or greater if the transmission does not drop below 98% for any relative position and orientation of an incident beam of 6 MeV photons having a 10 cm by 10 cm crosssection. The incident beam may be conical and diverging. The 10 cm by 10 cm cross section may correspond to the iso-centre of the beam. The iso-centre of the beam refers to the cross section at a distance of 100 cm from an implied point source of the incident beam (along the beam path / axis). For practical purposes, only a path having the largest volume integral of density may be considered. Alternatively, having a transmission of 98% or greater for 6 MeV photons may correspond to the transmission remaining at 98% or greater when the headrest is rotated through a range of solid angle(for example corresponding to a range of motion of a gantry) relative to a beam of 6 MeV photons directed such that the centre of the 10 cm by 10 cm field (the iso-center) coincides with the centre of mass of the headrest. Equivalently, the source of the incident beam may be rotated about the headrest through the complete range of motion of a gantry (or similar) used to direct and position the incident beam. The 6 MeV photons may be generated by a linear accelerator (LINAC). The headrest may be formed from polymeric materials and / or polymer composite materials. Polymeric materials may include, or take the form of, polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), polycarbonate (PC), polylactic acid (PLA), polyethylene terephthalate (PET), glycol modified polyethylene terephthalate (PETG), or any other structural polymers. Polymeric materials may include mixtures, blends and / or co-polymers of the polymers listed herein. Polymer composite materials may include fibre-reinforced polymer composites. Reinforcing fibres may include, or take the form of, glass fibres, carbon fibres, high-density polyethylene (HDPE) fibres, cellulose fibres, or any other fibres used in the field of structural polymer composites. Polymer composite materials may include a foam core. The fluid actuator assembly may include, or take the form of, a number of fluid actuators configured such that, when the head of the patient is stably seated in the support, the fluid actuators are annularly arranged about the centre of mass of the head when viewed from above. Above may be relative to gravity when in use. The fluid actuator assembly may include, or take the form of, a number of fluid actuators, and each fluid actuator may be configured such that a volume containing an operating fluid is hollow along an actuation direction. Each fluid actuator may not include a through-hole when viewed along the actuation direction. Each fluid actuator may be a fluid actuator according to the second aspect. A system may include the headrest. The system may also include one or more pumps operatively coupled to drive the fluid actuator assembly with an operating fluid. The system may also include a controller configured to control the one or more pumps. In this way, the controller may move the support of the headrest to a desired position and orientation by controlling the pumps to actuate the fluid actuator assembly. The controller may include a digital electronic processor operatively coupled to random-access memory and non-volatile computer readable storage which stores program code which, when executed by the digital electronic processor, causes the system to carry out the functionalities described herein. The controller may take the form of a micro-controller storing suitable program code. Alternatively, the controller may take the form of a field programmable gate array, an application specific integrated circuit, or any other suitable data processing device. The system may also include user input controls operatively coupled to the controller and configured to allow a user to reposition the headrest support. The system may be configured to allow a user to store the current position of the headrest support to the non-volatile computer readable storage. The system may be configured to allow a user to recall a stored position, and to control the fluid actuator assembly to move the headrest support to the stored position. A stored position which is recalled may be a position previously stored by a user, for example corresponding a specific patient. The stored position which is recalled may be one of a number of pre-set positions. The user input controls may be provided as part of the controller. The user input controls may be provided via a separate data processing device operatively coupled to the controller (for example a laptop, desktop or tablet computer, or a smartphone). The user input controls may include, or take the form of, physical input controls, such as one or more buttons, joysticks, sliders and so forth. Alternatively, the user input controls may include, or take the form of, a graphical user interface being executed by the controller, or by a separate data processing device operatively coupled to the controller (for example a laptop, desktop or tablet computer, or a smartphone). The system may also include one or more sensors operatively coupled to the controller. The one or more sensors may be configured to detect changes in the position and / or orientation of the head of a patient seated in the support. In a locked mode of the system, the controller may be configured, based on inputs from the one or more sensors, to control movement of the headrest support to maintain a fixed position and orientation of the head of the patient relative to the isocentre of an incident beam for radiotherapy and / or imaging. The one or more sensors may detect the position and / or orientation of the head of the patient. Changes in the changes in the position and / or orientation of the head may be determined by comparing a current measurement of the position and orientation of the head to a reference measurement obtained at, or before, initiation of the locked mode. The one or more sensors may include, or take the form of, a stereoscopic surface contour sensor. The stereoscopic surface contour sensor may take the form of a Surface Guided Radiation Therapy (SGRT) system. The stereoscopic surface contour sensor may take the form of an arrangement or two or more spaced apart cameras. Images from the two or more spaced apart cameras may be processed to determine the stereoscopic surface contour by the controller, or by another data processing apparatus operatively coupled to the controller. The stereoscopic surface contour sensor may be configured to measure a stereoscopic contour of at least the head of the patient. In the locked mode of the system, the controller may be configured to move the headrest support to maintain alignment between a reference stereoscopic surface contour and a stereoscopic surface contour measured by the stereoscopic surface contour sensor. The reference stereoscopic surface contour may be obtained at, or before, initiation of the locked mode. The system may be set to the locked mode using the user input controls. The fixed position and orientation may refer to the position and orientation relative to, for example, equipment for radiotherapy and / or imaging being used or intended to be used on the patient. Alternatively, the fixed position and orientation may refer to the position and orientation of the patient's head relative to the baseplate of the headrest. In the locked mode of the system, the controller may be configured to move the headrest support such that any measured displacement from the fixed position and orientation decays over time. For example, the movement of the headrest support to correct a small movement of a patients head may be implemented so as to approximate the mechanical response of a critically damped system or an overdamped system. The position and orientation of the patient's head may be denoted by vector: R = (x y z e cp) in which x, y and z are the coordinates of the centroid of the patients head relative to a global coordinate system fixed relative to an installation location of the system (for example a room of a building), 9 is the angle of R to the z-axis and cp is the angle between the x-axis and the projection of R into the x-y plane. The fixed position and orientation may be denoted by Ro, and the displacement from this position may be denoted by AR. The controller may be controlled to move the headrest so that the movement of the patients head following the displacement may be approximated by: R(t) = Ro + AR x e~kt in which t is the time elapsed since the displacement from Ro was measured and k is a decay constant. If a further displacement occurs before the head returns to the fixed position and orientation Ro, then the decay may be reset from the new displacement and total displacement AR. Alternatively, each separately occurring displacement may be tracked and decay independently. For example, if a sampling interval of the one or more sensors is denoted by St, so that a time t after a reference time to may be denoted tn = to + n.St (with n a positive integer), then for a first displacement of ARi occurring at time ti and a second displacement of AR? occurring at time t2, the controller may be configured to control the movement of the headrest support such that the position of the head may be approximated by: R(t„) = Ro + ARi x + AR2 X e~k<-tn~t2^ Once a given period