Patent Publication Number: US-2021162241-A1

Title: Improvements to linear accelerators

Description:
FIELD OF THE INVENTION 
     The present invention relates to improvements in linear accelerators (“LINAC”). In particular, the present invention relates to modifications to linear accelerators to improve the isocentric accuracy of the linear accelerator. 
     BACKGROUND OF THE INVENTION 
     Radiotherapy refers to a regime of treatments which involve exposing cancer cells to intense X-ray radiation. The desired outcome of these treatments is that cancer cells will be damaged beyond repair (“killed”), and will therefore be unable to further reproduce within the patient&#39;s body. A linear accelerator is the device most commonly used for external beam radiation treatments for patients with cancer. The linear accelerator can be used to treat all parts or organs of the body, and functions by delivering high-energy X-rays to the region of the patient&#39;s tumour. 
     Typically, however, cancerous growths are embedded within healthy tissue. As a result, some of the healthy tissue is also damaged during the treatment of the cancer cells, as the radiation must pass through the healthy tissue in order to reach the cancerous cells or tumour. 
     Depending on the sensitivity of the surrounding tissue, the effect of damage to healthy tissue from the treatment of the cancer ranges from minimal to life-threatening. For example, treatments close to the brain, spinal cord, optic nerve, lung, kidneys, liver, reproductive organs, eyes, prostate gland, bladder and rectum have potentially serious implications if the nearby organs are not adequately protected from exposure to the damaging radiation. In some patients, exposure of healthy tissue to radiation may also lead to the formation of secondary cancers. As a result of the radiation treatment the primary cancer should be killed, however, any damage to surrounding tissues, or formation of secondary cancers is clearly highly undesirable. The patient should be able to be treated successfully, without causing damage to tissues other than the tumour, and without an increased risk of further cancer formation. 
     It is of great importance, therefore, not only to make use of a beam of radiation which can be accurately shaped, but also to provide a mechanically stable platform for the radiation beam in order to provide pinpoint accuracy in three dimensions, up/down, forwards/backwards, and sideways. 
     In a LINAC, all moving and rotating parts are centred about a single point in space. This point in space, relative to the treatment machine and about which various components of the LINAC rotate, is referred to as the “isocentre”. The composite parts of the LINAC are mechanically adjusted during installation, to minimise the volume of the isocentre, which is a result of these mechanical alignments and the positioning of the LINAC as required for patient treatment and any mechanical forces active on the structure of the treatment unit, that may also act to distort the intended positioning. The location and volume of the radiation isocentre plays a critical role in treatment planning as the radiation beam needs to be accurately located with respect to the centre of the target tumour, and any surrounding critical organs at risk. The intention is, of course, to target and kill tumour cells and minimise damage to healthy tissues. A smaller volume isocentre creates greater accuracy of the radiation isocentre, which is paramount to improve patient treatments and results. 
     Since most linear accelerators weigh approximately 7 to 8 tonnes, some distortion of the isocentre occurs due to the weight of equipment in association with the required movements and rotations. The distortion is typically caused by sagging in the arm, plus wear in any associated bearings and means that the accuracy of the radiation isocentre may be decreased, thereby increasing the chance of damage to healthy tissue surrounding the tumour. As equipment ages and is used repeatedly, the likelihood of some sagging is increased, and thereby the likelihood of some inaccuracy in the radiation isocentre increases over time. 
     Specifically, distortion of the isocentre can occur in three directions: the X axis, the Y axis and the Z axis (as shown in  FIG. 5  PRIOR ART) of the linear accelerator. As would be understood, the X axis is a horizontal axis that extends perpendicular to the direction from which the arm extends from the linear accelerator, the Y axis is a horizontal axis that extends parallel to the direction from which the arm extends from the linear accelerator, while the Z axis is a vertical axis that extends perpendicular to the direction from which the arm extends from the linear accelerator. 
     In some LINACs, the diameter of the isocentre volume may be greater than 1.5 millimetres due to these unintended distortions, meaning that the effectiveness of providing pinpoint accuracy in the targeting of cancer cells is dramatically reduced. 
     There would, therefore, be a clear and beneficial advantage if it were possible to improve the isocentric accuracy of a LINAC, so as to more accurately target a patient&#39;s tumour, while minimising the damage to the surrounding healthy tissue. The inventor has conducted dedicated research and development over the preceding years, seeking to address the serious problem of isocentre distortion, due to sagging of LINAC support structures. Until now no suitable method to address the problem has been found. The invention is a significant and surprising improvement over the prior art, which is likely to be universally adopted by the industry, both for new equipment and for existing equipment. Through use of the invention the isocentre accuracy is dramatically improved, leading to less damage of healthy tissue and a reduction in the risk of damaged healthy tissues leading to secondary cancer development. All in all the outcome for the patient should be improved through use of the invention, in an easy to use and economic to produce apparatus when considering the significant patient benefit. 
     It will be clearly understood that, if a prior art publication is referred to herein, this reference does not constitute an admission that the publication forms part of the common general knowledge in the art in Australia or in any other country. In general terms, the present invention is directed to improvements to linear accelerators, which may at least partially overcome at least one of the abovementioned disadvantages or provide the consumer with a useful or commercial choice. 
     It is an object of the present invention to provide an improvement to a linear accelerator that at least ameliorates one or more of the aforementioned problems of the prior art. 
     DISCLOSURE OF THE INVENTION 
     The present invention, in one form, resides broadly in a drum assembly for a linear accelerator, the drum assembly comprising a drum having a front face including a front rim and a rear face including a rear rim, one or more support wheels supporting the drum, an arm extending from the front face of the drum and including a beam collimator through which a beam of radiation is emitted to form a radiation isocentre and one or more rear rim members associated with the rear rim, the rear rim members being adapted to substantially offset distortion to the isocentre due to unintended movement of the drum assembly. 
     The present invention, in one form, resides broadly in a drum assembly for a linear accelerator, the drum assembly comprising a drum having a front face including a front rim and a rear face including a rear rim, one or more support wheels supporting the drum, an arm extending from the front face of the drum and including a beam collimator through which a beam of radiation is emitted to form a radiation isocentre and one or more rear rim members associated with the rear rim, the rear rim members being adapted to substantially offset distortion to the isocentre in the Y direction due to unintended sagging of the beam arm which occurs with rotation of the drum assembly. 
     The present invention, in one form, resides broadly in a drum assembly for a linear accelerator, the drum assembly comprising a drum having a front face including a front rim and a rear face including a rear rim, one or more support wheels supporting the drum, an arm extending from the front face of the drum and including a beam collimator through which a beam of radiation is emitted to form a radiation isocentre and one or more front rim members associated with the front rim, the front rim members being adapted to substantially offset sagging of the arm. 
