Patent Description:
Radiotherapy uses ionising radiation to treat a human or animal body. In particular, radiotherapy is commonly used to treat tumours within the human or animal body. In such treatments, cells forming part of the tumour are irradiated by ionising radiation in order to destroy or damage them. However, in order to apply a prescribed dose of ionising radiation to a target location or target region, such as a tumour, the ionising radiation will typically also pass through healthy tissue of the human or animal body. Therefore, radiotherapy has the desirable consequence of irradiating and damaging a target region, but can also have the undesirable consequence of irradiating and damaging healthy tissue. In radiotherapy treatment, it is desirable to align the dose received by the target region with a prescribed dose and to minimise the dose received by healthy tissue.

<CIT> is directed to a radiotherapy unit equipped with an arm rotatable centering on the horizontal rotation axis supported by a rack, an X-ray irradiating head attached to the arm to irradiate X-rays to the isocenter equipped on the axis line of the horizontal rotation axis, and a treatment bed on which a patient is laid so as to match the patient's target volume with the isocenter. A guide means to support the rack rotatable centering on the vertical axis passing the isocenter and a rotation drive means to drive and rotate the rack centering on the vertical axis are further equipped.

Modern radiotherapy treatment uses techniques to reduce the radiation dose to healthy tissue and thereby provide a safe treatment. For example, one approach to minimising a radiation dose received by healthy tissue surrounding a target region is to direct the radiation towards the target region from a plurality of different angles, for example by rotating a source of radiation around the patient by use of a rotating gantry. In this case, the angles at which radiation is applied are selected such that each beam of radiation passes through the target region. In this way, a cumulative radiation dose may be built up at the target region over the course of a treatment arc in which the radiation source rotates through a certain angle. Radiation is emitted in a radiation plane which is coincident with the plane of the gantry around which the radiation source rotates and radiation may thus be delivered to a radiation isocentre at the centre of the gantry regardless of the angle to which the radiation head is rotated around the gantry. Because the radiation is applied from a plurality of different angles, the same, high, cumulative radiation dose is not built up in the healthy tissue since the specific healthy tissue the radiation passes through varies with angle. Therefore, a unit volume of the healthy tissue receives a reduced radiation dose relative to a unit volume of the target region. Treatments that utilise rotation of the gantry in this manner are known as coplanar. However, after the radiation source has been rotated <NUM>°, it will be appreciated that any subsequent radiation beams begin to pass through regions of healthy tissue which have already been irradiated. This increases the radiation dose applied to healthy tissue. Accordingly, when using such a method the volume of healthy tissue available to spread the radiation dose is relatively small, thus imposing restrictions on the treatment which can be provided by such devices.

Therefore, an alternative approach to minimising the radiation dose received by healthy tissue surrounding a target region is to rotate the patient relative to the plane of radiation. As the angle of the patient varies relative to the plane of the gantry, so does the healthy tissue the radiation passes through. In order to further reduce the radiation dose relative to a unit volume of the target region, it is desirable to provide a treatment that combines both of these rotations. An example of a known device that combines the rotation of the patient with the rotation of the radiation source is shown in <FIG>. This shows that the patient <NUM>, who is supported on the subject support surface <NUM>, which is also referred to herein as a patient support surface <NUM>, can be rotated whilst the gantry <NUM> may also rotate about the patient support surface <NUM>. The gantry <NUM> shown in <FIG> is a C-arm gantry or open gantry. The rotation mechanism <NUM> rotates the gantry <NUM> about a fixed axis <NUM>. As the gantry <NUM> is rotated, radiation emitted by a radiation source <NUM> can sweep out a circle. Radiation can be applied to the patient <NUM> from a plurality of angles around the circle. The circle may be described as lying in a radiation plane. The radiation axis lies in the radiation plane. The radiation axis makes an angle of <NUM>° with respect to the fixed axis <NUM>.

The rotation mechanism <NUM> for the patient support surface <NUM> is located underneath the gantry <NUM> of the radiotherapy device, while a rotation mechanism for the gantry <NUM> is located opposite the patient support surface <NUM>. The rotation mechanism <NUM> for the patient support surface <NUM> is located underneath the gantry <NUM> so that the axis of rotation of the patient support surface <NUM> will be in the radiation plane. In particular, the axis of rotation of the patient support surface <NUM> passes through the isocenter <NUM> of the radiotherapy device, so that the patient support surface <NUM> is rotated about the isocenter <NUM>. When the patient support surface <NUM> is in its neural position, the axis of rotation of the patient support surface <NUM> is substantially vertical (perpendicular to the plane of the floor) and this can also be called a vertical axis <NUM>. The longitudinal axis <NUM> is parallel to long side of the patient support surface <NUM> in its neutral position and the transverse axis <NUM> is parallel to the short end of the patient support surface <NUM> in its neutral position. The rotation mechanism <NUM> is located within the plane of radiation. Treatments utilising both the rotation of the radiation and the patient <NUM> are known as non-coplanar treatments.

Some recently developed radiotherapy devices comprise ring-based gantries (or bores), such as that shown in <FIG>. Typically, the bore of a radiotherapy device is cylindrical. A patient support surface <NUM> is positioned in the bore such that radiation can be directed toward a patient <NUM> positioned on the support surface <NUM>. The bore of the apparatus can be formed by a framework, which may otherwise be described as a chassis, a shielding structure, a shell, or a casing. The framework defines the outer surface of the device which the patient <NUM> sees upon entering the treatment room, as well as defining the inner surface of the bore which the patient <NUM> sees when positioned inside the bore. The framework also defines a hollow region of annular cross-section in which the gantry <NUM> can be both rotated and tilted. Thus, the patient <NUM> is shielded from the rotatable gantry <NUM>. Movement of the gantry <NUM> is hidden from the patient's view, reducing intimidation and distress which may otherwise be caused if the patient <NUM> were able to see rotation of the large gantry <NUM>, as they would for an open gantry as shown in <FIG>, and also reducing the likelihood that the patient can accidentally touch or otherwise interfere with the movement of the gantry <NUM>. This means that the gantry <NUM> can be rotated quickly, efficiently and safely. Ring-based gantries are also desirable because they increase device stability. The ring-based gantry is supported by the floor and rests upon it. However, the geometry of a ring-based gantry and its connection to the floor makes it impossible to rotate the patient support surface <NUM> about the isocenter <NUM>.

Another problem that arises when attempting to minimise the radiation dose received by healthy tissue surrounding a target region can be found in accurately locating the position of the target region relative to the device. For example, movement of the patient can cause movement of unhealthy tissue such as a tumour and thus the dose applied to the target region may be decreased and the dose applied to the healthy tissue may be increased. In other words, if a patient moves during or prior to radiotherapy treatment, this can cause a high cumulative dose to build up in a region of healthy tissue instead of in a target region. This can reduce the effectiveness of the radiotherapy for treating the target region and can cause damage to otherwise healthy tissue.