has elapsed since a given displacement such that the exponent may be considered negligible (for example less than a measurement error of the one or more sensors), the corresponding term may be removed from a model being used to by the controller to re-position the headrest support. The controller may be configured, in a case where the one or more sensors measure a displacement from the fixed position and orientation which exceeds a threshold displacement, to turn off a beam emitted by a radiotherapy or imaging system operatively coupled to the system. In this way, critical anatomical structures in the patients head and neck which are not the intended target for radiotherapy or imaging may be protected from accidental exposure. The system does not include any mask or headband for restraining the head of the patient. The system may also include a patient positioning couch configured for at least three degree-of-freedom movement. The headrest may be supported by the patient positioning couch. Alternatively, the headrest may be integrally formed with the patient positioning couch. The headrest may be configured to allow movement of the headrest support relative to the patient positioning couch. The patient positioning couch may be configured for six degree-of-freedom movement. The patient positioning couch may include, or be supported on, a second hexapod / Stewart platform. The controller may also control movement of the patient positioning couch. Alternatively, a further, separate data processing device may control movement of the patient positioning couch. In the latter case, the controller may be operatively coupled to the data processing device which controls movement of the patient positioning couch, in order to receive information about the position and / or movement of the patient positioning couch. The system may be configured to be operable in a first relative motion mode and a second relative motion mode such that, when the patient positioning couch is moved, the controller is configured to control movement of the headrest support such that: in the first relative motion mode, the headrest support maintains a fixed position and orientation relative the patient positioning couch; and in the second relative motion mode, the headrest support maintains a fixed position and orientation relative to the isocentre of an incident beam for radiotherapy and / or imaging. In other words, in the first relative motion mode the fluid actuator assembly is not actuated in response to movement of patient positioning couch, so that that the headrest support remains in a fixed relative position and orientation relative to the patient positioning couch. Similarly, in the second relative motion mode, the headrest support is controlled to remain at a fixed (global) location and orientation as the patient positioning couch is moved / tilted beneath it. The second relative motion mode may be useful for orienting the and manipulating the neck of the patient, by keeping their head positioned for radiotherapy or imaging whilst moving their body. Alternatively, in the second relative motion mode the headrest support may maintain a fixed position and orientation relative to the baseplate. According to a second aspect of the invention, there is provided a hydraulic actuator. The hydraulic actuator includes an exterior barrel, tapering to an outlet at a first end and open at a second end. The hydraulic actuator also includes an interior barrel received coaxially within the exterior barrel and connected to the exterior barrel such that a prismatic volume defined between the exterior barrel and the interior barrel is in fluid communication with the outlet. Once assembled, the combination of the exterior barrel and the interior barrel may be referred to as a "barrel assembly". The hydraulic actuator also includes a plunger having at least a first end slidably received within the prismatic volume and forming a seal between the exterior barrel and the interior barrel, such that depressing the plunger towards the first end of the exterior barrel forces fluid between the exterior barrel and the interior barrel out through the outlet. Equivalently, forcing fluid in via the outlet will cause the plunger to extend away from the first end of the exterior barrel. Every part of the hydraulic actuator is formed from non-metallic materials. The hydraulic actuator may include features corresponding to any features of the headrest of the first aspect and / or systems including the headrest. Definitions applicable to the headrest of the first aspect (and / or features thereof) and / or systems including the headrest (and / or features thereof), may be equally applicable to the hydraulic actuator (and / or features thereof). The hydraulic actuator may have a transmission of 98% or greater for 6 MeV photons having a 10 cm by 10 cm field size. The hydraulic actuator may include no through-hole when viewed along an axial direction. The hydraulic actuator may be cylindrical. However, the hydraulic actuator need not be cylindrical. The prismatic volume may be a hollow cylinder. According to a third aspect of the invention, there is provided a method for execution by the controller of the system including the headrest of the first aspect. The method includes controlling the one or more pumps to actuate the fluid actuator assembly to move the support of the headrest to a desired position and orientation. The method may also include features corresponding to any features of the headrest of the first aspect and / or a system including the headrest. Definitions applicable to the headrest of the first aspect (and / or features thereof) and / or a system including the headrest (and / or features thereof) may be equally applicable to the method (and / or features thereof). The method may be for execution by a system including one or more sensors operatively coupled to the controller and configured to detect changes in the position and / or orientation of the head of a patient seated in the support. The method may also include, based on the inputs from the one or more sensors, controlling movement of the headrest support to maintain a fixed position and orientation of the head of the patient relative to the isocentre of an incident beam for radiotherapy and / or imaging. Movement of the headrest support may be controlled such that any measured displacement from the fixed position and orientation decays over time. The method may also include, in response to the one or more sensors measure a displacement from fixed position and orientation which exceeds a threshold displacement, turning off a beam emitted by a radiotherapy or imaging system operatively coupled to the system. Brief Description of the drawings Certain embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 schematically illustrates a system including a headrest for use in radiotherapy and / or imaging; Figure 2 is a process-flow diagram for a method of calibrating the system and headrest shown in Figure 1; Figure 3 schematically illustrates positioning of a patient head and neck anatomy using a headrest support; Figure 4A is a process-flow diagram for a first method of using the system shown in Figure 1; Figure 4B is a process-flow diagram for a second method of using the system shown in Figure 1; Figures 5A to 5E schematically illustrate an exemplary headrest for the system shown in Figure 1; and Figures 6A to 6M schematically illustrate a hydraulic actuator operable using a hollow column of working fluid. Detailed description In the following description, like parts are denoted by like reference numerals. The present specification describes headrests which are configured to be manoeuvred in 6 degrees-of-freedom and upon which a patient places their head and neck for radiotherapy and / or imaging. The headrest can be used in conjunction with an existing patient couch without the need for modification of the patient couch surface. For example, the headrest may simply be indexed to the patient couch (i.e. located in a repeatable way). The patient's head is placed upon a support which, by virtue of its design, causes the head to remain centred and stably seated under the force of gravity (see for example the ring-shaped portion 9 and bridge shaped neck support portion 29, 37 in Figures 5A to 5E). The support is coupled to a baseplate by an actuator arrangement which provides the functionality of manoeuvring in 6 degrees of freedom. The support is able to be moved and oriented in such a way that the desired treatment position, patient-specific neck contour and comfort for a specific patient can be achieved. Additional functionalities and control methods are described herein. For example, a user is able to save / store the geometric position of the support, to be recalled again at subsequent sessions for the same patient. The patient will typically be instructed to try and stay as motionless as possible throughout all sessions. However, in practice some movement is inevitable. The headrest of the present specification may be controlled based on inputs from sensors tracking patient position so as to provide dynamic stabilisation of the position of the patients head and neck. In normal radiotherapy practice, a series of X-ray, computed tomography (CT), or magnetic resonance imaging (MRI) images will also be obtained with which the