GB2522914A

GB2522914A - Image guided radiotherapy

Info

Publication number: GB2522914A
Application number: GB1402272.7A
Authority: GB
Inventors: Per Carlsson
Original assignee: Elekta AB
Current assignee: Elekta AB
Priority date: 2014-02-10
Filing date: 2014-02-10
Publication date: 2015-08-12
Also published as: GB201402272D0

Abstract

A radiotherapy apparatus comprising: a source rotatable about at least one axis; an associated imaging device; and a control means adapted to detect the position of a patient using the imaging device and to calculate a rotation of the patient around the axis relative to a predetermined position and to offset the source by an equal rotation. The source may rotate by at least 360 degrees. The apparatus may determine a rotational difference an insert a corresponding offset into the source rotation. The apparatus may comprise a treatment planning computer that accepts a patient image where the predetermined position is a position of the patient in the image and where the image may be a three dimensional (3D) CT or MRI scan. The imaging device and the source may both be mounted on a rotatable gantry. The imaging device may be integrated with the source. Preferably the control means determines the rotation by comparing the image with a further image of the patient.

Description

Image Guided Radiotherapy

FIELD OF THE INVENTION

The present invention relates to image guided radiotherapy.

BACKGROUND ART

Radiotherapy is a process for the treatment of lesions such as cancers, which involves directing a beam of ionising radiation (typically high-energy x-rays) towards the lesion. Care is taken to maximise the dose that is applied to the lesion and minimise the dose that is applied to the areas of healthy tissue around the lesion, mainly by directing the beam toward the lesion from a multitude of directions and collimating the beam (i.e. shaping its cross-section) as required depending on the shape of the lesion. Various protocols exist for determining the beam directions, shapes, strengths and times to deliver a specific three-dimensional dose distribution to a specific region of the patient.

Clearly, it is necessary to ensure that the dose distribution is positioned correctly within the patient, i.e. that the patient and the radiotherapy apparatus are correctly aligned.

To do this, it is now common to take a cone-beam CT scan ("CBCT") before treatment starts, and use that scan to compare the actual patient position with the treatment coordinate system to detect any mismatch. That is measured and calculated in 6 degrees of freedom, and the patient position is adjusted so as to match that which is expected by the treatment co-ordinate system. For this purpose, patient table are known which are adjustable in 6 degrees of freedom, i.e. three translational directions and three rotational directions. This allows the patient to be brought into register with the apparatus, placing the lesion at the location expected by the apparatus and at which the dose distribution will be delivered.

SUMMARY OF THE INVENTION

A table with 6 degrees of freedom imposes a significant cost on the apparatus, and S inevitably has a redundant axis. The provision for multiple degrees of freedom also adds to the volume of the table and its supporting structure, which can give clearance problems during treatment.

It is also inherently undesirable to rotate the patient on the table. Significant rotations are disconcerting for the patient, and any rotation will result in gravity acting differently on the patient, which could give rise to positional deviations in the patient anatomy relative to the bed.

The present invention allows a positional error of the patient consisting of a rotation around an axis about which the source can also rotate around to be corrected without needing a rotational adjustment of the table. The invention provides a radiotherapy apparatus comprising a source able to rotate about at least one axis, and an associated imaging device, both under the control of a control means and wherein the control means is adapted to detect the patient position using the imaging device, calculate a rotation of the patient around the axis relative to a predetermined position, and offset the source by an equal rotation. This is particularly useful where the source can rotate by at least 360 degrees. Essentially, the apparatus determines a rotational difference and inserts a corresponding offset into the source rotation.

This can be combined with a treatment planning computer that accepts an image of the patient and creates a treatment plan based thereon, in which case the predetermined position can be a position of the patient in the image. The image of the patient can be a CT scan or such other scan as is useful and available, such as MRI or ultrasound. The image can be obtained via the imaging device or via a separate imaging system.

The imaging device is preferably integrated with the source, and they are ideally mounted on a rotatable gantry. If they are not integrated, it is at least preferred that both are mounted on the same rotatable gantry.