This problem is also caused by flexing of the table top of the patient support surface when the table top is extended into the device. In normal operation, the table top will initially be positioned substantially outside the plane of the gantry to enable the patient to easily position themselves onto the table. The table top will then be extended into the plane of the gantry and, in particular, such that that the target region is aligned with the isocenter of the device. In the extended position, the table top will flex with a magnitude dependent on the position of the table top, the position of the patient on the table top and the weight of the patient. Due to the table top flex, the target region will move relative to the isocenter and this will result in healthy tissue receiving a higher dose of radiation than is necessary. Furthermore, during spiral treatments, which are used to target a larger target region, the table top is moved during the treatment. Spiral treatments involve the patient being moved, by movement of a table top, whilst the radiation source moves around the gantry and emits radiation. Accordingly, the amount of table top flex will vary during the treatment and so the position of the target region relative to the isocenter will vary during the treatment, resulting in healthy tissue receiving a higher dose of radiation than is necessary. This problem occurs for all radiotherapy devices with an extendable table top. However, this problem is particularly significant for radiotherapy devices with a bore solution, because these devices will often have a longer top extension. The longer the table top extension, the more table top flex will occur and the greater will be the change in position of the target region.

Previous solutions to this problem involve manually positioning the patient, for example with the assistance of lasers. However, particularly for automatic and spiral treatments this does not work without repeatedly stopping the treatment and thereby resulting in longer treatment times and lower efficiency. It is possible to detect the position of the target region by taking an image, however doing so is harmful to the patient. It would be desirable to know the position of the treatment region as accurately as possible at all times during the treatment without the need to take additional images. Accurately knowing the position of the target region allows the radiation to be focused where it is needed, ideally within <NUM> of the target region, thereby minimising the radiation dose received by the healthy tissue surrounding the target region.

Specific embodiments are now described, by way of example only, with reference to the drawings, in which:.

By providing a radiotherapy apparatus for delivering radiation to a subject, with the apparatus comprising a subject support surface configured such that a portion of the subject support surface can be located substantially at the isocenter and a subject support surface rotation mechanism configured to rotate the subject support surface about an axis of rotation that passes through the isocenter, wherein the subject support surface rotation mechanism is located outside the radiation plane, a number of benefits are provided. The apparatus provides means for allowing the dose received by healthy tissue during a radiotherapy treatment to be minimised. By rotating the subject support surface, for example with a patient positioned on it, it is possible to spread the radiation through the healthy tissue while rotating about the isocenter ensures that the maximum amount of radiation still passes through the target region which maximises the efficiency of the treatment and allows the treatment time to be reduced. However, if in order to rotate a couch about the isocenter, the rotation mechanism is located within the plane of the radiation, then it is not possible to use the couch and rotation mechanism in radiotherapy device using a bore because the gantry and the rotation mechanism would obstruct one another. Locating the subject support surface rotation mechanism outside the radiation plane allows the dose received by healthy tissue of the subject during the radiotherapy treatment to be minimised for a wide range of radiotherapy apparatuses with different geometries. For example, these benefits can be achieved in radiotherapy apparatuses that comprise a bore for receiving the subject.

By providing a system for positioning a subject in a radiotherapy apparatus, with the system comprising a subject support surface with an extendable table top and one or more sensors that are configured to measure a vertical position of the extendable table top, as well as a processor configured to determine a deflection of the extendable table top using the measured position and control a treatment of the radiotherapy apparatus according to the deflection, a number of benefits are provided. Using this system to determine a deflection profile allows treatments, such as spiral treatments for example, to be performed accurately without the need for re-imaging the patient during treatment. This reduces the amount of imaging radiation that the patient is exposed to which reduces the harm done to a patient. Removing the requirement for re-imaging increases the speed at which a treatment can be performed, thereby increasing patient throughput and improving the efficiency of the radiotherapy apparatus. The system also enables the position of the target region to be known with greater certainty and accuracy, which enables the treatment to be performed with greater accuracy and confidence in treating the target region. This also minimises the radiation received by healthy tissue.

When administering a treatment to a subject or patient <NUM> with a radiotherapy apparatus comprising a source of radiation <NUM> configured to rotate about an isocenter <NUM> and emit radiation in a radiation plane containing said isocentre <NUM>, rotating the subject about the isocenter <NUM> allows the dose received by healthy tissue during the radiotherapy treatment to be minimised. This can be achieved by providing a subject support surface rotation mechanism <NUM> connected to the subject support surface <NUM> and configured to rotate the subject support surface about the isocenter <NUM>. Rotating the subject support surface <NUM> about an axis of rotation that passes through the isocenter <NUM> ensures that the radiation will pass through the same point, regardless of the rotation angle of the subject support surface <NUM>. This is advantageous because, for example, by locating a target region of a patient <NUM> at the isocenter <NUM>, it is possible to ensure that the radiation passes through the target region for all rotation angels of the subject support surface <NUM>. By rotating the subject support surface <NUM> (and therefore a patient <NUM>), it is possible to spread the radiation through the healthy tissue while rotating about the isocenter <NUM> ensures that the maximum amount of radiation still passes through the target region which maximises the efficiency of the treatment and allows the treatment time to be reduced. Locating the subject support surface rotation mechanism <NUM> outside the radiation plane allows the dose received by healthy tissue of the subject <NUM> during the radiotherapy treatment to be minimised for a wide range of radiotherapy apparatuses with different geometries. In particular, radiotherapy apparatuses that comprise a bore for receiving the subject <NUM>. By way of background, in known devices the rotation mechanism is located within the plane of radiation, as shown in <FIG>, which make them unsuitable for radiotherapy apparatuses that comprise a bore.

In accordance with one embodiment, <FIG> depicts a radiotherapy device suitable for delivering a beam of radiation to a patient during radiotherapy treatment. The device and its constituent components will be described generally for the purpose of providing useful accompanying information for the present invention. The device depicted in <FIG> is in accordance with the present disclosure and is suitable for use with the disclosed systems and apparatuses, although not all of the features are necessarily present, or as depicted in <FIG>. While the device in <FIG> is an MR-linac, the implementations of the present disclosure may be any radiotherapy device, for example a linac device. <FIG> shares features common with known devices such as Versa HD™ in particular, the features involved in producing the treatment beam <NUM>. The embodiment shown in <FIG> is modified over known devices in accordance with the invention by the provision of a subject support surface rotation mechanism <NUM>, as will be described in more detail below.

The device depicted in <FIG> is an MR-linac. The device comprises both MR imaging apparatus <NUM> and radiotherapy (RT) apparatus which may comprise a linac device. In operation, the MR scanner produces MR images of the patient <NUM>, which can be used to determine the position of the patient <NUM> on the couch <NUM> and also the position of a target region, such as a tumour, within the patient <NUM> so that a target region's position relative to the couch <NUM> may be determined. The linac device produces and shapes a beam of radiation and directs it toward a target region within a patient's body in accordance with a radiotherapy treatment plan. The usual 'housing' which would cover the MR imaging apparatus <NUM> and RT apparatus in a commercial setting such as a hospital is not depicted in <FIG>.