user (e.g. a clinician) can verify the correct anatomical position of the internal structures of the head and neck. In the event of the internal anatomy not being correctly aligned to a planned position and / or orientation, the user will be able to adjust the anatomy using the headrest. Using the headrests described herein, these adjustments may be made in the range from submillimetre scales up to millimetre scales, or even centimetre scales. In this way, any changes of the patient's anatomy over time (e.g. weight loss / gain, swelling increase / decrease, tumour volume changes, etc), may be mitigated. The headrest may be particularly useful when coupled to sensors monitoring the position of the patients head and neck, for example surface contour detectors used for surface guided radiation therapy SGRT. Once the user is satisfied that the patient is in the correct position for treatment after adjusting the head and neck, the user may activate a dynamic stabilisation (or "locked") mode, in which as the patient tries to remain motionless, if any movements are detected, the headrest can be controlled to move to compensate. In this way, any gradual muscle relaxation of the patient, subconscious head motion, excessive respiratory motion or essential tremor may be reduced or removed. The headrest described herein, and systems and methods using the headrest, may enable a patient's anatomy to be effectively "dynamically immobilised" and may provide a solution for head and neck radiotherapy and / or imaging without the need for external immobilisation such as thermoplastic masks. Referring to Figure 1, a system 1 including a headrest 2 for use in radiotherapy and / or imaging is shown. The headrest includes a support 3, a baseplate 4, and a fluid actuator assembly 5 operatively coupling the support 3 to the baseplate 4. Every part of the headrest 2, including the support 3, baseplate 4 and fluid actuator assembly 5, is formed from non-metallic materials. Headrest In the example shown in Figure 1, the fluid actuator assembly 5 takes the form of a six-actuator hexapod, sometimes also termed a "Stewart platform". However, in general the fluid actuator assembly 5 may be of any type configured for six degree-of-freedom movement of the support relative to the baseplate. For example, three translational degrees of freedom Ti, T2 and T3 and three rotational degrees of freedom Ri, R2, R3. The six degrees-of-freedom should be independent over at least an operating portion of the range of movement of the support 3. In other words, a rotation Ri, R2, R3 of the support 3 relative to the baseplate 4 should be possible without a corresponding (non-negligible) translation Ti, T2, T3. For reference during the following explanations, let x, y, z denote a first set of right-orthogonal axes of a global (fixed) coordinate system which does not move with the support 3. For example, fixed relative to the room in which the system 1 is installed, preferably fixed relative to the isocentres of one or more radiation beams 7 used for radiotherapy and / or imaging. In the example shown in Figure 1, the z-axis is aligned vertically (relative to gravity) and the x-axis extends generally along the height of a patient (equivalently parallel to the longitudinal axis of a patient couch 7). Let x', y', z' denote a second set of local right-orthogonal axes which are fixed relative to the support 3. In other words, the x', / 'and z'axes move and rotate with the support 3, relative to the global axes x, y, z. In the example shown in Figure 1, the origin 8 of the second axes x', y', z' is within the bounds of the support 3, at the centre of a ringlike portion 9 of the support 3 which receives the head 10 of the patient. The orientation of the local axes x', y', z' is aligned with the global axes x, y, z when the support 3 is in an initial (or "neutral") reference position relation the baseplate 4. Depending on the configuration of the fluid actuator assembly 5, the relative orientation of the degrees-of-freedom Ti, T2, Ts, Ri, R2, R3 may be pre-defined by the design of the fluid actuator assembly 5 (and any bearings included therein), or may be dynamically adjustable. For example, when the fluid actuator assembly 5 takes the form of a six actuator hexapod (as shown in Figure 1), the translational degrees-of-freedom Ti, T2, T3 may be controlled along any three non-coplanar directions, and the axes of rotation for rotational degrees-of-freedom Ri, R2, R3 do not need to intersect at a point or lie parallel to the axes of the translational degrees-of-freedom Ti, T2, T3. For the purposes of explanations of the headrest 2 herein, it shall be assumed that translational degrees of freedom Ti, T2, T3 are controlled to be aligned with the global axes x, y, z such that Ti = Tx, T2 = Ty and T3 = Tz. The rotational degrees of freedom Ri, R2, R3 shall be assumed to be controlled to act about axes of rotation corresponding to the local axes x', y', z', such that Ri = RX', R2 = Ry and R3 = Rz'. Although this is only one example of how the six degrees-of-freedom may be configured, this is a convenient and readily understood selection, having a clear connection to the corresponding movements of patients' head 10. In the general case, it is preferred that all of the rotational degrees-of-freedom Ri, R2, R3 of the support 3 relative to the baseplate 4 should act about respective axes of rotation which intersect the support 3 when viewed from above, i.e. along the z-axis looking towards the baseplate 4. This is advantageous to help minimise any translations of the patients' head 10 which accompany a pure rotation RX', Ry, Rz' of the support 3. In other words, the headrest 2 is configured so the support 3 is not rotated about axes-of rotation which are offset outside the footprint of the support 3 (and hence the head 10 of the patient in use). This may allow for greater control in positioning the anatomy of a patient, because for example a rotation of the head may be substantially decoupled from the angle and / or extension of the neck 11 of the patient. The preceding discussion does not mean that the headrest 2 cannot be controlled with a combination of translations Tx, Ty, Tz and rotations RX', Ry, Rz', simply that these degrees-of-freedom are preferably independent. In some examples (including that shown in Figure 1), all of the rotational degrees-of-freedom Ri, R2, R3 (for example RX', Ry, Rz) of the support 3 relative to the baseplate 4 may act about respective axes which meet at a point (for example origin 8). Every part of the headrest 2, including the support 3, baseplate 4, fluid actuator assembly 5 and any further, optional components, is formed from non-metallic materials. Suitable materials for the headrest 2 and component parts thereof include polymeric materials such as, for example, polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), polycarbonate (PC), polylactic acid (PLA), polyethylene terephthalate (PET), glycol modified polyethylene terephthalate (PETG), or any other structural polymers. Polymeric materials may include mixtures, blends and / or co-polymers of the polymers listed herein. Other materials for the headrest 2 and component parts thereof include polymer composite materials such as, for example, fibre-reinforced polymer composites including a matrix of any of a polymer (including any described herein) and reinforcing fibres such as, for example, glass fibres, carbon fibres, high-density polyethylene (HDPE) fibres, cellulose fibres, or any other fibres used in the field of structural polymer composites. Polymer composite materials may include a foam core. For example, composites including a foam core surrounded by a fibre-reinforced polymer composite "skin" provide a combination of high stiffness / rigidity with low density. The formation of the headrest 2 (and components thereof) from non-metallic materials does not exclude the presence of thin metal coatings or similar structures which provide no, or negligible, mechanical structure or function to the headrest. In this way, attenuation or scattering of radiation being used for radiotherapy (or imaging) by the headrest may be reduced, whilst also providing precision control over head 10 and neck 11 of a patient. The support 3 is shaped for stably seating the head 10 of the patient. In other words, when the patient's head 10 is received by the support 3, the weight of the head 10 under gravity and the shape of the support 3 discourage relative movement between the head 10 and the support 3. In practice, this means that the shape of the support 3 is such that a movement (translation or rotation) of the head 10 relative to the support would result in raising the centre of gravity of the head 10 (positive z-direction in Figure 1). In this way, the head 10 is stably supported in the sense that a perturbation in the position or orientation of the head 10 relative to the support 3 will result in a net restoring force (due to gravity) urging the head 10 back towards its initial position. The support 3 is shaped for stably seating the head 10 of the patient across a practical range of motion of the fluid actuator assembly 5 - at a high enough angle to the vertical the head 10 could no longer be supported, but this is not relevant to any practical configuration of a patient for radiotherapy and / or imaging. One example of