     The present invention, in one form, resides broadly in a drum assembly for a linear accelerator, the drum assembly comprising a drum having a front face including a front rim and a rear face including a rear rim, one or more support wheels supporting the drum, an arm extending from the front face of the drum and including a beam collimator through which a beam of radiation is emitted to form a radiation isocentre and one or more front rim members associated with the front rim, the front rim members being adapted to substantially offset distortion to the isocentre in the Z direction due to unintended sagging and twisting of the beam arm which occurs with rotation of the drum assembly. 
     Preferably, the front rim members may also be adapted to substantially offset a distortion of the isocentre associated with sagging of the arm. 
     Accordingly, the present invention provides a drum assembly for a linear accelerator, the drum assembly comprising a drum having a front face including a front rim and a rear face including a rear rim, one or more support wheels supporting the drum, an arm extending from the front face of the drum and including a beam collimator through which a beam of radiation is emitted to form a radiation isocentre and one or more rear rim members associated with the rear rim, the inner surface of the rear rim members being provided with one or more projections extending therefrom, that extend between the rear rim members and the inside edge of the rear rim, and the rear rim members and projections being adapted to substantially offset distortion to the isocentre due to unintended movement of the drum assembly and further wherein the drum assembly also comprises one or more locating members associated with the rear rim, and the rear rim members and locating members together are adapted to create and or limit movement of the drum assembly in a direction that is substantially parallel to the axis of rotation of the drum assembly (i.e. in the direction of the Y axis) the created movement of the drum assembly will substantially offset the equal and opposite distortion of the isocentre caused by the deflection of the beam arm, at particular gantry angles. 
     Accordingly, the invention provides in a variant, a drum assembly for a linear accelerator, the drum assembly comprising a drum having a front face including a front rim and a rear face including a rear rim, one or more support wheels supporting the drum, an arm extending from the front face of the drum and including a beam collimator through which a beam of radiation is emitted to form a radiation isocentre and one or more rear rim members associated with the rear rim, the inner surface of the rear rim members being provided with one or more projections extending therefrom, that extend between the rear rim members and the inside edge of the rear rim, and the rear rim members and projections being adapted to substantially offset distortion to the isocentre due to unintended movement of the drum assembly. 
     The drum may be of any suitable shape, size and configuration. For instance, the drum may have a square, triangular, circular, oval or rectangular cross-sectional shape. Preferably, however, the drum may be substantially circular in cross-section and substantially cylindrical in shape. In a preferred embodiment of the invention, the drum may be oriented so that the front and rear faces of the drum are positioned substantially vertically in use. It will be understood that the term “front face” in reference to the drum is intended to indicate the face of the drum from which the wave accelerator, X-ray target and beam collimator extend, while the term “rear face” is intended to indicate the opposite side of the drum to that from which the wave accelerator extends and where the electron gun is located. 
     As previously stated, the front face of the drum includes a front rim, while the rear face of the drum includes a rear rim. The front and rear rims may simply comprise an edge of the front and rear faces. Alternatively, the front and rear faces may be provided with one or more lip members at the outer edges thereof that extend at least partially about the circumference of the front and rear faces. A skilled addressee will understand, however, that the exact nature of the front rim and the rear rim is not critical to the invention. 
     The one or more support wheels may be of any suitable form, and may be positioned in any suitable orientation relative to the drum. For instance, the support wheels may be positioned substantially vertically so that the axis of rotation of the support wheels is substantially parallel to the axis of rotation of the drum. Alternatively, the support wheel may be positioned substantially horizontally so that the axis of rotation of the support wheels is substantially perpendicular to the axis of rotation of the drum. In other embodiments of the invention, a combination of vertical and horizontal support wheels may be provided. 
     It will be understood that the purpose of the vertical support wheels is to support the drum and facilitate rotation of the drum about a horizontal axis (the Y axis), while the horizontal support wheels act to limit the movement of the gantry drum across the surface of the vertical support wheels (movement along the Y axis). Preferably, the purpose of any vertical support wheels is to support the drum and facilitate rotation of the drum about a horizontal axis (the Y axis), while any horizontal support wheels act to limit the movement of the gantry drum across the surface of the vertical support wheels. This may assist in maintaining the drum in an essentially fixed position in the direction of the Y axis. The support wheels are conventional within the present art and no further description of the support wheels is required. It is worth noting, however, that, in conventional LINACs, the front and rear rims of the drum are in abutment with the support wheels such that rotation of the drum assembly causes rotation of the support wheels and vice versa. Thus, in the present invention, the front rim members (and rear rim members as described later) are in contact with the support wheels and are positioned between the support wheels and the respective rims. 
     It is envisaged that the drum assembly may comprise a base portion to which the support wheels may be mounted. It is envisaged that the vertical support wheels may be mounted to the base portion in a vertical position (such as to a side or edge of the base portion), while the horizontal support wheels may be mounted in a horizontal position (such as to an upper or lower surface of the base portion). 
     The one or more front rim members may be of any suitable size, shape or configuration. In some embodiments of the invention, the one or more front rim members may extend at least partially about the circumference of the front rim. Alternatively, however, the one or more front rim members extend substantially about the entire circumference of the front rim. 
     In some embodiments of the invention, one front rim member may be present. More preferably, however, two or more front rim members are present. The two or more front rim members may be of the same dimensions as one another, or may be of different dimensions in that each front rim member extends for a different length about the circumference of the front rim. Preferably, the total length of the two or more front rim members is less than the circumference of the front rim, although it is envisaged that the total length of the two or more front rim members may be substantially equal to the circumference of the front rim. It is envisaged that, when two or more front rim members are present where the total length of the front rim members is less than the circumference of the front rim, one or more gaps between front rim members may be formed about the circumference of the front rim. 
     Preferably, each of the front rim members defines an arc of a circle having a diameter substantially equal to, or slightly larger than, that of the front face. 
     In a most preferred embodiment of the invention, a pair of front rim members may be provided on the front rim. Preferably, the length of the pair of front rim members is such that the front rim members together extend about approximately 50 percent of the circumference of the front rim. 