The MR-linac device depicted in <FIG> comprises a source of radiation <NUM>. The source of radiation <NUM> may comprise beam generation equipment, such as one or more of: a source of radiofrequency waves <NUM>, a circulator <NUM>, a source of electrons <NUM>, a waveguide <NUM>, and a target (not shown). The MR-linac may also comprise a collimator <NUM> such as a multi-leaf collimator configured to collimate and shape the beam, MR imaging apparatus <NUM>, and a patient support surface <NUM>. The device also comprises a housing which, together with the ring-shaped gantry defines a bore. The moveable subject support surface <NUM> can be used to move a patient, or other subject, into the bore when an MR scan and/or when radiotherapy is to commence or during treatment. The MR imaging apparatus <NUM>, RT apparatus, and a subject support surface actuator are communicatively coupled to a controller or processor. The controller is also communicatively coupled to a memory device comprising computer-executable instructions which may be executed by the controller.

The RT apparatus comprises a source of radiation <NUM> and a radiation detector (not shown). Typically, the radiation detector is positioned diametrically opposed to the radiation source <NUM>. The radiation detector is suitable for, and configured to, produce radiation intensity data. In particular, the radiation detector is positioned and configured to detect the intensity of radiation which has passed through the subject. The radiation detector may also be described as radiation detecting means, and may form part of a portal imaging system.

The radiation source <NUM> defines the point at which the treatment beam <NUM> is introduced into the bore. The radiation source <NUM> may comprise a beam generation system, which may comprise a source of RF energy <NUM>, an electron gun <NUM>, and a waveguide <NUM>. The beam generation system is attached to the rotatable gantry <NUM> so as to rotate with the gantry <NUM>. In this way, the radiation source <NUM> is rotatable around the patient <NUM> so that the treatment beam <NUM> can be applied from different angles around the gantry <NUM>. In a preferred implementation, the gantry <NUM> is continuously rotatable. In other words, the gantry <NUM> can be rotated by <NUM> degrees around the patient, and in fact can continue to be rotated past <NUM> degrees. The gantry <NUM> rotates about a mechanical isocenter, which is the point in space about which the gantry <NUM> rotates and about a fixed axis <NUM>. The radiation isocenter can be defined as the point where the radiation beams intersect. These two isocenters <NUM> need not be the same, although they can be. In this disclosure, the term isocenter <NUM> can refer to either or both of these. The isocenter <NUM> is located within the radiation plane. The gantry <NUM> may be ring-shaped. In other words, the gantry <NUM> may be a ring-gantry with a bore. The gantry <NUM> may also not be ring-shaped and may instead be an open gantry such as that shown in <FIG>.

The source <NUM> of radiofrequency waves, such as a magnetron, is configured to produce radiofrequency waves. The source <NUM> of radiofrequency waves is coupled to the waveguide <NUM> via circulator <NUM>, and is configured to pulse radiofrequency waves into the waveguide <NUM>. Radiofrequency waves may pass from the source <NUM> of radiofrequency waves through an RF input window and into an RF input connecting pipe or tube. A source of electrons <NUM>, such as an electron gun, is also coupled to the waveguide <NUM> and is configured to inject electrons into the waveguide <NUM>. In the source of electrons, 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 <NUM> is synchronised with the pumping of the radiofrequency waves into the waveguide <NUM>. The design and operation of the radiofrequency wave source <NUM>, electron source and the waveguide <NUM> is such that the radiofrequency waves accelerate the electrons to very high energies as the electrons propagate through the waveguide <NUM>.

The source of radiation <NUM> is configured to direct a beam <NUM> of therapeutic radiation toward a patient positioned on the patient support surface <NUM>. The source of radiation <NUM> may comprise a heavy metal target toward which the high energy electrons exiting the waveguide are directed. When the electrons strike the target, X-rays are produced in a variety of directions. A primary collimator may block X-rays travelling in certain directions and pass only forward travelling X-rays to produce a treatment beam <NUM>. The X-rays may be filtered and may pass through one or more ion chambers for dose measuring. The beam can be shaped in various ways by beam-shaping apparatus, for example by using a multi-leaf collimator <NUM>, before it passes into the patient as part of radiotherapy treatment.

In some implementations, the source of radiation <NUM> is configured to emit either an X-ray beam or an electron particle beam. Such implementations allow the device to provide electron beam therapy, i.e. a type of external beam therapy where electrons, rather than X-rays, are directed toward the target region. It is possible to 'swap' between a first mode in which X-rays are emitted and a second mode in which electrons are emitted by adjusting the components of the linac. In essence, it is possible to swap between the first and second mode by moving the heavy metal target in or out of the electron beam path and replacing it with a so-called 'electron window'. The electron window is substantially transparent to electrons and allows electrons to exit the flight tube.

The radiotherapy apparatus / device depicted in <FIG> also comprises MR imaging apparatus <NUM>. The MR imaging apparatus <NUM> is configured to obtain images of a subject positioned, i.e. located, on the subject support surface <NUM>. The MR imaging apparatus <NUM> may also be referred to as the MR imager. The MR imaging apparatus <NUM> may be a conventional MR imaging apparatus <NUM> operating in a known manner to obtain MR data, for example MR images. The skilled person will appreciate that such a MR imaging apparatus <NUM> may comprise a primary magnet, one or more gradient coils, one or more receive coils, and an RF pulse applicator. The operation of the MR imaging apparatus is controlled by the controller.

The controller is a computer, processor, or other processing apparatus. The controller may be formed by several discrete processors; for example, the controller may comprise an MR imaging apparatus processor, which controls the MR imaging apparatus <NUM>; an RT apparatus processor, which controls the operation of the RT apparatus; and a subject support surface processor which controls the operation and actuation of the subject support surface. The controller is communicatively coupled to a memory, i.e. a computer readable medium.

The linac device also comprises several other components and systems as will be understood by the skilled person. For example, in order to ensure the linac does not leak radiation, appropriate shielding is also provided.

The patient support surface <NUM> may serve to support an object. The object may be a human body (such as a patient), an animal body or a material sample. The subject support surface <NUM> is configured to move parallel to the longitudinal axis <NUM> between a first position substantially outside the bore, and a second position substantially inside the bore. In the first position, a patient <NUM> or subject can mount the subject support surface <NUM>. The subject support surface <NUM>, and patient <NUM>, can then be extended inside the bore, to the second position, in order for the patient <NUM> to be imaged by the MR imaging apparatus <NUM> and/or imaged or treated using the RT apparatus. The movement of the subject support surface <NUM> is effected and controlled by a subject support surface actuator, which may be described as an actuation mechanism. The actuation mechanism is configured to move the subject support surface <NUM> in a direction parallel to, and defined by, the longitudinal axis of the subject support surface <NUM>. The terms subject and patient are used interchangeably herein such that the subject support surface <NUM> can also be described as a patient support surface <NUM>. The subject support surface <NUM> may also be referred to as a moveable or adjustable couch or table.

The present invention is distinguished over known devices as follows. The subject support surface <NUM> is connected to a subject support surface rotation mechanism <NUM>. The rotation mechanism <NUM> can be attached to the floor as shown or, for example, can be attached to the device housing or gantry <NUM> (as shown in, for example, <FIG>). The rotation mechanism <NUM> is configured to rotate the patient support surface <NUM> with the axis of rotation of the patient support surface <NUM> passing through the isocentre <NUM> of the gantry <NUM>. The patient support surface <NUM> or part thereof can be rotated around (or about) the longitudinal axis <NUM> (roll), around the transverse axis <NUM> (pitch), or about an axis perpendicular to the floor <NUM> (yaw), or any combination of these.