a practical shape for the support 3 is a ring-like portion 9 (or "annulus") as illustrated in Figure 1. A further example including an additional support 29 for the patient's neck 11 is illustrated in Figures 3 and 5A to 5E and described hereinafter. The headrest 2 does not include any part which passes across or restrains the front of the head 10, face or neck 11 of the patient. For example, the headrest does not include a mask, a headband and so forth. Such restraints can produce significant discomfort and distress in many patients, and the capability to provide stable and accurate patient positioning without such confining features is considered a key advantage of the presently described headrest 2 and systems 1 using it. The headrest 2 does not support the torso 12 or limbs (not shown) of the patient. These are supported by a patient positioning couch 7 as usual. The headrest 2 may be integrally formed with a patient positioning couch 7, but this is not necessary. For example, the headrest may be operatively coupled to a patient positioning couch 7 to augment its existing functionality. The fluid actuator assembly 5 is preferably a hydraulic actuator assembly as the working fluid is incompressible. Pneumatic actuators could be considered if sufficient precision and stability is obtainable. For example, when the head 10 of the patient may be lighter, such as in paediatric settings. The fluid actuator assembly 5 preferably takes the form of a hexapod actuator, for example as illustrated in Figure 1 and Figures 5A to 5E. However, other suitable actuator configurations may be used, provided that the required degrees-of-freedom may be implemented using components which do not include any metals. The fluid actuator assembly 5 may preferably include a number (six or more) of fluid actuators 13 arranged such that, when the head 10 of the patient is stably seated in the support 3, the fluid actuators 13 are annularly arranged about the centre of mass of the head when viewed from above (see also Figure 5B). In other words, the fluid actuators 13 form a "ring" around the support 3 and patient head 10 viewed from above. This arrangement helps to ensure that the rotational degrees-of-freedom RX', Ry', Rz' may be substantially independent from translations Tx, Ty, Tz of the patient's head 10 in use as described hereinbefore. Suitable fluid actuators 13 for the headrest 2 are, in common with the other components of the headrest 2, formed without metallic materials. This includes barrels, plungers and so forth. In this way, the attenuation of radiation beams 6 used for therapy and / or imaging may be minimised, whilst keeping the actuators close to the support 3. The working fluid of the fluid actuators 13 may take the form of water, oil, or any other fluid which is chemically compatible with the non-metallic materials used to form the fluid actuators 13. Although such working fluids typically have low attenuation, radiation transparency may be further improved if each fluid actuator 13 is configured such that a volume containing an operating fluid is hollow along an actuation direction. In other words, the fluid column used for actuation takes the form of a hollow cylinder or similar so as to reduce a total mass of operating fluid within the fluid actuator 13 (see for example the fluid actuator 13 structure described hereinafter with reference to Figures 6A to 6M). Radio transparency A headrest 2 composed entirely of practical non-metallic materials will have a reduced attenuation compared to a comparable headrest including, for example, conventional steel fluid actuators. For example, the headrest preferably has a transmission of 98% or greater for 6 MeV photons (for example generated by a linear accelerator, LINAC) having a 10 cm by 10 cm field size at the isocentre of the radiation beam 6. The isocentre of a radiation beam 6 corresponds to the cross-section off the beam 6 at a distance of 100 cm from an implied point source of the incident beam (along the beam path / axis. In practice, most radiation beams 6 are diverging, often having an approximately conical shape. The condition of the transmission of 98% or more is preferably satisfied for any given orientation of the headrest 2 relative to the incident beam 6 (impractical orientations need not be taken into account). Consequently, for practical purposes only the path having the largest volume integral of density across the headrest 2 needs to be considered. Alternatively, the condition that the transmission is 98% or greater for 6 MeV photons may correspond to the transmission remaining at 98% or greater whilst the headrest is rotated through a possibly range of solid angle (for example corresponding to a range of motion of a gantry) relative to an incident beam 6 of 6 MeV photons directed such that the isocentre of the 10 cm by 10 cm field coincides with the centre of mass of the headrest 2. Equivalently, and more practically, the source of the incident radiation beam 6 may be rotated about the headrest 2 through the complete range of motion of a gantry used to direct and position the radiation beam 6. System In addition to the headrest 2, the system 1 also includes one or more pumps 14 operatively coupled to drive the fluid actuator assembly 5 with an operating fluid. In the example illustrated in Figure 1, the pump(s) 14 are coupled to the fluid actuator assembly 5 via tubes 15. The tubes 15 are also composed of non-metallic materials, or at a minimum the portions of the tubes 15 which will potentially intersect radiation beams 6 are formed from non-metallic materials. For example, polymers, rubbers (natural or synthetic), in either case reinforced, and so forth. The length of the tubes 15 is such that the pump(s) 14 do not need to be metal free for radio transparency, since the pump(s) 14 may be situated well outside a volume in which intersection with radiation beams 6 is possible. The pump(s) 14 are controlled by a controller 16. In this way, the controller 16 can move the support 3 of the headrest 2 to a desired position and orientation relative to the baseplate 4 by controlling the pumps 14 to actuate the fluid actuator assembly 5. The controller 16 includes a digital electronic processor 17 (or equivalent) operatively coupled to random-access memory 18 (RAM) and non-volatile computer readable storage 19 such as, for example, a hard disc drive (magnetic or solid state), a read only module (ROM) and so forth. The storage 19 contains program code 20 which, when executed by the digital electronic processor 17, causes the system 1 to carry out the functionalities described herein. The controller 16 also includes user input device(s) 21 (or "controls"), such as, for example, buttons, keyboards, keypads, touch-screen interfaces and so forth. The user input controls 21 allow a user to reposition the headrest 2 support 3. The user input controls 21 may be provided as part of the controller 16, or may form part of a separate data processing device operatively coupled to the controller 16 such as, for example, a laptop, desktop or tablet computer, or a smart-phone. In other examples, the user input controls 21 may by physical input controls, such as one or more buttons, joysticks, sliders and so forth. Alternatively, the user input controls may be provided via a graphical user interface being executed by the controller 16 (and presented on a display 22), or by a separate data processing device operatively coupled to the controller such as, for example, a laptop, desktop or tablet computer, or a smart-phone. A display 22 is preferably included to display feedback on the relative position of the headrest 2, and optionally to allow display of status information and / or control of other components such as a LINAC (not shown), imaging system (not shown) and so forth. The display 22 and user input device(s) 21 may be a single, integrated device such as a touchscreen. Control of the pumps 14 is provided via an input / output ("I / O") interface 23 and a first wired or wireless link 24. Optionally, one or more positioning / imaging sensors 25 may be coupled to the controller 16 via a second wired or wireless link 26 provided through the same I / O interface 23. Alternatively, the second link 26 may be connected via a second I / O interface (not shown). Sensors 25 may include surface contour detectors, for example as already used for surface guided radiotherapy (SGRT). The positioning / imaging sensors 25 may include, or take the form of, a stereoscopic surface contour sensor. A stereoscopic surface contour sensor uses an arrangement or two or more spaced apart cameras. Images from the two or more spaced apart cameras are processed to determine the stereoscopic surface contour by the controller 16, or by another data processing apparatus operatively coupled to the controller 16 (for example integrated into the sensor 25). A sensor 25 in the form of a stereoscopic surface contour sensor may be configured to measure a stereoscopic contour of at least the head 10 of a patient. The system 1 may also be configured to allow a user to store the current position of the headrest support 3 to the non-volatile computer readable storage 19. For example, the system may be configured to allow a user to store patient settings 27i, 272, 27n. When the same patient attends for a subsequent treatment / scan, the