as noted above , the field size of current epids is smaller than the maximum apertures of most mlcs , although larger than most actual dose shapes . we have realised that this limitation becomes a problem when the epid is used for portal dosimetry or to capture a dynamic mlc video under certain treatment conditions . as an example , we propose to discuss a volumetric modulated arc therapy ( vmat ) delivery to an off - axis target such as a tumour present in one breast or lung . this is not the only example , however , and the invention is applicable in other contexts . fig1 shows such a situation . a rotatable gantry 10 is set in an upright orientation , perhaps recessed into a wall or projecting through a false wall . it is in the form of a drum , rotatable around a horizontal axis ; fig1 is a view along that axis . the gantry 10 carries a radiation source 12 which can emit a collimated beam of therapeutic ionising radiation around a central axis 14 , which intersects with the horizontal axis around which the gantry 10 rotates . as the gantry 10 rotates and carries the source 12 around , the central axis 14 of the beam sweeps out a vertical plane and approaches the point of intersection ( often referred to as the isocentre ) from all possible directions . this forms the basis of a radiotherapy treatment ; by placing the tumour or other lesion at the isocentre and irradiating the tumour from a range of directions , a dose can be delivered to the tumour which is substantially greater than the dose delivered to tissue around the tumour . the tumour 16 is positioned relative to the beam by placing the patient 18 on a patient support 20 . this is separate to the gantry , usually supported on a floor in front of the gantry . many modern patient supports offer adjustment in six axes , i . e . three translational axes and three rotational axes . in this way , after the patient 18 has been helped onto the support , their position can be adjusted via the patient support 20 so as to locate the tumour as desired . some limits will of course be imposed by the range of motion of the patient support 20 . an epid 22 is also carried by the gantry 10 , located diametrically opposite the radiation source 12 to as to capture an image from the radiation beam after attenuation by the patient 18 , as described above . as the gantry rotates , this will rotate with the gantry so as to maintain its position relative to the radiation source 12 . the epid could alternatively be carried by a separate structure , arranged to support the epid in a suitable location opposite the source , but mounting it on the same gantry is likely to be easier and more accurate . fig1 illustrates a situation as outlined above , in which the tumour 16 is significantly off - centre within the patient 18 . with the patient on the patient support 20 , the tumour is then away from the central axis 14 . the beam can however be collimated to compensate for this , thus producing an off - axis beam 24 which will deliver radiation to the tumour 16 . this off - axis beam 24 will miss the epid 22 if in its usual or default position 26 ( shown in dotted lines ), so the epid 22 is manually adjusted into a suitable position as shown . however , as the gantry rotates ( fig2 ) and the patient 18 remains stationary on the patient support 20 , the degree of offset of the tumour 16 from the central axis 14 will vary . considered geometrically , there will be a vector 28 from the isocentre to the tumour , and when the central axis 14 is perpendicular to the vector 28 ( as in fig1 ) then the offset as viewed along the central axis will be at is maximum . equally , after a 90 ° rotation ( as in fig2 ), the central axis 14 will be parallel to the vector 28 and the offset will be at a minimum , possibly zero . this means that the epid 22 in its offset position will no longer capture an image of the beam 24 , as shown in fig2 . in addition , at some point in the rotation between the states shown in fig1 and 2 , the beam 14 will leave the active imaging region and strike the edge of the epid 22 ; this may damage neighbouring electronic components which are often much more radiation sensitive than the imaging panel itself , and are not intended to be exposed to the therapeutic beam . thus , although the actual field size ( projected to isocentre ) is likely to be smaller than the 26 × 26 cm aperture of the epid 22 , the offset position of beam may track across the full 40 × 40 cm beam aperture of the collimator as the gantry rotates . thus , if the epid is being used to collect portal dosimetry data , then some data will be lost and this will compromise the ability to accurately calculate the portal dose , as well as potentially causing damage to the epid . fig3 and 4 show the alternative , to position the patient off - axis via adjustment of the patient support 20 ( or otherwise ). thus , the tumour is placed at or nearer the isocentre . as shown in fig3 , this means that distance between the isocentre and the tumour 16 is small , and the epid 22 does not need to be offset . as shown in fig4 , rotation of the gantry 10 leaves the beam 24 still within the effective aperture of the epid 22 . however , this raises a risk of collision between the radiation head 12 rotating around the patient support 20 and the off - axis patient support 20 . any further rotation in an anti - clockwise direction beyond that shown in fig4 will lead to a collision and possible injury to the patient , as well as damage to the apparatus . as a result , such treatment may have to be planned around a limited range of rotational movement of the head in order to prevent collision . this is possible within the bounds of treatment planning , but may lead to a sub - optimal treatment plan . alternatively , the treatment can be interrupted in order to reposition the patient and / or imaging panel , but this will prolong the duration of the treatment session , which is undesirable for the patient and hospital . so , the treatment can be planned with the tumour placed away from the isocentre on the machine , to allow the patient to be positioned centrally , avoid collision risk , and allow use of the full 360 ° rotation of the source , but the epid will not be available . alternatively , the tumour can be positioned