     It is envisaged that, in embodiments of the invention in which a pair of front rim members is present, the front rim members may be spaced apart from one another around the circumference of the front rim. The front rim members may be spaced apart from another at any suitable distance. Preferably, however, the pair of front rim members are spaced apart approximately 180 degrees from one another about the circumference of the front rim. 
     The pair of front rim members may be located at any suitable location on the rim relative to the arm. Preferably, however, the front rim members are spaced approximately 90 degrees about the circumference of the front rim from the arm (or at least the closest point on the front rim to the location of the arm). In this way, when the beam of radiation issues from the arm in a substantially horizontal position, sagging of the arm (and therefore distortion of the isocentre) may be substantially precluded due to the positioning of one of the pair of front rim members in a lower region of the drum (i.e. located at least partially between the support wheel and the front rim). 
     By locating the front rim members at this point, the effect of gravity on the drum assembly (which causes sagging of the arm and distortion of the isocentre) may be offset such that the volume of the isocentre is substantially minimised. 
     In a preferred embodiment of the invention, the front rim members may be provided with inner and outer surfaces, with the inner surface of the front rim members being located in abutment with, or in close proximity to, the front rim. 
     In some embodiments of the invention, the thickness of the front rim members may be substantially constant along their entire length. Alternatively, the thickness of the front rim members may vary along their length. In this embodiment of the invention, the inner and/or upper surface of the front rim members may be provided with one or more peaks and/or troughs such that the thickness of the front rim members changes along its length. By providing one or more peaks and/or troughs on the front rim members, a substantially constant distance between the support wheels and the front rim may be maintained, and the likelihood of compression of the front rim members by the weight of the drum may be minimised. In other embodiments of the invention, two or more front rim members may be located so as to at least partially overlap one another, thereby increasing the strength and further reducing the effect of sagging of the arm. 
     It is envisaged that, in some embodiments, the inner surface of the front rim members may be provided with one or more projections extending therefrom. The one or more projections may be of any suitable size, shape and configuration, and may extend from the inner surface of the front rim members at any suitable angle. Preferably, however, the one or more projections extend between the inner surface of the front rim members and the front rim such that the one or more projections abut the front rim in use so as to support the front rim members and fix them in position on the flat face of the front rim. 
     Any suitable number of projections may be provided, although it is envisaged that a plurality of projections spaced about the circumference of the front rim may be provided. The projections may be spaced at any suitable intervals, and may be spaced evenly about the circumference of the front rim or may be spaced apart at varying intervals. 
     In some embodiments of the invention, the front rim members may extend outwardly beyond the front face of the drum. In this embodiment of the invention, the front rim members may be provided with one or more projections that extend between the front rim members and the front face of the drum. In this way, the front rim members may be substantially fixed in position around the flat face of the front rim. 
     The front rim members may be associated with the front rim in any suitable manner. For instance, the front rim members may be attached to the front rim using one or more fasteners. Any suitable fasteners may be provided, such as mechanical fasteners including one or more nails, bolts, rivets, screws or the like, or any combination thereof, or chemical fasteners such as an adhesive. Alternatively, the front rim members may be connected to the front rim using a frictional fitting, one or more loops, ties, hooks, hook and loop fasteners (e.g. Velcro) or the like, or any combination thereof. In other embodiments, the front rim members may be connected to the front rim using a snap fitting. 
     In alternative embodiments of the invention, the front rim members may be fixedly connected to the front rim, such as by welding, brazing or any other suitable technique. Still further, the front rim members may be integrally formed with the front rim of the drum. 
     The front rim members may be fabricated from any suitable material. Preferably, however, the front rim members may be fabricated from a material capable of withstanding being crushed by the weight of the drum when in use. Thus, in a preferred embodiment of the invention, the front rim members may be fabricated at least partially from a metal, such as steel, titanium (or alloys thereof) or the like. By fabricating the front rim members from a material capable of withstanding being crushed by the weight of the drum, sagging of the LINAC, (and therefore distortion of the isocentre) may be reduced or even eliminated. Thus, it is envisaged that the front rim members may act as shims. 
     In some embodiments of the invention, the drum assembly may further comprise one or more rear rim members. It is envisaged that the rear rim members may be associated with the rear rim using any suitable technique. For instance, the rear rim members may be attached to the rear rim using one or more fasteners. Any suitable fasteners may be provided, such as mechanical fasteners including one or more nails, bolts, rivets, screws or the like, or any combination thereof, or chemical fasteners such as an adhesive. Alternatively, the rear rim members may be connected to the rear rim using a frictional fitting, one or more loops, ties, hooks, hook and loop fasteners (e.g. Velcro) or the like, or any combination thereof. In other embodiments, the rear rim members may be connected to the rear rim using a snap fitting. 
     In alternative embodiments of the invention, the rear rim members may be fixedly connected to the rear rim, such as by welding, brazing or any other suitable technique. Still further, the rear rim members may be integrally formed with the rear rim of the drum. 
     The one or more rear rim members may be of any suitable size, shape or configuration. In some embodiments of the invention, the one or more rear rim members may extend at least partially about the circumference of the rear rim. 
     Alternatively, the one or more rear rim members may extend substantially about the entire circumference of the rear face of the rear rim. 
     In some embodiments of the invention, one rear rim member may be present. More preferably, however, two or more rear rim members are present. The two or more rear rim members may be of the same dimensions as one another, or may be of different dimensions in that each rear rim member extends for a different length about the circumference of the rear face of the rear rim. 
     Preferably, each of the rear rim members defines an arc of a circle having a diameter substantially equal to, or slightly larger than, that of the rear face. 
     In a most preferred embodiment of the invention, multiple rear rim members may be provided on the rear rim. Preferably, the length of the rear rim members is such that the rear rim members extend up to approximately 80 percent of the circumference of an edge of the rear rim. 
     The rear rim members may be located at any suitable location on the rim relative to the arm. Preferably, however, the rear rim members are spaced symmetrically around the lowest point of the rear rim when the drum is at zero degrees of rotation, and extend up to approximately 80 percent of the circumference of the rear edge of the rear rim. In this way, when the beam of radiation issues from the arm in a substantially vertical position, the axial movement (in the direction of the Y axis) of the drum is affected in such a way as to offset the sagging of the arm (and therefore distortion of the isocentre which is moved back towards the front face of the drum as a result of the typical sag in the arm at that angle). 