Although in <FIG> the plane of the rotation of the patient support surface <NUM> is illustrated as being parallel to the illustrated floor (as is defined by the xy plane, which corresponds to the plane of the patient support surface <NUM> in its neutral position where x is the longitudinal axis <NUM> and y is the transverse axis <NUM>), with rotation as yaw about the axis <NUM>, by way of example, the angle of the plane of rotation relative to the floor could be at an angle of <NUM>, <NUM>, <NUM> or <NUM> degrees to the floor. However, for reasons of patient comfort, the angle will usually be kept fairly low. It is also possible for the tilt to be changed either prior to, or during, treatment. The subject support surface rotation mechanism is configured to rotate the subject support surface +/- <NUM>-<NUM> degrees about the subject support surface axis of rotation, more preferably <NUM>-<NUM> degrees, most preferably <NUM> degrees. The rotation mechanism <NUM> and/or the patient support surface <NUM> may also be connected to an additional rotation mechanism (not shown) configured to rotate the rotation mechanism <NUM> and/or the patient support surface <NUM> in a different plane, wherein the axis of rotation also passes through the isocenter <NUM>. In this way, the patient support surface <NUM> may be connected to more than one rotation mechanism, each configured to move the patient support surface <NUM> in a different plane. Alternatively, a single rotation mechanism <NUM> may be configured to rotate the patient support surface <NUM> in more than one plane with the axis of rotation of each of the rotation planes of the patient support surface <NUM> passing through the isocenter <NUM>. The primary consideration is that the centre of rotation (about whichever axis) is located at the isocenter <NUM>, or close to the isocenter <NUM>. As a result, the treatment beam can be consistently focussed on the area requiring treatment.

By rotating the couch and hence the patient around the isocenter, the radiation dose can be spread through the healthy tissue so that the radiation dose received by healthy tissue surrounding a target region is minimised. This improves patient <NUM> wellbeing. If the rotation of the couch <NUM> was not about the isocenter <NUM>, then the location of the target region would move with respect to the isocenter <NUM> (and focus of the radiation) and, accordingly, this would result in an increased dosage of radiation being received by healthy tissue. Furthermore, this would result in a longer treatment time because the target region would not receive the intended dosage of radiation.

The disclosure provides rotation means for rotating a patient support surface around an isocenter <NUM>, whilst locating the patient support surface rotation mechanisms <NUM> outside the radiation plane. This is particularly useful for ring gantry/bore solutions or devices with <NUM>° rotation of the gantry <NUM>, for which it is problematic to position the rotation mechanism <NUM> within the radiation plane without interfering with the gantry <NUM>. However, this disclosure is applicable to any radiotherapy device. Whilst the disclosure is not limited to bore solutions (ring gantries), bore solutions offer improved device stability. Furthermore, bore solutions are less imposing or alarming for patients. Bore solutions therefore may be desirable. The disclosure provides means to supply non-coplanar treatments (in which both gantry <NUM> and patient support surface <NUM> are rotated) in a radiotherapy device with a bore solution.

The disclosure provides rotation means which are located outside of the plane of the gantry <NUM> and therefore the plane of radiation, or isoline. Positioning the rotation means <NUM> outside the plane of radiation minimises radiation interference.

Examples of specific linkages and structures for rotating a subject support surface about an axis of rotation that passes through the isocenter, wherein the subject support surface rotation mechanism is located outside the radiation plane, will now be described.

One embodiment is shown from three different perspectives in <FIG> and <FIG>. These figures show a patient support surface <NUM> (which may also be described as a couch or patient positioning system) supported by and connected to a rotation mechanism <NUM>. The couch <NUM> is connected directly to the rotation mechanism <NUM> or via an intermediary and can be connected by any suitable means, for example, mechanically. The couch <NUM> may include a number of rollers, a table top <NUM>, a table base, or other parts. In these figures, the rotation mechanism <NUM> is connected to the gantry <NUM> but it could be connected to a floor, a wall or other support structure instead or as well. The rotation mechanism <NUM> shown here makes use of two curved guides or rails <NUM>, with the centre of curvature for both curved guides, which may be curved guide rails <NUM> being located substantially at the isocenter <NUM>. For example, the center of curvature (and the center of rotation of the patient support surface <NUM>) could be within <NUM> to <NUM>, more preferably <NUM>, <NUM> to <NUM>, more preferably <NUM>, <NUM> to <NUM>, more preferably <NUM>, <NUM> to <NUM>, more preferably <NUM>, <NUM> to <NUM>, more preferably <NUM>, <NUM> to <NUM>, more preferably <NUM>, <NUM> to <NUM>, more preferably <NUM>, or another distance of the isocenter <NUM>. Ideally, the centre of curvature of each curved rails <NUM> and the center of rotation of the couch <NUM> will be as close to the isocenter <NUM> as possible. There could be one curved rail <NUM> or any larger number. In one example in which there are two curved rails <NUM>, both of the curved rails <NUM> have the same radius. In another example, the two curved rails <NUM> have different radii but, the centre of curvature for both of the curved rails <NUM> is still the same.

The rotation mechanism <NUM> itself can be moved up and down in any direction, such as vertically as shown in <FIG>. The patient support surface <NUM> can move in any direction. Alternatively, or as well, the patient support surface <NUM> may comprise a table top <NUM> which can move independently from the rest of the patient support surface <NUM>, such as a table base, and in any direction, for example, a longitudinal direction (along a longitudinal axis <NUM> of the patient support surface <NUM> ), a lateral direction (along a transverse axis <NUM> of the patient support surface <NUM> ), a vertical direction (along a vertical axis <NUM> that is an axis perpendicular to the floor), or a direction oblique to any of these directions. In some embodiments, the longitudinal direction <NUM> may be described as Y direction <NUM>. The lateral or transverse direction <NUM> may be described as the X direction <NUM>. The vertical direction <NUM> may be described as the Z direction <NUM>. The rotational, vertical or other movements can be driven manually or by, for example, one or more motors.

In the example illustrated in <FIG> and <FIG>, the plane of rotation of the couch <NUM> is shown as being parallel to the floor. This may commonly be the case but it is not limited to this. The curved rails <NUM> themselves may be fixed at an incline to the floor or the tilt may actually be altered before, during or after the rotation of the couch <NUM>. Furthermore, the couch <NUM> may comprise a table top <NUM> which is itself configured to rotate, for example about the axis of the bore or the longitudinal axis <NUM>. This also serves to minimise the radiation dose received by healthy tissue surrounding a target region.

The rotation of the patient support system <NUM> can occur before, during or after treatment. Rotation can be continuous or discrete/static. Rotation of the couch <NUM> may also occur with the table top <NUM> extended or not extended. Rotation of the couch <NUM> can also occur at the same time as table top <NUM> is being extended. In one example, a patient <NUM> lies on the couch <NUM> in its non-extended position. The couch <NUM> is then extended, the patient is scanned and exposed to radiation. The radiation is then stopped, the couch <NUM> is rotated (yawed) manually by sliding the couch <NUM> along the curved rail <NUM> and the patient is then exposed to further radiation. In another example, the radiation is not stopped and the rotation of the couch <NUM> happens automatically and at the same time as the patient is exposed to radiation.