stored position may be recalled, and the fluid actuator assembly 5 controlled to move the headrest 2 support 3 to the stored position for that patient. In addition to any stored positions corresponding to patient settings 27i, 272, 27n, the storage 19 may also store a number of pre-set positions, for example to use as initial starting points to determine and store specific patient settings 27i, 272, 27n. The internal components 17 to 19, 21 to 23 of the controller are interconnected by a bus 28. The controller 16 may take any suitable form such as, for example, a computer, a micro-controller, a field programmable gate array, an application specific integrated circuit, or any other suitable data processing device. The functionalities of the controller 16 as described herein may be provided by a single device, or may be divided across two or more devices. Referring also to Figure 2, a method of calibrating the system 1 and headrest 2 is schematically illustrated. With a patient lying on the patient positioning couch 7 and their head 10 (and preferably also neck 11) supported by the support 3 of the headrest 2, the positioning of the support 3 relative to the baseplate 4 is manually adjusted using the user input device(s) 21 to find an initial or reference position which is suitable for the intended radiotherapy treatment and / or imaging and also acceptable for the patient (step SI). The patient settings 27 for that patient are stored (step S2). Planning scan(s) are then obtained (step S3). When the intended application is a single imaging scan or a series of imaging scans, this may be sufficient. For applications in radiotherapy, the scans may include computer tomography and / or magnetic resonance scans to allow planning treatment sites, calculating treatment fractions and so forth. In systems 1 including position sensors 25, baseline data may also be obtained, for example, a reference surface contour for SGRT. Referring also to Figure 3, a schematic representation is shown of patent head 10 and neck 11 anatomy positioned using the support 3 of a headrest 2. The support 3 illustrated in Figure 3 also includes a neck supporting portion 29 including a channel 37 (Figure 5A) to stably seat the patient's neck 11 and assist in positioning (see also the description of Figures 5A to 5E hereinafter). Although presented as a simplified wireframe, the example shown in Figure 3 may aid understanding of how the positioning and orientation of the support 3 may be adjusted to position patient anatomy for radiotherapy and / or imaging. The mapping of support 3 position and orientation to patent anatomy positioning is established in use, for example at step S3 of the method explained in relation to Figure 2. Although the system 1 is useful for both imaging and radiotherapy applications, the benefits of repeatable and adjusting positioning, and optionally dynamic stabilisation, may be particularly advantageous for radiotherapy treatments in which a patient is attending regularly (sometimes daily) to receive radiation fractions. This leads to a need for reliable and repeating positioning, and additional complications also arise such as changes in the patient's weight throughout a course of treatment. Beyond this, potential changes in the internal volume and position of a tissue such as a tumour which is being treated may also need to be taken into account. The system 1 and headrest 2 can help with these problems. Referring also to Figure 4A, a first method of using system 1 is illustrated. Optionally, , previously stored patient settings 27 are retrieved, and the support 3 is moved to the stored relative position and orientation relative to the baseplate 4 (step S4). This may be done prior to a patient taking their position with their head 10 stably seated within the support 3. Alternatively, the patient may seat their head 10 in the support 3 at a neutral / starting position, before the patient settings 27 are loaded and applied by controlling the fluid actuator assembly 5 to position the support 3. Verification imaging is performed to confirm the accurate positioning of the patient head 10 and / or neck 11 (step S5). Verification imaging may include one of more different types of imaging including, but not limited to, X-ray, MRI, surface contour and so forth. These imaging methods are some examples of sensors 25. The verification imaging is checked to confirm whether the patient head 10 and / or neck 11 are positioned within acceptable tolerances (step S6). Due to the abundance of delicate and important anatomical structures within the head 10 and neck 11, the tolerances for positioning are typical strict, for example within ±1 mm. If the positioning is not within required tolerances (step S6|No), then the position and / or orientation of the support 3 are adjusted (step S7). The adjustment may be manual, for example using the user input device(s) 21 in response to verification imaging presented to a user on the display 22 of controller 16. Alternatively, the adjustment of the support 3 may be performed automatically by the controller 16, for example based on the inputs from sensors 25. After the position and / or orientation of the support 3 is changed, the verification imaging is repeated (step S5). Once the positioning is correct within required tolerances (step S6|Yes), the patient setting 27 may optionally be updated (step S8). This may be done automatically, or at the discretion of a user. For example, small, random variations in positioning from treatment to treatment may not necessitate updating patient settings 27. In comparison, patient settings 27 should be updated to account for a systematic drift over time due to, for example, loss of patient weight throughout a course of treatment. One or more radiation beams 6 are applied to commence the treatment (step S9), and kept on until a pre-calculated dose has been delivered (step S10|Yes). Multiple radiation beams 6 may be applied from different directions, intersecting within a target volume 30 (see Figure 1). In this way, the total dose in the volumes outside the target volume 30 may be relatively reduced. Accurate definition and application of a dose to the target volume requires very accurate 3D positioning of the head 10 and / or neck 11, especially when the converging radiation beams 6 are preferably directed at the treatment volume 30 whilst minimising or avoiding exposure to delicate and important anatomical structures. The improvements in positioning accuracy obtainable using the headrest 2 and system 1 in conjunction with verification imaging may help to improve the reliability and repeatability of positioning, enabling improvements in accuracy, repeatability and control of applying a dose to the target volume 30. Referring also to Figure 4B, a second method of using system 1 is shown. The second method includes the same steps as the first method (steps S4 through S1O), and also includes additional steps. The second method is applicable to systems 1 which include one or more sensors 25 suitable to detect changes in the position and / or orientation of the head 10 of a patient seated in the support 3. For example, sensors 25 capable of determining a surface contour of the patient's head 10, such as using stereo imaging or by imaging structured light patterns (visible or invisible) projected onto the patient's head 10. A dynamic stabilisation (or "locked") mode may be used during which, based on inputs from the sensors 5, the controller 16 controls movement of the headrest 2 support 3 to maintain a fixed position and orientation of the head 10 of the patient relative to the isocentre of an incident radiation beam 6. Although described hereinafter in the context of radiotherapy, the locked mode may be equally applicable and useful to provide dynamic stabilisation for a one-off imaging. A dynamic stabilisation (or "locked") mode is activated (step Sil) after the patient's head 10 and / or neck 11 are positioned within required tolerances (step S6|Yes) and before (or contemporaneously) with activating radiation beam(s) 6 (step S9). The system 1 may be set to the locked mode using the user input device(s) 21. Whilst in the locked mode, the controller 16 controls the fluid actuator assembly 5 so that the support 3 is moved to maintain alignment between a fixed, reference position and orientation of the head 10 and a current position and orientation measured by the sensors 25. The fixed position and orientation may refer to the position and orientation in the global coordinates (x, y, z) relative to, for example, equipment for radiotherapy and / or imaging being used or intended to be used on the patient, or the room in which it is located. Alternatively, the fixed position and orientation may refer to the position and orientation of the patient's head 10 relative to the baseplate 4 of the headrest 2. In some examples, the fixed position and orientation may take the form of a reference stereoscopic surface contour measured using a sensor 25 in the form of a stereoscopic surface contour sensor. Such sensors may be re-purposed from existing uses in SGRT applications. During delivery of the radiation beam(s) 6, the sensors 25 continuously monitor the position and orientation of the head 10 to detect any changes / movements (step S12). For example, by comparing a current measurement of the position