     By locating the rear rim members at this point, the effect of gravity on the arm (which causes distortion of the isocentre), may be offset such that volume of the isocentre is substantially minimised. 
     In a preferred embodiment of the invention, the rear rim members may be provided with inner and outer surfaces, with the inner surface of the rear rim members being located in abutment with, or in close proximity to, the rear edge of the rear rim. 
     Thus, in this embodiment of the invention, at least a portion of the rear rim members may be located on the rear face of the drum. 
     In some embodiments of the invention, the thickness of the rear rim members may be substantially constant along their entire length. Alternatively, the thickness of the rear rim members may vary along their length. In this embodiment of the invention, the inner and/or upper surface of the rear rim members may be provided with one or more peaks and/or troughs such that the thickness of the rear rim members changes along its length. In other embodiments of the invention, two or more rear rim members may be located so as to at least partially overlap one another, thereby increasing the strength and further reducing the likelihood of sagging of the arm. 
     It is envisaged that, in some embodiments, the inner surface of the rear rim members may be provided with one or more projections extending therefrom. The one or more projections may be of any suitable size, shape and configuration, and may extend from the inner surface of the rear rim members at any suitable angle. Preferably, however, the one or more projections extend between the inner surface of the rear rim members and the rear rim such that the one or more projections abut the rear rim in use so as to support the rear rim members in a fixed position. 
     Any suitable number of projections may be provided, although it is envisaged that a plurality of projections spaced about the circumference of the rear rim may be provided. The projections may be spaced at any suitable intervals, and may be spaced evenly about the circumference of the rear rim or may be spaced apart at varying intervals. 
     In some embodiments of the invention, the rear rim members may extend outwardly beyond the rear edge of the rear rim. In this embodiment of the invention, the rear rim members may be provided with one or more projections that extend between the rear rim members and the inside edge of the rear rim. 
     In some embodiments of the invention, the drum assembly may further comprise one or more locating members. Preferably, the one or more locating members may be associated with the rear rim. Thus, in embodiments of the invention in which rear rim members and locating members are both present, the rear rim members and locating members may together be adapted to create and/or limit movement of the drum assembly in a direction that is substantially parallel to the axis of rotation of the drum assembly (i.e. in the direction of the Y axis). This created movement will substantially offset the equal and opposite distortion of the isocentre caused by the deflection of the beam arm at particular gantry angles. 
     In a preferred embodiment, the one or locating members are positioned so as to act upon an inner edge of the rear rim. It will be understood that the term “inner edge of the rear rim” is intended to refer to the edge of the rear rim that is closest to the front rim of the drum. Thus, it is envisaged that the locating members may be positioned at least partially between the front and rear rims of the drum. 
     Preferably, the locating members act upon the rear rim such that a force is applied to the rear rim to control and/or limit forward movement of the drum assembly (i.e. in the direction of the Y axis). The locating members may be of any suitable size, shape or configuration, and may include one or more blocks adapted to be located in abutment with the inner edge of the rear rim so as to limit forward movement of the drum assembly to the thickness of the rear rim members on the rear edge of the rear rim. 
     In an alternative embodiment of the invention, the locating members may include one or more locating wheels located in abutment with, or in close proximity to, the inner edge of the rear rim. The locating wheels may act in concert with the horizontal support wheels to support the drum. Preferably, however, the locating wheels may be biased into position using one or more biasing members (or may be associated with one or more biasing members), such that the drum is substantially unable to overcome the bias of the biasing members and unintended forward movement of the drum is reduced or eliminated. 
     Any suitable biasing member may be used, such as, but not limited to, one or more springs or spring-loaded plates, discs or the like. In this embodiment of the invention, the biasing member may be located between the locating wheels and the rear rim such that the drum assembly is unable to overcome the bias of the biasing members, and therefore unintended forward movement of the drum assembly is substantially precluded. 
     Alternatively, the biasing member may include one or more elongate members that bias the locating wheels into abutment with (or close proximity to) the rear rim. Preferably the one or more elongate members are spring-loaded. Thus, while the inner edge of the rear rim may contact the locating wheels, the drum is unable to overcome the bias of the elongate members and unintended forward movement of the drum assembly may be substantially precluded. 
     Preferably the elongate members comprise a first end associated with the locating wheels and an opposed fixed second end. The elongate members may be of any suitable length, cross-sectional, shape and so on. 
     In a preferred embodiment of the invention, the drum assembly of the present invention produces an isocentre of less than about 1 millimetres. More preferably, the drum assembly of the present invention produces an isocentre of less than about 0.5 millimetres. 
     In another aspect, the invention resides broadly in in a drum assembly for a linear accelerator, the drum assembly comprising a drum having a front face including a front rim and a rear face including a rear rim, one or more support wheels supporting the drum, an arm extending from the front face of the drum and including a beam collimator through which a beam of radiation is emitted to form an isocentre, and one or more rear rim members associated with the rear rim, and one or more locating members associated with the rear rim, the rear rim members and the locating members together being adapted to substantially offset deflection and/or movement of the beam collimator. 
     Preferably, the rear rim members and the locating members may also be adapted to substantially offset and reduce the volume of the isocentre associated with deflection and/or movement of the beam collimator. 
     In another aspect the invention resides broadly in a drum assembly for a linear accelerator, the drum assembly comprising a drum having a front face including a front rim and a rear face including a rear rim, one or more support wheels supporting the drum, an arm extending from the front face of the drum and including a beam collimator through which a beam of radiation is emitted to form an isocentre, and one or more front rim members associated with the front rim, one or more rear rim members associated with the rear rim, and one or more locating members associated with the rear rim, the front rim members, the rear rim members and the locating members together being adapted to substantially offset sagging of the arm and deflection and/or movement of the beam collimator. 
     Preferably, the front rim members, the rear rim members and the locating members may also be adapted to substantially offset and reduce the volume of the isocentre associated with deflection and/or movement of the beam collimator and/or movement of the beam collimator. 
     The present invention provides numerous advantages over the prior art. For instance, the presence of the front rim members offsets sag in the arm predominantly when the arm is in a horizontal position, and the associated distortion of the isocentre. Similarly, the presence of the rear rim members and/or the locating members offsets currently experienced isocentre (focal point) movement in the direction of the Y axis as the drum rotates, and thereby reduces the associated volume of the isocentre. 