This rotation may be controller by a processor which may be comprised in the patient support surface <NUM> or may be found elsewhere. For example, the processor can control the speed of rotation, the angle of rotation or the amount of rotation. This processor may also be used to control the radiation emission, radiotherapy treatment or other operation of the radiotherapy device. This can allow the rotation of the couch <NUM> to be synchronized with the operation of the radiotherapy device or delivery of the radiotherapy treatment.

In a bore solution, such as that shown in <FIG>, the rotation of the couch <NUM> may be inhibited at some angles by the gantry <NUM> or gantry cover. For example, the couch <NUM> may be rotated by +/- <NUM>° from the neutral position. The neutral position is when the couch <NUM> is aligned with the axis of the bore and parallel to the floor. When the patient support system <NUM> is fully extended into the bore, there may be less rotation possible compared to when the patient support system <NUM> is not extended, or only partially extended, into the bore. As a result, this system is particularly well suited to treatments for head and neck.

The one or more curved rails <NUM> may be made from the same materials or different materials. For example, each curved rail <NUM> could be made from a metal, for example steel. The curved rails <NUM> can be fixed to the floor or another support surface. The rails comprise a track and slider. The slider can be attached to the table top or on the frame. The slide position can be controlled by a linear motor, timing belt or direct drive. A direct drive is a separate cog track or an integrated cog track onto the rail. To help prevent the sliders crabbing between the rails, some flexures can be used to compensate tolerances.

Alternatively, the rotation mechanism <NUM> may not in fact comprise a curved rail <NUM> but may comprise one or more curved trenches, with the center of curvature of the one or more curved trenches substantially located at the isocenter <NUM>, wherein the one or more curved trenches serves to guide the rotation of the patient support surface <NUM> about the isocenter <NUM>. Alternatively, the rotation mechanism <NUM> may comprise one or more curved rails <NUM> and one or more curved trenches, with the center of curvature of both substantially located at the isocenter <NUM>. Where the centre of curvature is referred to as being substantially located at the isocenter <NUM>, this includes any point that falls substantially along a vertical axis passing through the isocenter <NUM>, as well as the isocenter <NUM> itself. Accordingly, it will be apparent that the particular means used to guide the rotation can be varied and the important concept is that the centre of curvature of the rotation guide is located substantially at the isocenter <NUM>.

The radiation source or gantry <NUM> itself may also be partially rotated about the transverse axis of the short end of the patient support surface <NUM> in its neutral position, although not necessary when the patient support surface <NUM> is in its neutral position, either at the same time, or a different time, synchronously or separately to the patient support surface <NUM>.

By using a rotation mechanism <NUM> comprising curved rails as described, it is possible to cause pure isocenter rotation of the patient support system <NUM> without the rotation mechanism <NUM> sharing a common mechanical axis with the gantry <NUM>. In other words, isocenter rotation and the benefits that come with that are achieved whilst keeping the rotation mechanism <NUM> outside the radiation unit, thereby not interfering with gantry rotation or interfering with the delivery of the radiation. Accordingly, the present disclosure allows the dose received by healthy tissue during radiotherapy treatment to be minimised.

Another example is shown from three different perspectives in <FIG>, <FIG> and <FIG>. These figures show a patient support surface <NUM> supported by and connected to a rotation mechanism <NUM>. The couch <NUM> is connected directly to the rotation mechanism <NUM> or via an intermediary and can be connected by any suitable means, for example, mechanically. The couch <NUM> may include a number of rollers, a table top <NUM>, a table base, or other parts.

In these figures, which show an example useful for understanding the invention, the rotation mechanism <NUM> is connected to the floor and, in particular, within a pit <NUM> that forms part of the floor. It could be connected to a gantry <NUM>, a wall, or other support structure instead or as well. The rotation mechanism <NUM> is shown as being partly comprised within the pit <NUM>, but may be formed completely inside the pit <NUM>. The rotation mechanism shown in <FIG>, <FIG> and <FIG> is similar to the rotation mechanism shown in <FIG> and <FIG>, as discussed above. For example, the rotation mechanism <NUM> makes use of two curved rails <NUM>, with the centre of curvature for both curved rails <NUM> being located substantially at the isocenter <NUM>. However, in this example, the curved rails <NUM> are stacked on top of each other, which is to say that they are parallel with one another but spaced apart from one another in a vertical direction, for example along a vertical axis. The vertical axis <NUM> is the axis of rotation. The rotation mechanism <NUM> is shown as being comprised within a box <NUM>.

In this example, the couch <NUM> is connected to the curved rails <NUM> by an arm <NUM>. The arm <NUM> may be connected to the curved rails <NUM> using any appropriate means. For example, the arm <NUM> may comprise first and second slots for the first and second curved rails <NUM> to engage with. The first and second slots may be straight or may be curved with a radius of curvature designed to match the radius of curvature of the curved rails <NUM>. In this example, the arm <NUM> also acts as the base for the couch <NUM> but the arm <NUM> may be separate from the base of the couch <NUM>. In one example, instead of using curved rails <NUM>, curved trenches are used. Any other appropriate curved guide may also be used instead of the curved rails <NUM> referred to in this disclosure. In another example, a curved trench is used in conjunction with a curved rail <NUM>, both of which have a centre of curvature that is the same and that is located at the isocenter <NUM> (or a point along a vertical axis passing through the isocenter <NUM>). In another example, the curved rail <NUM> has a different radius to the curved slot but, the centre of curvature for both of the curved rail <NUM> and the curved slot is still the same.

By separating curved rails <NUM> (or a curved rail <NUM> and a curved trench) vertically, the rotation mechanism <NUM> can be kept compact, thereby saving horizontal space.

As described above, the patient support surface <NUM> may comprise an extendable table top <NUM> which can move independently from the rest of the patient support surface <NUM>, such as a table base, and in any direction, for example, a longitudinal direction (along a longitudinal axis <NUM> of the patient support surface <NUM>). This can be extended from a first position, for example, a position outside the plane of the gantry <NUM> (as shown in Fig. 9A) to a second position, for example, a position that results in a portion of the couch <NUM> or table top <NUM> being inside the plane of the gantry <NUM> (as shown in <FIG>). This serves multiple purposes which include enabling a patient <NUM> to easily climb onto the couch <NUM> when it is in a first position, before positioning the patient <NUM> so as to receive the treatment beam <NUM> in the second position. This extension can also be done to compensate for the movement of the couch <NUM>. This extension can also be performed as part of a spiral treatment, as described in the background section.

As illustrated in <FIG>, when the table top <NUM> is in the second (extended) position, the weight of the patient <NUM>, the table top <NUM> itself or both of these weights, cause the table top <NUM> to flex (also interchangeably referred to herein as bend or deflect). The amount of flex depicted in <FIG> is exaggerated for illustrative purposes. An embodiment of the invention provides a system that allows this table top <NUM> bending to be compensated for in such a way as to enable the radiation to be more accurately focused on the position of the target region, as shall be explained by reference to the structure and operation of the system below.