centrally , but the full range of rotation may not be available , so the treatment may be sub - optimal . in a further alternative , according to us2007 / 0195936a1 , the mlc leaf positions are adjusted to ensure that the epid is protected and usable , but this will also affect the treatment and render it sub - optimal . fig5 and 6 illustrate a further type of radiotherapy apparatus . efforts are currently being made to integrate mri scanning with radiotherapy ; at present ct scanning ( usually cone - beam ct ) is easily integrated simply by adding a lower - energy diagnostic source to the gantry together with an opposing detector , usually located 90 ° away from the therapeutic beam and the epid . the integration of mri scanning is more complex as a design needs to be found that allows for the substantial magnets required by mri systems , although once this is done there are benefits in that the background dose of ionising radiation given to the patient is reduced as compared to a ct scan . fig5 and 6 show such a system ; the patient 50 ( not shown in fig6 ) lies on a patient support 52 that can be translated longitudinally into and out of a bore 54 . a pair of primary magnet windings 56 , 58 are arranged concentrically around the bore 54 , spaced longitudinally along the central axis 60 of the bore 54 so that each winding extends from a respective end of the bore 54 towards the centre of the bore . a gap 62 is left at that centre of the bore between the two windings , and a rotating gantry 64 fits within that gap 62 and is able to rotate around the bore 54 . a therapeutic source 66 is mounted on the gantry 64 and is therefore rotateable around the bore 54 together with the gantry 64 ; collimators 68 are provided within the source 66 so that a radiotherapeutic dose can be delivered to the patient 50 in an otherwise known manner . an epid ( not shown ) is mounted on the gantry 64 , opposite the source 66 and is used in a manner corresponding to that of the apparatus of fig1 to 4 . an apparatus of this type presents additional difficulties , in that where the target is offset it will not generally be possible to offset the patient as shown in fig3 and 4 , as the patient must be located within the defined bore 54 and cannot be displaced significantly . therefore , the only option is to offset the beam by use of the collimator 68 , leading to the difficulties illustrated in relation to fig1 and 2 . both problems can be addressed using the apparatus of fig7 in conjunction with the control schema of fig8 . fig7 shows a radiotherapy apparatus of the same general type as fig1 to 4 , although the invention is also applicable to apparatus of the type shown in fig5 and 6 by making a corresponding change . thus , a patient 100 is supported on a patient support 102 in front of a gantry 104 that is rotatable around a horizontal axis 106 . in practice , the gantry 104 is in the form of a cylindrical drum resting on its circular face on a number of supporting wheels 108 , and driven to rotate by a motor ( not shown ) that engages with an edge of the drum . the drum thus rotates around its centre , through which the axis 106 passes . a radiotherapeutic source 110 is mounted on the drum , offset from its front face 112 by a gantry arm 114 . the source 110 is aimed towards the rotation axis 106 , thus defining an isocentre where the rotation axis 106 meets the central axis of the beam emitted by the source 110 . a collimator arrangement 116 is included in the radiation source 110 so as to shape the beam as desired and allow the required dose distribution to be built up . an epid 118 is also mounted on the gantry 104 , via a gantry arm 120 that extends transversely from the front face 112 of the gantry 104 so as to place the epid 118 generally opposite the source 110 , with the patient 100 and the patient support 102 between them . this allows the epid 118 to capture an image of the beam as attenuated by the patient 100 . the epid 118 is connected to its gantry arm 120 via a series of servo - controlled linkages 122 that allow x - y movement of the epid 118 relative to the gantry arm 120 . the two translation axes of the linkages 122 are arranged transverse to the beam direction , so the effect of translating the epid 118 is to scan it across the field of the beam . the gantry arm 120 and / or the linkages 122 may be also able to move the epid 118 in a z - direction , i . e . towards or away from the source 110 . however , for the purposes of the present invention we are principally concerned with movement in the × and y directions ; motion in the z direction is in principle irrelevant to the invention apart from its influence on the effective image size within the beam aperture . the movement of the epid 118 in the y direction ( i . e . parallel to the rotation axis 106 ) will usually be in a straight line . its movement in the × direction ( i . e . perpendicular to both the rotation axis 106 and the beam axis , into and out of the page in fig7 ) is ideally along a circular path centred on the rotation axis 106 , in order to maintain a fixed distance from the source 110 . however , for ease of manufacture it is likely that movement in a straight line will be a good first approximation to circular movement , especially for smaller translational movements . it is likely that correction of the images to account for the difference between circular and linear movement will be straightforward . fig8 shows the control schema for the apparatus of fig7 . a physician prepares a prescription 124 setting out a dose and a dose distribution that are to be delivered to the patient . a separate document 126 sets out the delivery limitations imposed by the apparatus to be used , such as the resolution of the collimators , speed of collimator movement and gantry movement , the dose characteristics of the beam , etc . a treatment planning computer 128 processes the prescription and the machine characteristics according to known computational methods , to produce a treatment plan 130 . this sets out a sequence of gantry movements , collimator movements , beam intensities etc . which the apparatus can then follow in order to create a dose distribution in the patient which corresponds to the prescription . this treatment plan 130 is then passed to the