     The advantage of reducing or eliminating the volume of the isocentre of a LINAC is that treatment plans for a patient may be more precise, and may allow for a more intense dose of radiation to the tumour without causing damage to the surrounding healthy tissue. This increases the likelihood of successfully treating the tumour and returning the patient to a healthy, productive life. 
     It is envisaged that the present invention may reduce the volume of the isocentre from current sizes of 1.5 millimetres or more to isocentres of less than 1 millimetre, and more preferably less than 0.5 millimetres. 
     Any of the features described herein can be combined in any combination with any one or more of the other features described herein within the scope of the invention. 
     The reference to any prior art in this specification is not, and should not be taken as an acknowledgement or any form of suggestion that the prior art forms part of the common general knowledge. 
     INDUSTRIAL APPLICABILITY 
     The invention may be applied industrially, through manufacture of improvements to linear accelerators for supply to organisations or suppliers in need of the equipment. The improvements may be incorporated into the linear accelerator at the time of manufacture, or provided as a separate assembly to be provided with new equipment or retrofitted to existing equipment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred features, embodiments and variations of the invention may be discerned from the following Detailed Description which provides sufficient information for those skilled in the art to perform the invention. The Detailed Description is not to be regarded as limiting the scope of the preceding Disclosure of the Invention in any way. The Detailed Description will make reference to a number of drawings as follows: 
         FIG. 1  illustrates a side view of front rim members according to an embodiment of the present invention; 
         FIG. 2  illustrates a front view of rear rim members according to an embodiment of the present invention; 
         FIG. 3  illustrates an isometric view of a drum assembly according to an embodiment of the present invention, including the front and rear rim members of  FIGS. 1 and 2 ; 
         FIG. 4  illustrates a plan view of a portion of a drum assembly according to an embodiment of the present invention; 
         FIG. 5  illustrates the direction of the axes of a linear accelerator (PRIOR ART) as a useful reference; 
         FIG. 6  illustrates a schematic partial view of a drum assembly variant to the first preferred embodiment, to highlight the rear rim members and projecting bosses in particular; 
         FIG. 7  illustrates the detailed view at the 3 O&#39;clock position of  FIG. 6  illustrating the cut-outs for LINAC apparatus; 
         FIG. 8  illustrates the detailed view at the 1 O&#39;clock position of  FIG. 6  illustrating the boss and support; 
         FIG. 9  illustrates a schematic partial view of the drum assembly of  FIGS. 6 to 8  with the rim member in place between the rim and bosses; 
         FIG. 10  illustrates a schematic partial view of the boss of  FIGS. 6 to 9 , with the detail shown including the boss support; 
         FIG. 11  illustrates a front view of the drum assembly of  FIG. 6 , indicating the rim and including the rim members and 8 bosses positioned about the inner circumference; 
         FIG. 12  indicates a first section of the rim of  FIG. 11  with an arc of 38 degrees; 
         FIG. 13  indicates a second section of the rim of  FIG. 11 , with an arc of 52 degrees; 
         FIG. 14  is a detailed view of the schematic view from the front of  FIG. 11  with the detail of the position of the boss, on the rim member, inside the rim; 
         FIG. 15  is a detailed view of the blocks at the 3&#39;Oclock position on  FIG. 11 ; and 
         FIGS. 16 a  and  b   , is a two part schematic diagram of the boss of  FIGS. 11 and 14 , from above and then separately, in plan view to show the boss support. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     With reference to  FIGS. 1 to 4 , embodiments of the invention are shown in various aspects, useful to the disclosure of the invention. With reference in particular to  FIG. 1 , there is illustrated a side view of front rim members  10  according to an embodiment of the present invention. In both  FIGS. 1 and 2  the respective components are shown clearly, to illustrate the detailed features of the rim members, important to the working of the invention. Front rim members  10 , each define an arc of a circle that is substantially the same diameter, or of a slightly larger diameter, to the diameter of the front rim of the drum assembly (neither shown in  FIG. 1 ). 
     In the embodiment of the invention shown in  FIG. 1 , front rim members  10  together, have a circumference that is approximately equal to that of the front rim so that front rim members  10  could form a continuous rim about the front rim, if used together. Alternatively, only one of front rim members  10  could be used on the front rim so that the full circumference of the rim is not covered by front rim members  10 . For instance, two of the smaller front rim members  10  could be used so that approximately 50 percent of the front rim is covered by front rim members  10 . 
     Front rim members  10  are provided with a plurality of projections  11  that extend outwardly from front rim members  10 , towards the front face (not shown in  FIG. 1 ) of the drum assembly. In use, projections  11  will abut the front face of the drum assembly. Alternatively, projections  11  could be used to locate and or retain t front rim members  10  on the front rim. 
     With reference in particular to  FIG. 2 , there is illustrated a front view of rear rim members  12  according to an embodiment of the present invention, similar to front rim members  10 , as described above. Rear rim members  12  each define an arc of a circle that is substantially the same diameter, or of a slightly larger diameter, to the diameter of the rear rim of the drum assembly (neither shown in  FIG. 2 ). 
     In the embodiment of the invention shown in  FIG. 2 , rear rim members  12 , together have a circumference that is approximately equal to that of the rear rim (not shown in  FIG. 2 ) so that rear rim members  12  could form a continuous rim about the rear rim, if used together. Alternatively, only one of rear rim members  12  could be used on the rear rim so that the full circumference of the rim is not covered by the rear rim members  12 . For instance, two of the smaller rear rim members  12  could be used so that approximately 50 percent of the rear rim is covered by the rear rim members  12 . Preferably, a rear rim member is located on the rear edge of the rear rim of the drum, as can be seen in  FIG. 3 . 
     Rear rim members  12  are provided with a plurality of projections  13  that extend inwardly from an inner surface of rear rim members  12  towards the rear rim of the drum assembly (neither shown in  FIG. 2 ). In use, projections  13  will abut the rear rim (not shown) of the drum assembly. Alternatively, the projections  13  could be used to locate and/or retain the rear rim members  12  on the rear rim. 
     In  FIG. 2  it may be seen that both inner and outer surfaces of rear rim members  12  are provided with a series of peaks  14  and troughs  15  that assist in preventing rear rim members  12  from being crushed by the weight of the drum (not shown). 
     With reference in particular to  FIG. 3 , there is illustrated an isometric view of drum assembly  16 , according to an embodiment of the present invention. The view of  FIG. 3  includes the front and rear rim components labelled  10 ,  11 ,  12  and  13  as described for  FIGS. 1 and 2 , as can be readily understood. In this Figure it may be seen that drum assembly  16  comprises drum  17  having front face  18 , rear face  19  and a plurality of support wheels  20  atop which drum  17  rotates. In use, drum  17  rotates about a rotation of axis extending horizontally through the centre of both front face  18  and rear face  19  of drum  17 . Drum  17  is positioned on its side so that the front face  18  and the rear face  19  are positioned substantially vertically, in use. 