<FIG> and <FIG> show a system for positioning a subject, such as a patient <NUM>, in a radiotherapy apparatus. The radiotherapy apparatus is similar to that described above in relation to <FIG>, however, the subject support surface <NUM> also comprises one or more sensors <NUM> configured to measure a vertical position of the table top <NUM>. In one embodiment, as depicted in <FIG> and <FIG>, the subject support surface <NUM> is connected to the gantry <NUM> and also comprises the rotation mechanism <NUM> (in this case one or more curved rails <NUM>) within the subject support surface <NUM>, although it is not necessary for it to comprise any rotation mechanism <NUM>. In this way and in relation to these Figs. , the subject support surface <NUM> refers to everything, including a rotation mechanism <NUM> (if present), that supports and positions the table top <NUM> in such a way as to position a subject <NUM> in such a way as to receive the treatment beam <NUM>. The subject support surface <NUM> may not be connected to the gantry <NUM> and be connected to a support surface such as a floor instead. As described previously, the table top <NUM> can be extended along the longitudinal axis <NUM> using one or more motors, which can be electric motors with absolute encoders or other encoders, although any other suitable drive mechanism can be used instead of one or more of the one or more motors. The table top <NUM> itself is supported in exactly the same way in the outer (first, non-extended) position and the inner (second, extended) position, so that the absolute table top <NUM> flex will be the same over the full stroke (the full range of the extension of the table top <NUM>).

<FIG> shows a magnified view of the area comprising the sensor <NUM>. The sensor <NUM> is located close to the entry of the bore of the gantry <NUM>. Although the embodiment depicted in <FIG>, <FIG>, <FIG> and <FIG> is of a radiotherapy apparatus with a bore, the system can also not have a bore and can instead have an open gantry.

The sensor <NUM> is communicatively coupled to a processor and is configured to send data to the processor either directly or indirectly. In one example, the sensor <NUM> comprises a linear variable differential transformer (LVDT) that is configured to convert mechanical motion into an electrical current. LVDT sensors are a known technology, the mode of operation of which will not be described here in great detail. However, physically, the LVDT construction is a hollow metallic cylinder in which a shaft of a smaller diameter moves freely along the cylinder's long axis. The sensor <NUM> also comprises a pressure wheel that contacts the underside of the table top <NUM> in the first position, in the second position, and at all times in between these positions. As the table top <NUM> is extended it flexes, as already described above. This results in the pressure wheel being compressed, causing the shaft of the LVDT with the smaller diameter to move inside the larger cylinder which in turn causes an electrical current that corresponds to the displacement of one cylinder relative to the other. In this way, the sensor <NUM> is used to measure a deflection of the table top <NUM>. Other appropriate sensors can also be used. In particular, other sensors that are known for providing accurate and easy measurements. For example, an alternative sensor could be a laser triangular measurement device of a type well known to the skilled person, so long as it is radiation hard due to the sensor's proximity to the beam in the scatter area. Alternatively, the sensor could comprise one or more ultrasonic sensors. The compression of the sensor <NUM> is related to a position of the table top <NUM> which in turn is related to a deflection or bend of the table top <NUM>. Any of these values or an electrical signal that can be used to calculate any of these values, is then communicated to the processor so that the processor can determine the deflection of the table top <NUM>.

The sensor <NUM> is comprised in the subject support surface <NUM> so as to only measure table top <NUM> flex. The sensor <NUM> is located outside the imaging/radiation volume and is attached to the couch <NUM> so that, when moving the patient <NUM> together with the table top <NUM> into the bore the structural flex of the rest of the couch <NUM>, is not taken into account. In this way, only the table top <NUM> flex is measured. Whilst only one sensor <NUM> is depicted, it should be understood that more than one sensor <NUM> can be used, for example, to provide redundancy. The same lateral position is measured to avoid any variation in measurements caused by lateral motion and/or unsmooth underside of the table top <NUM>.

When the table top <NUM> is in the second longitudinal position (which is an extended position), there is less bending measured compared to in the first longitudinal position (which is a non-extended position). The amount of bending will be increased at both longitudinal positions when the table top <NUM> is loaded, i.e. a patient <NUM> is positioned on the table top <NUM>. In the loaded state, the weight of the patient increases the bending moment on the table top <NUM>. The table top <NUM> is effectively a cantilever, supported at two points towards one end of the table top. The bending moment will be zero at the free end and it will be maximum towards the supported end. By measuring a deflection of the table top <NUM> in its first position and also in its second position, the relative deflection, change in deflection and/or increase in deflection can be determined. It should be understood that the deflection is a value that is equivalent to a relative position and is calculated based on a change in the position of the table top <NUM>, in the vertical <NUM> direction, at the location of sensor <NUM> along the longitudinal axis <NUM> between an unloaded reference state and a loaded state and/or between a first position and a second position or, as will be explained below. Furthermore, whilst for simplicity the deflection is discussed here in relation to a first position and a second position, the position of the table top <NUM> can be measured at more than two positions. For example, the position of the table top <NUM> can be measured at <NUM>, <NUM>, <NUM>, <NUM> or some other number of positions along the extension of the table top <NUM> along the longitudinal axis <NUM>. As another example, the position of the table top <NUM> can be measured at different levels of extension along the longitudinal axis <NUM>, for example, every <NUM>, <NUM>, <NUM> or other interval. The position of the table top <NUM> in the vertical direction, relative to the first position (which is also referred to as the deflection) can therefore be determined for a number of different positions, each corresponding to a particular extension of the table top <NUM> on a scale from no extension to full or maximum extension, which may refer to the maximum extension used for a particular treatment rather than to a maximum possible extension. In one example, the measurement zone is the full treatment zone and the deflection is measured in at least three different places to determine the tilt angle of the table top <NUM>. When the treatment zone is longer, more measurements can be taken because the tilt angle will vary.

In operation, a patient <NUM> climbs on to the subject support system <NUM> when the table top <NUM> is in its non-extended position. The table top <NUM>, with the patient <NUM> on it, is then extended into the bore and the table top <NUM> deflection is then measured at several positions during the transport of the table top <NUM> into the bore. The amount of deflection (or the vertical position of the table top <NUM> at the location of the sensor <NUM>) is measured by the sensor <NUM> at each position and is saved in a memory associated with the processor. In this way, a deflection profile can be generated and recorded by the processor. For example, the profile may resemble a portion of a negative exponential curve or some other shape, as illustrated in <FIG>.

The subject <NUM> is then imaged to locate the position of the target region, so that the treatment beam <NUM> can be focused on this area. The subject support surface <NUM> can also be controlled and extended in the vertical <NUM> or lateral <NUM> direction so that the target region is located at or close to the radiation isocenter <NUM> or other desired location.

A spiral treatment is then performed in which the table top <NUM> is retracted (the opposite of the earlier extension) whilst the treatment beam <NUM> is administered. As the table top <NUM> is retracted, the deflection of the table top <NUM> will reduce. The deflection profile that has been determined by the processor is used to predict the changing vertical position of the location of the target region, according to the current level of extension (or amount of retraction). Knowing that the target region is not moving (retracting) perfectly parallel to the axis of the bore <NUM> but is instead following a particular (e.g. banana shaped) profile relative to the axis of the bore allows the treatment to be adjusted accordingly.