and orientation of the head 10 to the fixed, reference position and orientation in the form of a reference measurement obtained at, or before, initiation of the locked mode (for example a reference surface contour). If there is no movement (step S12|No) and the required dose is not yet delivered (step SlO|No), beam delivery continues (step S9). However, if there is movement exceeding a minimum threshold (step S12|Yes), the position and / or orientation of the support 3 are adjusted (step S15) back towards the fixed, reference position and orientation as the beam delivery continues (step S9). Optionally, the controller 16 may be configured with a safety interlock which activates in response to detection of excessive movement. Let the position and orientation of the patient's head be denoted by vector: R = (x y z e cp) in which x, y and z are the coordinates of the centroid of the patients head relative to the global coordinate system, 9 is the angle of R to the z-axis and cp is the angle between the x-axis and the projection of R into the x-y plane. If the fixed position and orientation is denoted by Ro, then displacement from this position is AR = R - Ro. In a case where the one or more sensors 25 measure a displacement AR from the fixed position and orientation Ro which exceeds a maximum threshold displacement rmax, i.e. | AR| >rmax (step S13|Yes), the controller 16 is configured to turn off the one or more radiation beam(s) 6 emitted by a radiotherapy and / or imaging system operatively coupled to the system 1 (step S14) In this way, critical anatomical structures in the patients head and neck which are not the intended target for radiotherapy and / or imaging may be protected from accidental exposure. Some small deviations |AR| <rmax may be tolerable since the movement of the headrest 2 support 3 may bring the patients head 10 back to the fixed position and orientation Ro (or at least, within an acceptable distance). However, excessively large or fast corrective movements of the support 3 may be uncomfortable or even injurious to the patient. Consequently, above a threshold displacement it is preferable to shut off the radiation beam(s) 6 and pause the procedure until the patients head 10 has been re-positioned. The headrest 2 may still be used for such re-positioning of course, since it may be conducted at a safe and comfortable speed with the radiation beam(s) 6 shutdown. Rather than a simple absolute displacement threshold, in some examples the maximum threshold displacement may alternatively be defined as a surface relative to the fixed position and orientation Ro, within which the head 10 displacement AR is required to remain to avoid shutdown of the radiation beams 6 (step S14). In this way, treatments in close proximity to a delicate structure may have reduced tolerance for displacements which would cause exposure to that structure. Preferably, when the controller 16 is repositioning the support 3 in the locked mode, the controller 16 is configured to move the headrest 2 support 3 such that any measured displacement AR from the fixed position and orientation Ro decays over time. For example, the movement of the headrest 2 support 3 to correct a small movement AR of the patient's head 10 may be implemented to approximate the mechanical response of a critically damped system or an overdamped system. For example, the controller 16 may be controlled to move the support 3 so that the movement of the patients head 10 following the displacement AR may be approximated by: R(t) = Ro + AR x e~kt In which t is the time elapsed since the displacement from Ro was measured and k is a decay constant. If a further displacement occurs before the head 10 returns to the fixed position and orientation Ro, then the decay may be reset from the sum of the new displacement and total displacement AR. Alternatively, each separately occurring displacement AR may be tracked and controlled to decay independently. For example, if a sampling interval of the one or more sensors 25 is denoted by 6t and a time t after a reference time to is denoted tn = to + n.5t (with n a positive integer), then for a first displacement of ARi occurring at time ti and a second displacement of AR? occurring at time t2, the controller 16 may be configured to control the movement of the support 3 such that the position of the head 10 is approximated by: R(t„) = Ro + AR, x + AR2 X Once a long enough period has elapsed since a given displacement AR that the exponent may be considered negligible (for example less than a measurement error of the one or more sensors 25), the corresponding term may be removed from a model being maintained by the controller 16 and used to re-position the headrest 2 support 3. Although the dynamic stabilisation method described in relation to Figure 4B has been described in relation to radiotherapy applications, such dynamic stabilisation is also useful for imaging. In particular, imaging a longer exposure or a longer sequence of images during which it is important that a patient be as still as possible. Exemplary headrest Referring also to Figures 5A to 5E, one example of implementing a headrest 2 in accordance with the present specification is shown. Figure 5A is a schematic projection view of an exemplary headrest 31 which is a headrest 2 according to the present specification. Figure 5B is a schematic plan view of the exemplary headrest 31. Figure 5C is a schematic side-view of the exemplary headrest 31. Figure 5D is a part-sectional view through the support 3 and baseplate 4 along the line labelled A-A in Figure 5C. Figure 5E is a schematic exploded projection view of the exemplary headrest 31, showing a single fluid actuator 13 for visual clarity. In Figures 5A to 5E, every element is not labelled in every Figure to avoid visual clutter. The baseplate 4 of the exemplary headrest 31 is formed from three struts 32. The struts 32 include flanges 33 at both ends, which are bolted to corresponding flanges of the other struts 32 to form the baseplate 4 have the approximate shape of an equilateral triangle. A socket 34 for receiving a ball portion 35 of a fluid actuator 13 is formed at each end of each strut 32. The support 3 of the exemplary headrest 31 is formed from a coupling plate 36 which supports a ring-like portion 9 (or "annulus") and a neck support 29. The annulus 9 provides for stably seating the head 10 of the patient. In the exemplary headrest 31 the annulus 9 is circular, however in other examples the annulus may by elliptical, toroidal, polygonal or irregularly shaped. The only requirement is that the head 10 should be stably seated such that a translation of the head 10 relative to the support 3 would result in movement of the head 10 against gravity and a restoring force towards the stably seated position. In the exemplary headrest 31, the annulus 9 is complete, i.e. without any gap, though in other examples the annulus 9 may take the form of a partial annulus (i.e. including a gap). The neck support 29 defines a channel 37 for stably seating the neck 11 of the patient. The channel 37 defined by the neck support 29 of the exemplary headrest 31 has a saddle-shape. In other examples, the channel 37 cross-section may be U-shaped, semi-circular and so forth. In the exemplary headrest 31 the annulus 9 and the neck support 29 defining the channel 37 are integrally formed as a union of the two shapes. In other examples, the neck support 29 may be separate from the annulus 9. The coupling plate 36 of the exemplary headrest 31 is approximately an equilateral triangle in shape (with rounded corners), which each corner defining a pair of sockets 38 for receiving the ball portions 35 of a pair of fluid actuators 13. In the exemplary headrest 31, the annulus 9 and neck support 29 are attached to a separate coupling plate 36 (see for example Figure 5E). In other examples, the coupling plate 36 may be integrally formed with one or both of the annulus 9 and the neck support 29 defining channel 37. Six fluid actuators 13 couple the struts 32 forming the baseplate 4 to the coupling plate 36 of the support 3 in a hexapod configuration. The pair of sockets 38 at each corner of the coupling plate 36 receive fluid actuators coupled to opposite ends of the same strut 32 of the baseplate. Each fluid actuator 13 includes an inlet 38 for coupling to a respective pump 14 via a tube 15. The barrel and piston of each fluid actuator 13 are each terminated in a ball portion 35. The ball portions 35 are each received by low-friction cups 40, and the cups 40 are received into the sockets 34, 38. In this way, ball-and-socket joints are formed coupling each fluid actuator 13 to a strut 32 of baseplate 4 at one end, and at the opposite end to the coupling plate 36 of support 3. The cups 40 are formed of low friction materials such as polytetrafluoroethylene (PTFE) or similar materials. The ball portions 35 may also be formed of low-friction material, but this is not essential. In other examples, if the friction and wear rate between the materials of the ball portions 35 and the material of the struts 32 and coupling portion 36 are acceptably low, the cups 40 may be omitted. The exemplary headrest 31 may be formed from any of the materials described herein, but as described hereinbefore includes no metallic materials. Fluid actuator with hollow fluid column Headrests 