     Extending from, or, more accurately, through, front face  18  is arm  21  housing a wave accelerator, an X-ray target (obscured) and beam collimator  22  from which beam of radiation  23  issues when the LINAC is in use. Beam of radiation  23  forms an isocentre about which drum assembly  16  rotates. 
     Front face  18  includes an annular front rim  24  that extends about the circumference of front face  18 . Similarly, rear face  19  includes an annular rear rim  25  that extends about the circumference of rear face  19 . In the embodiment of the invention shown in  FIG. 3 , front rim members  10  and rear rim members  12  are located on front rim  24  and rear rim  25 , respectively, with rear rim members  12  located at least partially on rear face  19  of drum assembly  16 . Front rim members  10  extend about a portion of the length of front rim  24 , and therefore about a portion of the circumference of front face  18 , while rear rim members  12  are located partly on rear face  19  of drum assembly  16 , and extend onto the rear edge of rear rim  25 . In this way, the rear rim members  12  extend about a portion of the circumference of the rear face  19 . 
     Front rim members  10  are positioned so as to be located between front rim  24  and support wheels  20  as can be seen in  FIG. 3 ; when arm  21  is positioned substantially horizontally. Front rim members  10  positioned between front rim  24  and support wheels  20  are adapted offset sagging of the arm  21  (i.e. movement in the direction of the Z axis) measured at the isocentre and therefore reduce distortion the isocentre volume, when arm  21  is positioned such that beam of radiation  23  is issued from beam collimator  22  in a substantially horizontal direction. Rear rim members  12  are positioned so as to prevent in-plane gantry movement of drum  17  along its axis of rotation (i.e. movement of the drum in the direction of the Y axis), which also would result in an increase in isocentre volume. 
     With reference in particular to  FIG. 4  illustrates a plan view of a base portion  30  of a drum assembly according to an embodiment of the present invention. In this Figure, base portion  30  includes three vertical support wheels  31  on which the drum (not shown) is supported for rotation and two horizontal support wheels  32   a ,  32   b , one of which  32   a  is fixed in position for use while the other  32   b  second is awaiting installation. 
     In use, the horizontal support wheels  32   a ,  32   b  are located in close proximity to an edge (and preferably a front edge) of the rear rim. In this way, the horizontal support wheels  32   a ,  32   b  control movement of the drum in the direction of the Y axis, thereby minimising the isocentre volume. The rear rim members may be located between the edge of the drum and the horizontal support wheels  32   a ,  32   b  to further prevent movement of the drum in the direction of the Y axis. 
     Use of the invention acts to control movement of the drum assembly in the direction of the Y and/or Z axes, in order to offset any movement in the location of the isocentre (and radiation isocentre), position, while the LINAC is in use. The different components act in such a way as to offset isocentre variations in separate axes caused mainly by the weight of the arm and gravitational forces at different gantry angles, plus to a lesser extent distortion and the weight and wear of the apparatus generally. The effect of these forces is to give rise to a predictable variation in the isocentre position, and by eliminating them through the offset produced by the additionally introduced members, the variations in isocentre position can be reduced and thereby the volume of the locus described by these positions is reduced and the accuracy of the isocentre is increased. As described elsewhere, even the smallest of inaccuracies in this regard can have unfortunate outcomes for the patient, losing healthy tissue unnecessarily, which may cause irreparable loss of function to organs, and clearly a degradation to the life of the patient in the short and longer term. The patient will be happy to have the tumour destroyed but will need to leave with the consequences of the treatment, if this involved loss of function through damage to healthy tissues. To avoid the inaccuracy, where very specific accuracy is possible through use of the same LINAC equipment, would be a hugely beneficial outcome of the invention. Use of the invention would remove the degradation of treatment through distortion to the isocentre over time as the LINAC equipment wears and ages through much repeated use. For new equipment, as the invention is likely to be fitted as standard, the accuracy and useful life of the equipment will be prolonged, a further beneficial outcome for the stakeholders, investing in this costly equipment for hospitals and other treatment centres. 
     The inventor has developed a clever and surprisingly useful invention, that works with LINAC apparatus to significantly improve the accuracy of the isocentre by acting against distortion due to sagging of the support assembly. 
     With reference to  FIGS. 6 to 16 , variants to the first embodiment are disclosed, in schematic form to show the best method of these aspects known—refer also the general description below. Various aspects are shown with much of the drum assembly and arm omitted, to assist illustration, and similar reference numerals are used throughout. In  FIG. 6  annular rear rim  25  and the rear face to which rear rim members  12  and projecting bosses  13  are installed to provide support to the drum assembly against unintended movement. Annular front rim  24  with front rim bosses  11  (not shown) may be arranged in a similar, or slightly different arrangement, as appropriate. The rear rim arrangement is of great benefit to act against unintended movement in the apparatus that may increase isocentre volume. 
     As shown in  FIG. 6, 8  projecting bosses  13  are shown within rim  26 , in the position to which they will be arranged in use. The 8 bosses  13  have been found to be a particularly suitable arrangement. Bosses  13  are indicated in  FIG. 6  are shown to indicate their actual position in use, spaced substantially evenly within rim  25 , and rim members  12 , rim members  12  being omitted form  FIG. 6 , for ease of illustration. An approximate horizontal or vertical spacing between neighbouring bosses  13  is shown as 300 millimetres. The outer dimension of rim  26  is shown as 1976 millimetres with an inner diameter 1946 millimetres as suitable for the particular drum assembly. Fitted with the inner rim of rear rim  26  and arranged circle of bosses  13  are rear rim members  12  which together substantially surround the inner circumference of rim  25 . 8 sections are included of rear rim members to strongly support and surround the assembly, as has been found to be of particular use. Partial rim members may be include instead, but it is beneficial to include a plurality of rear rim members  12  to generally surround the inner surface of rim  25  and provide support thereto. A single rim member could be used around most or some of the inner circumference or this ring may be made of one or more overlapping section to create the strong support to the drum assembly. 
     The particular dimensions of these components may be readily adapted to the particular drum assembly as will be readily understood by the person skilled in the art. The arrangement of rim members  12  and bosses  13  is an illustrative example, in which 8 bosses are included spaced about the inner circumference, as indicated by the round circles. But more or less projections or bosses may be included and these may be spaced evenly to assist to distribute pressure or unevenly to suit the particular application. 
     Detailed image of part of  FIG. 7  at the 3 O&#39;clock position of  FIG. 6 , the detailed rebate to accommodate existing LINAC items, cut to be 60 by 15 millimetres in this example by 6 millimetres deep, as is self-explanatory. 
     Detailed image of part of  FIG. 8  at the 1 O&#39;clock position of  FIG. 6 , illustrates boss  13  and distance of inner diameter of the rim, relative to rim  25  and support  26 . The detail of the boss, support and positions around the rim/rim members is given further below. 