For example, the treatment can be offset in such a way that the radiation isocenter <NUM> can be kept precisely within the target region over the full distance that the table top <NUM> moves along the longitudinal axis <NUM> during the treatment. In one example, the vertical position of the table top <NUM> can be raised or lowered along the vertical axis <NUM> to coordinate the absolute vertical position of the table top <NUM> with the deflection profile so as to maintain the target region substantially at the isocenter and optimise the treatment by reducing the amount of healthy tissue exposed to harmful radiation. This vertical movement is actuated and controlled by the subject support surface <NUM> which, as described above, is configured to extend the table top <NUM> along the vertical axis <NUM>. In other words, the treatment can be correlated to, and using, the predicted deflection profile during retraction of the table top <NUM>, which is the reverse of the deflection profile determined during extension of the table top <NUM>. The processor that determines the deflection profile can be used to synchronise the treatment according to the deflection profile or it can supply the information required to do so to another processor that is used to control the treatment. In this way, it is possible to perform an accurate spiral treatment whilst only taking initial images, rather than during the treatment. This reduces the harmful effects of the imaging, whilst also increasing the speed of the treatment.

In one example, a lung spiral treatment is performed using the disclosed apparatus. This treatment is performed by scanning a treatment beam <NUM> over the target region, starting from towards the end of the patient <NUM> that comprises the patient's head and moving down the patient's body for a distance of <NUM>. The scanning is achieved by physically moving the patient <NUM>, on the table top <NUM>, through the treatment beam <NUM>, as shall now be explained. In one example, the treatment beam is also moved. For example, a tilted beam can be used.

To get the patient <NUM> into position, the patient <NUM> first climbs onto the table top <NUM> in a first, non-extended longitudinal position. The table top <NUM> is connected to the couch <NUM> by two connection points, also referred to as the table top attachment points <NUM> (although this number may be greater or smaller). These two table top attachment points <NUM> are fixed on the table top <NUM> so that the table top <NUM> is attached by the same two points <NUM> in both the first position and a second position. However, the connections <NUM> to the subject support surface <NUM> are moveable from a first longitudinal position to a second longitudinal position relative to the subject support surface <NUM>. This longitudinal movement can be achieved using any appropriate means. For example, the table top <NUM> may be connected to the couch <NUM> in such a way as to be configured to move along it in a longitudinal direction along a set of rollers, sliders, or along a rail. The movement along the longitudinal axis <NUM> from a first position to a second position can be driven manually but can also be driven by, for example, one or more electric motors or by any other suitable means.

The weight of the patient <NUM> on the table top <NUM>, particularly if the patient <NUM> is large, will cause the table top <NUM> to flex by an amount, for example, <NUM>. When the table top <NUM> has a subject <NUM> positioned on it, it is to be considered as in a loaded state. The amount of flex will vary along the length of the table top <NUM>, with the flex being greater further along the table top <NUM> away from the table top attachment points <NUM>. However, once the patient <NUM> is on the table top <NUM>, if the patient does not move relative to the table top <NUM>, the absolute amount of flex of the table top <NUM> will not change over the course of the treatment or with the extension of the table top <NUM>. The table top <NUM> may comprise additional means to prevent the patient <NUM> from moving on it during the treatment or transport into the bore, for example, straps, blocks, braces or other suitable means. In other words, the flex of the table top <NUM> as a whole does not change as the table top <NUM> is extended, but only the amount of flex measured relative to a particular longitudinal point (i.e. relative to the sensor <NUM>).

Before the patient <NUM> is positioned on the table top <NUM>, the sensor <NUM> is used to provide a first reference value or signal. This value or signal and the subsequent values or signal generated by the sensor <NUM> can be considered to be representative of the height of the table top <NUM> at the position of the sensor <NUM> in an unloaded state. As the patient <NUM> is positioned on the table top <NUM> in its first (non-extended) position (i.e. the loaded state), the table top <NUM> flexes and the sensor <NUM> is compressed, resulting in an electric signal or a change in electrical signal that is communicated to the processor so that the processor can determine a new height of the table top <NUM> and therefore, the amount of deflection caused by the particular patient <NUM> in the first position along the table top <NUM>. For example, the sensor <NUM> may determine that there has been a change in height of the table top <NUM> (in other words a deflection) of <NUM> when comparing the height of the table top <NUM> with the patient <NUM> on it in its first position, to the height of the table top <NUM> without the patient <NUM> on it in its first position.

In this example, the table top <NUM> is then extended along the longitudinal axis <NUM> into the bore so as to position the inferior end of the target region beyond the location that will be the centre of radiation during treatment. At this point, the location of the target region may be known approximately due to previous diagnostics. The approximate position target region may be physically marked on the patient <NUM> by use of a pen or a tattoo. This can be used to assist the patient <NUM> being moved to approximately the correct location for receipt of the start of the treatment, for example, manually by an operator. This can be assisted by the use of laser positioning. However, the position of the actual target region can move inside the patient <NUM> relative to the marked position on the outside of the patient <NUM> due to swelling or for other reasons. In order to accurately know the position of the target region at the time or nearer to the time of treatment and so that the patient can be precisely positioned and thereby reduce any unnecessary exposure of healthy tissue to the treatment beam, the patient <NUM> is scanned using an MR imaging apparatus <NUM> to obtain initial CBCT images. This allows the precise location of the target region to be determined so that the couch <NUM> can then be adjusted further, for example by extension in a vertical <NUM>, lateral <NUM> or longitudinal <NUM> direction so as to precisely position the patient <NUM> and particularly the target region, relative to the radiation isocenter or isocenter <NUM>. For example, the target region is positioned within <NUM>, <NUM> or <NUM> of the desired location. In one example, it is desirable to have the target region within <NUM> of the intended position. The table top extension in this position will be referred to as the second (extended) position, although the second position may also be at a greater extension than that suitable for the start of the treatment. Whilst the table top <NUM> is in the second position, the sensor <NUM> will provide a second height measurement for the table top <NUM> which, when compared with the reference value, enables a deflection of the table top <NUM> in the second position, at the location of the sensor <NUM>, to be determined. This second deflection can be compared by the processor to the first deflection to determine the change in flex between the first longitudinal position and the second longitudinal position. Because in the second longitudinal position the sensor <NUM> is measuring the height of the table top <NUM> at the end of the table top <NUM> that is closer to the table top attachment points <NUM>, the amount of flex determined for the second longitudinal position is expected to be less than for the first longitudinal position.

Between the first position and the second position, the flex will vary by an unknown amount that will depend on how close the position is to the attachment points <NUM> of the table top <NUM> and cannot easily be calculated without knowing the centre of mass of the subject <NUM>, which is something that will vary between subjects <NUM> and is hard to determine. As a result, the sensor <NUM> is used to measure the height of the table top <NUM> at one or more positions (which correspond to different levels of extension of the table top <NUM> along the longitudinal axis <NUM>) between the first position and the second position. The first position is often the non-extended position and the second position is often the maximally extended position (for that treatment). In this example, the sensor <NUM> measures the height of the table top <NUM> for the first <NUM> of extension, every <NUM>. The sensor <NUM> data is then used to determine the amount of deflection every <NUM> and, in this way, a deflection profile is generated by the processor. This deflection profile can be generated during or after transport of the table top <NUM> into the bore (or patient <NUM> loading).