2, 32 formed without metals can obtain improved radio-transparency (transmission). The columns of working fluid within fluid actuators 13 represent a significant fraction of the remaining absorption of the headrest 2, 32. A new type of hydraulic actuator shall be described which uses a hollow column of working fluid to permit actuation whilst reducing a volume of the working fluid within the headrest 2, 31. In this way, transmission of radiation through the headrest may be further improved. Referring also to Figures 6A to 6M, an example of the hollow-column hydraulic actuator 41 (hereinafter "hydraulic actuator") is shown. Figure 6A is schematic projection view of the hydraulic actuator 41. Figure 6B is a schematic side-view of the hydraulic actuator 41. Figure 6C is a cross-section along the line labelled B-B in Figure 6B. Figure 6D is a schematic, exploded side-view of a barrel assembly 42 of the hydraulic actuator 41. Figure 6E is a cross-section along the line labelled C-C in Figure 6D. Figure 6F is a cross-section along the line labelled D-D in Figure 6D. Figure 6G is a schematic side view of the barrel assembly 42 of the hydraulic actuator 41. Figure 6H is a cross-section along the line labelled E-E in Figure 6G. Figure 61 is a cross-section along the line labelled F-F in Figure 6G. Figure 6J is a schematic projection view of a plunger 43 of the hydraulic actuator 41. Figure 6K is a schematic side-view of the plunger 43 of the hydraulic actuator 41. Figure 6L is a cross-section along the line labelled G-G in Figure 6K. Figure 6M is a cross-section along the line labelled H-H in Figure 6K. Every element is not labelled in every one of Figures 6A to 6M to avoid visual clutter. The hydraulic actuator 41 includes barrel assembly 42 and plunger 43. The barrel assembly 42 includes an exterior barrel 44, tapering to an outlet 45 at a first end 46 and open at a second end 47. The barrel assembly 42 also includes an interior barrel 48 received coaxially within the exterior barrel 44 and connected to the exterior barrel 44 such that a prismatic volume 48 defined between the exterior barrel 44 and the interior barrel 48 is in fluid communication with the outlet 45. The hydraulic actuator 41 includes no through-hole when viewed along an axial direction. In other words, there is no hole passing axially all the way through the hydraulic actuator 41. In the illustrated example, the prismatic volume 49 formed between the exterior barrel 44 and the interior barrel 48 takes the form of a hollow cylinder. In the illustrated example, the interior barrel 44 includes fins 50 which extend for connection to the tapering first end 46 of the exterior barrel 44. The fins 50 include through holes 51 and leave gaps 52 to allow fluid communication between the prismatic volume 49 and the outlet 45. The plunger 43 has at least a first end 53 slidably received within the prismatic volume and forming a seal 54 between the exterior barrel 44 and the interior barrel 48, such that depressing the plunger 43 towards the first end 46 of the exterior barrel 44 forces working fluid between the 44 exterior barrel and the interior barrel 48 out through the outlet 45. In the illustrated example, the plunger 43 takes the form of a hollow-cylinder to match the cross-sectional shape of the prismatic volume 49, and also includes four ribs 55 extending along an axial direction and distributed about the perimeter at 90° intervals. The ribs 55 serve to increase the rigidity of the plunger 43 and improve resistance against local buckling. Every part of the hydraulic actuator 41 is formed from non-metallic materials. Suitable materials include structural or engineering polymers and / or polymer composites described herein. The hydraulic actuator 41 may have a transmission of 98% or greater for 6 MeV photons having a 10 cm by 10 cm field size when the isocentre intersects the hydraulic actuator, even when filled with working fluid such as water, oil and so forth. Only the core components of the hydraulic actuator 41 are shown in Figures 6A to 6M for visual clarity. When integrated into a headrest 2, 31, additional structures will be included to form appropriate couplings between the support 3 and baseplate 4. For example, ball portions 35, inlets 39 and so forth. In example illustrated in Figures 6A to 6M, the prismatic volume 49 is a hollow cylinder. However, the hydraulic actuator 41 need not be cylindrical and the prismatic volume 49 need not be a hollow cylinder. Any other sectional shape may be used, though the configuration must be prismatic along the axial direction, to allow for depression / extension of the plunger 43 relative to the barrel assembly 42 by pumping working fluid out / in via the outlet 45. The prismatic volume 49 and plunger 43 are not required to form an enclosed, hollow cross-section. For example, the prismatic volume 49 and plunger 43 may take a "U" shaped cross-section, an "I" shaped crosssection (similar to structural beams), or any other shape which allows actuation by retaining rigidity whilst reducing a volume of working fluid. Modifications It will be appreciated that many modifications may be made to the embodiments hereinbefore described. Such modifications may involve equivalent and other features which are already known in the design, manufacture and use of headrests and systems for radiotherapy and / or imaging and which may be used instead of or in addition to features already described herein. Features of one embodiment may be replaced or supplemented by features of another embodiment. The system 1 and headrests 2, 31 described herein may be useable with any existing type of patient positioning couch 7, including static, adjustable and / or dynamically movable patient positioning couches 7. Additional movement modes may be enabled when both the headrest 2, 31 and the patient positioning couch 7 are controllable. For example, the system 1 may be used in conjunction with a patient positioning couch 7 configured for at least three (and up to six) degree-of-freedom movement of the patient positioning couch 7. For example, the patient positioning couch may include, or be supported on, a second hexapod / Stewart platform. The patient positioning couch 7 and any associated actuator assembly providing it with movement do not need to be metal-free, because they are offset relative to the patient head 10 and / or neck 11. The headrest 2, 31 may be supported by the patient positioning couch 7, or alternatively may be attached or integrally formed with the patient positioning couch 7. In this way, the patient positioning couch 7 may provide movement of the patient and headrest 2, 31 relative to an installation room, whilst the headrest 2, 31 allows movement of the support 3 relative to the patient positioning couch 7. Movement of the patient positioning couch 7 may be controlled by the controller 16 of the system 1. Alternatively, a further, separate data processing device (not shown) may control movement of the patient positioning couch 7. In the latter case, the controller 16 should preferably be operatively coupled to the data processing device (not shown) which controls movement of the patient positioning couch 7, to receive information about and / or coordinate the movement of the patient positioning couch 7. When the patient positioning couch 7 is movable, the system 1 may be configured to operate in at least a first relative motion mode and a second relative motion mode. In the first relative motion mode, when the patient positioning couch 7 is moved, the controller 16 may be configured to control movement of the headrest 2, 31 support 3 such that the support 3 maintains a fixed position and orientation relative the patient positioning couch 7. In the second relative motion mode, the controller 16 may instead move the support 3 so as to maintain a fixed position and orientation relative to the isocentre of one or more incident radiation beam(s) 6 for radiotherapy and / or imaging. In other words, in the first relative motion mode the fluid actuator assembly 5 is not actuated when the patient positioning couch 7 is moved, so that that headrest 2, 31 support 3 remains in a fixed relative position and orientation relative to the patient positioning couch 7. Alternatively, in the second relative motion mode, the headrest 2, 31 support 3 remains at a globally fixed location and orientation as the patient positioning 7 couch is moved / tilted beneath it. The second relative motion mode may be especially useful for orienting and manipulating the neck 11 of a patient, by keeping their head 10 positioned for radiotherapy or imaging whilst moving their body. For convenience of understanding, examples have been described herein in relation to sets of right-handed orthogonal axes x, y, z and x', y', z'. However, the relative movements described herein may be described using any suitable 3D coordinate system. Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel features or any novel combination of features disclosed herein either explicitly or implicitly or any generalization thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. The applicant hereby gives notice that new claims may be formulated to such features and / or combinations of such features during the prosecution of the 5 present application or of any further application derived therefrom.