apparatus control unit 132 . the control unit 132 is arranged to control the gantry drive motor 134 , the radiation source 136 , and the multi - leaf collimator (“ mlc ”) drives 138 . thus , once initiated by a clinician , the apparatus control unit 132 can deliver the radiotherapy treatment . in practice , the apparatus control unit 132 may comprise several sub - modules , each attending to different functional aspects of the apparatus , which may be distributed around the apparatus as required . also shown in fig8 is a link from the apparatus control unit 132 to the servo drive motors 140 of the epid linkages 122 ( fig7 ), thus allowing the apparatus control unit to control the x - y position of the epid 118 . this permits the apparatus control unit 132 to adjust the position of the epid within the beam aperture so as to maintain the beam within the bounds of the epid , thus obtaining an accurate image of the attenuated beam and also protecting the epid from the beam . this control can be done in one of two ways . a first way is to predict the necessary epid movements and adjust it accordingly . accordingly , the treatment planning computer 128 can use its a priori knowledge of the collimator positions during the treatment to calculate the required position of the epid 118 at each point during the treatment . this can be done using simple ray projection methods , either to determine where the beam will fall in the plane containing the epid , or to determine a correlation between epid positions and mlc leaves and positions , from either of which the necessary epid position for each collimator shape during the treatment can be determined . if a collimator shape is called for during treatment that is so large or unusual that the epid cannot accommodate it , then the system can either issue a warning to the clinician or can incorporate the epid size as an apparatus limitation within document 126 and then calculate or re - calculate the treatment plan , as necessary . the epid positions during treatment that are determined in this way can then be incorporated into the treatment plan 130 and passed to the apparatus control unit 132 . during the treatment , the apparatus control unit 132 can then control the epid drive motors 140 as required in order to achieve this . adjustments to the planned epid positions may of course be needed in view of any adjustments made to the planned collimator positions , such as to compensate for movement of the target . of course , the epid position calculations could be done by a separate module within the treatment planning computer 128 , or by a different computing element , or by the apparatus control unit 132 . in the latter case , the necessary epid positions could be calculated in real time while acting on the collimator positions contained in the treatment plan . the alternative control mechanism for the epid drive motors is a reactive feedback method which uses the images obtained from the epid in order to determine a necessary movement . this is shown in fig9 to 11 . fig9 shows an image 150 obtained from the epid ; the predominant feature in the epid is the shape 152 of the beam as shaped by the multi - leaf collimator . outside this outline 152 , the image is essentially dark other than any inevitable leakage ; within this outline there will be information as to the attenuation by the patient , but for the purposes of clarity this detail is omitted from fig9 to 11 . in fig9 , the beam shape 152 is in the centre of the epid aperture 154 and so no action needs to be taken . as the treatment progresses , with rotation of the gantry and adjustment of the collimator shape , the beam shape 152 may well move as shown in fig1 . the apparatus control unit 132 therefore monitors the image 150 to detect when the beam shape 152 reaches the edge of a pre - defined margin 156 around the edge of the image . fig1 shows that the beam shape 152 has moved sufficient to touch the edge of the margin 156 at 158 . the apparatus control unit 132 therefore instructs the servo drive motors 140 of the epid linkages 122 to move the epid in the appropriate direction ( s ) to bring the beam shape 152 back to the centre of the image as shown in fig1 . in this way , a negative feedback loop is created which serves to keep the beam shape 152 within the bounds of the image 150 , responding both to gross movements of the beam shape ( such as following rotation around an offset target ) and to changes in the beam shape that take it towards an edge ( such as due to reconfiguration of the collimator shape ). as illustrated in fig9 to 11 , the margin 156 is quite narrow . in practice , the choice of margin size will be a balance between factors , to be assessed by the skilled person . a narrow margin will reduce the number of movements of the epid , thus reducing wear on the epid drive motors and ( possibly ) reducing the complexity of correcting the images for position , but will require faster movement of the epid and less lag in initiating movement . equally , a larger margin means that the epid drive motors need not react as promptly or move as quickly , but raises the possibility of very large beam shapes causing problems if they approach the margin on two or more sides . one possibility is a variable margin , chosen according to how large the beam shape is , with smaller beam shapes implying a larger margin and large beam shapes prompting a relaxation of the margin size . in either case , the epid position is ideally recorded during the treatment , such as in conjunction with the images obtained from it . this then allows the images to be corrected for the epid offset . alternatively , the images could be processed in real time by offsetting them against ( say ) a plain background by an amount corresponding to the epid offset when they were captured . the saved set of images are then in a comparable format for later analysis . accordingly , the present invention allows the epid to be used for real - time diagnostic purposes , regardless of the type of treatment , and without having to make potentially detrimental adjustments to the treatment plan . it will of course be understood that many variations may be made to the above - described embodiment without departing from the scope of the present invention .