     With reference in particular to  FIG. 9 , an arc of rim  25  is shown, with rear rim members  12  between two bosses  13  positioned spaced about the arc. The defined 3 rear rim members  12  can be seen abutted one with the other and the two boss  13  sections. The inner surface of rim member  12  as shown has an outer circumference in the region of 1944 millimetres, within rim  25  to an inner dimension of 1880 millimetres. Bosses  13  are in close proximity to rear rim members  12 . Again, clearly variation may be made to suit the particular application of these precise measurements but are given for illustrative purposes. 
     With reference to  FIG. 10 , boss  13  is shown with support  27  used to support boss  13  in its position and in its function to act against unintended movement. 
     Support  27  has a 22.5 millimetre width on either side of the aperture and an internal hole for a M4 grub screw. The width of boss support  27  can be seen to be approximately 160 millimetres (horizontal measurement across the arc) as suitable. There will be 8 boss  13  and corresponding 8 boss supports  27 , in use. 
     8 Support segments as shown elsewhere are supported on top of 8 Boss Flanges  FIG. 10 , and two support segments join on each Boss flange. Boss flange  27  insertion on to Boss  13  is to be adjusted (then tightened with Grubb screws), to present flush face between face of Support Segments/rear rim members  12  and rim face  25 , to allow for mounting of offset shims (not shown here). Shims to be locked in location by 8 addition clamping segments (not shown), which are tightened onto the shims by bolts through holes in the shims to sandwich the shims between the 8 Support Segments and the 8 Clamping segments. 
       FIG. 11  illustrates much the same view as  FIG. 6 , however the rear rim members are indicated showing bosses  13  in contact therewith, and supports  27 . The spacing between the bosses has been found to be beneficial. The spacing between the top two bosses of  FIG. 11  is at approximately a 38 degree angle, as shown in the arc of  FIG. 12 . Similarly,  FIG. 13  illustrates the 52 degree angle defined in an arc between the boss to the right of the 12&#39;Oclock position and the next one around, before the 3&#39;Oclock position. These angles are working examples of the invention but can be varied to suit different apparatus and would be understood by the person skilled in the art. 
       FIG. 14  is a further schematic of boss  13  on an arc, a close up from  FIG. 11 .  FIG. 15  gives a close up of the cut-out blocks of  FIG. 7 , useful in the invention. 
       FIG. 16   a , and  FIG. 16   b , illustrate boss  13  from above and then in profile showing boss support  27 . In use boss  13  act against the movement in the apparatus that may be caused by the weight of the arm, or other pressures or sagging biasing the drum assembly away from where it should be positioned. The precise apparatus is aligned to a particular, precise low volume isocentre, and the patient aligned thereto for targeted tumour treatment. Any unintended movement in the drum assembly is acted against by the rim members taking the pressure and the bosses acting on the drum. There may be slippage in any direction, slight twists or distortion to any of the parts of the assembly, leading to an increase in volume/width of the isocentre applied to the patient which is undesirable. 
     In general terms, and as illustrated somewhat in  FIGS. 6 to 11 , versions of the invention which are currently under development and testing involve the use of 8 ‘Flange supports’ which fit around the 8 bosses on the LINAC drum face, and onto which are attached 8 supporting segments which abut to the internal diameter of the gantry rims, providing a stable mounting surface against which the ‘high tensile, laser cut stainless steel shims can be sandwiched. These shims are accurately cut to specific shapes and lengths so that the resulting displacement of the gantry drum that they cause, directly offsets the predictable distortions experienced at the isocentre due to the effects of gravity on the structure of the LINAC. Hence when limited offset is required, potential a small shim thickness will be used, and this will increase as the amount of offset required increases with variation in gantry angle. In this way, using combinations of variable shim thicknesses on the front and/or rear rims as required or requested by the Customer, the accuracy of the isocentre of the LINAC can be accurately controlled. The Shims are currently secured in their locations by a second supporting segment which clamps the shims against the supporting segment, using a combination of bolts and washers plus threaded and clearance penetrations in the support members. 
     When the shims move the drum upwards in the vertical direction, gravity ensures the return of the gantry drum to the lower position as the shim moves away from the contact point between the face of the front rim and the vertical supporting wheel on the gantry base. 
     However, when the shims move the drum in the Y axis, a spring loaded plunger is currently placed against the inside edge of the rear rim to cause the gantry drum to be biased in a direction keep the rear horizontal positioning wheel firmly in contact with however many shims are between the rear face of the rear rim and the horizontal wheel. When there are no shims present, the horizontal wheel is kept firmly pressed against the rear face of the rear rim. Hence the gantry drum has a biasing spring force to keep it pushed rearwards. The Biasing spring force is currently produced by a spring-loaded bolt action plunger which is secured to the topside of the gantry base, between the front and rims. The end of the bolt action plunger is capped with a Teflon based material, to minimise friction and wear on the face of the gantry rim, as it rotates with the bolt action plunger pressing against it. In later versions it is anticipated that this spring force will be generated using reciprocating cupped washers in association with linear bearings and plungers. 
     Because the horizontal support wheels will need to have greater clearance to the faces of the rear rim to allow for the movement of the gantry drum in the Y axis, it is anticipated that an additional clamping members, which are already designed and produced by the LINAC manufacturer, will need to be fitted to the Gantry base to secure the gantry drum to the gantry base in case of seismic activity and due to reduced lateral pressure due to the increased spacing of the horizontal support wheels. 
     When the modification is installed the rear horizontal support wheel will need to be exchanged from a centric bearing to an eccentric bearing. This will allow for the contact point of the rear horizontal support wheel to be moved rearwards by approximately 1.1 to make suitable clearance for the movement of the gantry drum structure in the Y axis. 
     Exchange of this horizontal support wheel will mean that the current isocentre position of the LINAC within the treatment room will remain unchanged and the lasers will not require any significant adjustment. 
     From a patient treatment aspect, there will be no difference to the existing patient setup procedures. The patient will enter the treatment room and be positioned on the treatment couch according to treatment tattoos (marked during treatment planning), and room lasers which accurately align to the LINAC isocentre. The Patient is generally planned so that the target tumour is located at the radiation isocentre of the LINAC. 
     Variation may be made to the forms of the inventive features, as would be understood by the person skilled in the art. The combination of features, rim members, projections/bosses and locator means, with the support wheels to press the apparatus act against distortion forces. The clever invention may be performed in many different ways using the same clear principles, as hereinbefore disclosed. 
     In the present specification and claims, the word “comprising” and its derivatives including “comprises” and “comprise” include each of the stated integers but does not exclude the inclusion of one or more further integers. 
     Reference throughout this specification to ‘one embodiment’ or ‘an embodiment’ means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearance of the phrases ‘in one embodiment’ or ‘in an embodiment’ in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more combinations. 
     It will be apparent to a person skilled in the art that changes may be made to the embodiments disclosed herein without departing from the spirit and scope of the invention in its various aspects. 
     