Having accurately positioned the patient <NUM> using the CBCT image and the couch <NUM> and table top <NUM>, the treatment is begun. The treatment beam <NUM> is turned on, as described previously. Either continuously as the treatment beam is administered, or in one or more intervals between the administering of the treatment beam, the table top <NUM> is retracted by a distance that corresponds to the desired length of the target region that is receiving the treatment beam, in this case, <NUM>. The deflection profile, saved in the memory associated with the processor, can be used to predict the amount of deflection of the table top <NUM> and therefore the change in the vertical position of the target region, at any particular time/position of the retraction. For example, if the deflection is <NUM> the vertical movement will raise the couch <NUM> or the table top <NUM> by <NUM> to compensate for the deflection.

This allows the treatment to be optimised by compensating for the changing height of the target region as the table top <NUM> is moved from the second longitudinal position back to the first longitudinal position (or some other position therebetween corresponding to the end of the target region and also referred to here as a third longitudinal position). The treatment can be adjusted by the processor that determines the deflection profile, or the deflection profile can be communicated to a separate processor that is configured to adjust the vertical height according to the deflection profile.

Once the table top <NUM> has reached the third longitudinal position, the treatment is stopped by turning the beam completely off. The table top <NUM> is then fully retracted to the first longitudinal position to enable the patient <NUM> to easily dismount from the couch <NUM>.

In one example, the processor is also configured to take deflection measurements whilst the table top <NUM> is being retracted. These measurements can be compared to those taken during the extension, to see if the deflection has varied, which could be indicative of a patient <NUM> having moved. By comparing the deflection at a particular point during retraction of the table top <NUM> to the corresponding measurement taken at the same point during the extension of the table top <NUM>, a change in deflection value can be calculated. In one example, the processor is configured to stop the radiation if the change in deflection value is greater than a change in deflection safety threshold value. In one example, after the radiation has been stopped in response to the change in deflection value being greater than the change in deflection safety threshold value, the processor is configured to instruct a CBCT scan to accurately check the patient <NUM> position and, for example, recalibrate the position of the target region accordingly. In one other example, in response to the change in deflection value being greater than the change in deflection safety threshold value, the processor is configured to alter the treatment in such a way as to compensate for the change in deflection caused by, for example, the movement of a patient <NUM>.

The sensor <NUM> that is measuring table top <NUM> position and flex needs to be fast and accurate. For example, the sensor can have an accuracy of around <NUM> or better to help ensure the tolerance is a reasonable level. As described above, an LVDT sensor can be used in which a magnet is moved inside a coil. LVDT don't have electronics near the sensor and are simple. As described above, the LVDT, for example an induSENSOR LVDT, may comprise a pressure wheel or a standard roller that is built into the LVDT. Alternatively, a triangulation sensor or a proximity sensor can be used. Any other appropriate type of sensor could also be used as the sensor <NUM>, as long as it is capable of accurately determining the position or deflection of the table top <NUM>. More than one sensor <NUM> can also be used to provide redundancy, to increase the reliability of the measurements and the deflection profile, or for any other reason. In one example, multiple sensors <NUM> are located along the lateral axis <NUM> of the couch <NUM> and are configured to determine a deflection of the table top <NUM> in the lateral direction. If multiple sensors <NUM> are used, the sensors <NUM> can be the same kind or could be different from each other. In one example, the one or more sensors <NUM> are chosen to be radiation hard to prevent damage from scattered radiation from the linac and CBCT over time.

The table top <NUM> is made from one or more rigid materials such as steel, aluminium, titanium, a composite or any other suitable material. The table top <NUM> may also comprise a softer material such as a foam, designed to improve the comfort or support of the patient <NUM>. The table top <NUM> may also comprise multiple layers, one of which includes a plastic. In one example, these materials are chosen to be radiation hard to prevent them from becoming damaged or brittle following repeated exposure to emitted radiation.

Whilst the materials chosen for the table top <NUM> will be chosen to provide an adequate degree of rigidity, the system disclosed allows the materials use to be less rigid than would otherwise be required in a radiotherapy apparatus without table top <NUM> bending compensation. This is because the bending caused by a choice of less rigid materials can nonetheless be compensated for by adjusting the radiotherapy or other treatment, as described above. This results in a wider range of materials being suitable for use in the table top <NUM>. For example, materials that are less rigid but cause less interference to the radiation can be used. This in turn reduces the amount of radiation that has to be generated, thereby saving power and minimising the damage done to any healthy tissue exposed to the radiation. Furthermore, rigid materials are often expensive and reducing the requirements for rigidity enables cheaper or more commonly available materials to be used for the table top <NUM>.

It is possible to mount more sensors to also measure the flex of the main body of the subject support surface <NUM>, but this flex will be less great than the table top flex. The structural flex will depend on the couch <NUM>/table top <NUM> position in the longitudinal direction <NUM>. There will be a flex in the main structure of the couch <NUM> that is related to the rigidity of the materials used for its construction, which is why rigid materials are desirable. However, the majority of the flex still comes from the deflection of the table top <NUM>. In one example, markers are added on to the table top <NUM> and this is then measured with cameras. For example, the camera or cameras could be placed on the floor and look upwards to the underside of the table top <NUM>, or could look from the side to determine the deflection. The camera need to process high resolution images to get the accuracy of the measurements and the determination of the deflection to within a range of <NUM>. In another example, a C-Rad scanner system is used to measure the deflection.

The processor may also be configured to use data from a memory that stores information such as the dimensions and configuration of the components so that these can be used in the calculations controlling the movement of the assorted components and to prevent, for example, the couch <NUM> from colliding with the gantry <NUM>.

It should be noted that the various embodiments may be implemented in hardware, software or a combination thereof. The various embodiments and/or components, for example, components and controllers for these, also may be implemented as part of one or more computers or processors or field-programmable gate arrays (FPGAs). The computer or processor or FPGA may include a computing device, an input device, a display unit and an interface, for example, for accessing the Internet. The computer or processor may include a microprocessor. The microprocessor may be connected to a communication bus. The computer or processor or FPGA may also include a memory. The memory may include Random Access Memory (RAM) and Read Only Memory (ROM). The computer or processor or FPGA further may include a storage device, which may be a hard disk drive or a removable storage drive such as an optical disk drive, and the like. The storage device may also be other similar means for loading computer programs or other instructions into the computer or processor.

Claim 1:
A radiotherapy apparatus for delivering radiation to a subject, the apparatus comprising:
a source of radiation configured to rotate about an isocenter (<NUM>) and emit radiation in a radiation plane containing said isocenter;
a subject support surface (<NUM>) configured such that a portion of the subject support surface can be located substantially at the isocenter; and
a subject support surface rotation mechanism (<NUM>) configured to rotate the subject support surface about an axis of rotation that passes through the isocenter, wherein the subject support surface rotation mechanism is located outside the radiation plane; and
wherein the subject support surface rotation mechanism comprises a first curved guide (<NUM>)
with a center of curvature located at a vertical axis that passes through the isocenter; and
wherein the radiotherapy apparatus comprises a gantry,
wherein the first guide is connected to the gantry.