Claims

1. A headrest for use in radiotherapy and / or imaging comprising: a support shaped for stably seating the head of a patient;a fluid actuator assembly operatively coupling the support to a baseplate and configured for six degree-of-freedom movement of the support relative to the baseplate;wherein the headrest is formed from non-metallic materials.

2. The headrest of claim 1, wherein the fluid actuator assembly is a hydraulic actuator assembly.

3. The headrest of claims 1 or 2, wherein the fluid actuator assembly comprises a hexapod actuator.

4. The headrest of any one of claims 1 to 3, wherein the support comprises an annulus for stably seating the head of the patient.

5. The headrest of any one of claims 1 to 4, configured such that rotational degrees-of-freedom of the support relative to the baseplate act about respective axes of rotation that intersect the support when viewed from above.

6. The headrest of any one of claims 1 to 5, wherein the support comprises a channel for stably seating the neck of the patient.

7. The headrest of any one of claims 1 to 6, having a transmission which is 98% or greater for 6 MeV photons having a 10 cm by 10 cm field size.

8. The headrest of any one of claims 1 to 7, formed from polymeric materials and / or polymer composite materials.

9. The headrest of any one of claims 1 to 8, wherein the fluid actuator assembly comprises a plurality of fluid actuators configured such that, when the head of the patient is stably seated in the support, the fluid actuators are annularly arranged about the centre of mass of the head when viewed from above.

10. The headrest of any one of claims 1 to 9, wherein the fluid actuator assembly comprises a plurality of fluid actuators, and wherein each fluid actuator is configured such that a volume containing an operating fluid is hollow along an actuation direction.

11. The headrest of claim 10, wherein each fluid actuator is a fluid actuator according to claims 20 or 21.

12. A system comprising:the headrest of any one of claims 1 to 11;one or more pumps operatively coupled to drive the fluid actuator assembly with an operating fluid;a controller configured to control the one or more pumps.

13. The system of claim 12, further comprising user input controls operatively coupled to the controller and configured to allow a user to reposition the headrest support.

14. The system of claims 12 or 13, comprising one or more sensors operatively coupled to the controller and configured to detect changes in the position and / or orientation of the head of a patient seated in the support;wherein in a locked mode of the system the controller is configured, based on inputs from the one or more sensors, to control movement of the headrest support to maintain a fixed position and orientation of the head of the patient relative to the isocentre of an incident beam for radiotherapy and / or imaging.

15. The system of claim 14, wherein in the locked mode, the controller is configured to move the headrest support such that any measured displacement from the fixed position and orientation decays over time.

16. The system of claims 14 or 15, wherein the controller is configured, in a case where the one or more sensors measure a displacement from the fixed position and orientation which exceeds a threshold displacement, to turn off a beam emitted by a radiotherapy or imaging system operatively coupled to the system.

17. The system of any one of claims 12 to 16, which does not comprise any mask or headband for restraining the head of the patient.

18. The system of any one of claims 12 to 17, comprising a patient positioning couch configured for at least three degree-of-freedom movement;wherein the headrest is supported by the patient positioning couch, or integrally formed with the patient positioning couch;wherein the headrest is configured to allow movement of the headrest support relative to the patient positioning couch.

19. The system of claim 18, wherein the system is configured to be operable in a first relative motion mode and a second relative motion mode such that, when the patient positioning couch is moved, the controller is configured to control movement of the headrest support such that:in the first relative motion mode, the headrest support maintains a fixed position and orientation relative the patient positioning couch;in the second relative motion mode, the headrest support maintains a fixed position and orientation relative to the isocentre of an incident beam for radiotherapy and / or imaging.

20. A hydraulic actuator, comprising:an exterior barrel, tapering to an outlet at a first end and open at a second end;an interior barrel received coaxially within the exterior barrel and connected to the exterior barrel such that a prismatic volume defined between the exterior barrel and the interior barrel is in fluid communication with the outlet;a plunger having at least a first end slidably received within the prismatic volume and forming a seal between the exterior barrel and the interior barrel, such that depressing the plunger towards the first end of the exterior barrel forces fluid between the exterior barrel and the interior barrel out through the outlet;wherein every part of the hydraulic actuator is formed from non-metallic materials.

21. The hydraulic actuator of claim 20, wherein the hydraulic actuator includes no through-hole when viewed along an axial direction.

22. A method for execution by the controller of the system according to any one of claims 12 to 19, comprising controlling the one or more pumps to actuate the fluid actuator assembly to move the support of the headrest to a desired position and orientation.

23. The method of claim 22, for execution by the system comprising one or more sensors operatively coupled to the controller and configured to detect changes in the position and / or orientation of the head of a patient seated in the support;the method further comprising, based on the inputs from the one or more5 sensors, controlling movement of the headrest support to maintain a fixed position and orientation of the head of the patient relative to the isocentre of an incident beam for radiotherapy and / or imaging.

24. The method of claim 23, wherein movement of the headrest support is10 controlled such that any measured displacement from the fixed position and orientation decays over time.

25. The method of claims 23 or 24, further comprising in response to the one or more sensors measure a displacement from fixed position and orientation which15 exceeds a threshold displacement, turning off a beam emitted by a radiotherapy or imaging system operatively coupled to the system.-37-

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