       
         
           
               
             
               
                   
               
               
                 REFERENCE SIGNS LIST: 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 10 
                 Front rim members 
               
               
                   
                 11 
                 Projections of 10 
               
               
                   
                 12 
                 Rear rim members 
               
               
                   
                 13 
                 Projections of 12 
               
               
                   
                 14 
                 Peaks of 12 
               
               
                   
                 15 
                 Troughs of 12 
               
               
                   
                 16 
                 Drum assembly 
               
               
                   
                 17 
                 Drum of 16 
               
               
                   
                 18 
                 Front face of 17 
               
               
                   
                 19 
                 Rear face of 17 
               
               
                   
                 20 
                 Support wheels 
               
               
                   
                 21 
                 Arm 
               
               
                   
                 22 
                 Beam collimator 
               
               
                   
                 23 
                 Beam of radiation 
               
               
                   
                 24 
                 Annular front rim of 18 
               
               
                   
                 25 
                 Annular rear rim of 19 
               
               
                   
                 26 
                 Support 
               
               
                   
                 27 
                 Boss support 
               
               
                   
                 28 
                 Blocks 
               
               
                   
                 30 
                 Base portion 
               
               
                   
                 31 
                 Vertical support wheels 
               
               
                   
                 32a 
                 Horizontal support wheel 
               
               
                   
                 32b 
                 Horizontal support wheel