The control means preferably determines the rotation by obtaining a further image of the patient using the imaging device; that further image can be a CT scan or can be via another modality such as one of those noted above. The control means can determine the rotation of the patient by comparing the image and the further image.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention will now be described by way of example, with reference to the accompanying figures in which; Figure 1 shows a radiotherapy apparatus according to W02005/041774, being one example of the type of apparatus to which the present invention can be applied; Figure 2 shows the apparatus of figure 1, in a horizontal section; Figure 3 shows the apparatus of figure 1 in a horizontal section, after rotation of the radiation source in a horizontal plane; Figure 4 shows the apparatus of figure 1, in a vertical section; Figure 5 shows the apparatus of figure 1 in a vertical section, after rotation of the radiation source in a vertical plane; Figure 6 shows a front view of an alternative form of radiotherapy apparatus for which the present invention is suitable; Figure 7 shows a side view of the radiotherapy apparatus of figure 6; and Figure 8 shows a process flowchart for the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Figure 1 illustrates the radiotherapy apparatus described in WO20051041774, to which the reader is referred for a fuller description. In brief, the apparatus 10 comprises a substantial vertical circular outer ring structure 12 which is rigidly fixed to a floor structure in order to provide a firm base. An inner ring 14 is journalled within the outer ring 12 so that it can rotate about a horizontal axis, transverse to the outer ring 12 and passing through the centre of the circular ring. A U-profile support 16 is attached at either end to the inner ring 14 and extends out of the plane of the inner ring 14.

A linear accelerator 18 is supported on a mounting 20 that is carried by the U-profile support 16. The mounting 20 is attached to the support 16 via a pair of pivot connections 22, defined on opposing arms of the U-profile support 16 that extend out of the plane of the inner ring 14. These allow the mounting 20 to rotate relative to the support 16 about an axis that is perpendicular to the horizontal axis of the inner ring 14, and offset from the inner ring 14. The orientation of the axis in the vertical plane will of course depend on the instantaneous rotational position of the inner ring 14. The two axes -i.e. the perpendicular axis of the mounting and the horizontal axis of the inner ring -meet at a defined point, which remains fixed due to the geometry described above. The linear accelerator 18 is attached to the mounting 20 and directs a beam of radiation towards the defined point.

Thus, it will be appreciated that whatever the rotational positions of the inner ring 14 and the mounting 20, the beam will be directed towards the defined point. However, the direction from which the beam arrives at the defined point will be dictated by those rotational positions.

Electrical motors are provided in order to drive the inner ring 14 relative to the outer ring 12, and the mounting 20 relative to the support 16. These are servo-controlled so that specific chosen positions can be instructed and maintained.

Figures 2 and 3 show the apparatus from above, with the inner ring 14 oriented so that the perpendicular axis is vertical and thus aligned with the view direction. Figure 2 shows the mounting 20 at its extreme anti-clockwise position and figure 3 at its extreme clockwise position. Figures 2 and 3 also show the beam stop 22 provided on the mounting opposite the linear accelerator 18 in order to absorb the beam 24 that is produced and prevent it from escaping the apparatus.

Figure 4 shows the apparatus from one side. Relative to the state shown in figures 2 and 3, the inner ring 14 has been rotated through 90° in order to place the perpendicular axis 22 horizontal, i.e. into and out of the page. Figure 4 shows the apparatus with the linear accelerator 18 at its highest point, emitting a near-vertical beam 24 through the patient's head and into the beam stop 22. Figure 5 shows the apparatus with the mounting rotated through a maximum angle of about 45°.

Other types of radiotherapy apparatus can also benefit from the present invention.

For example, a rotatable C-arm structure may be used, or a rotateable gantry as shown in figures 5 and 6.

Figures 6 and 7 show an alternative radiotherapy system for which the present invention is suitable. Structurally, the system is similar to a conventional radiotherapy apparatus. The system comprises a gantry 110 on which is mounted a source of radiation 112 and, diametrically opposite the source 112, a radiation detector 116. Such detectors are commonly referred to as portal imagers and may be adapted to detect high-energy x-rays of the type used for therapy, or lower-energy diagnostic x-rays that some sources 112 are capable of emitting as an alternative to therapeutic radiation. Where the source 112 is only able to emit therapeutic radiation, the gantry 110 often carries a second diagnostic source, positioned at an angular spacing of about 90° from the therapeutic source. This emits a low-energy beam orthogonal to the therapeutic beam, which is detected after passing through the patient by a diagnostic imager positioned opposite the diagnostic source. The radiation source 112 is typically a linear accelerator producing x-rays or other penetrating radiation.

Control and processing circuitry 126 is in communication with the gantry 110, the source 112 and the collimator 114 and controls their operation.

The gantry is rotatable about an axis 122. In figures 6 and 7, the gantry 110 is depicted as a disc-shaped support, but in practice there will usually be a substantial structure behind the illustrated disc in order to provide balance for the projecting arm 128 carrying the source 112 and for the imager 116, and to allow rotation of the gantry under control of the control and processing circuitry 126. The isocentre of the system is defined as a plane running through the rotation axis 122 of the gantry 110 perpendicular to the instantaneous axis of the radiation beam.

A collimator 114 is coupled to the radiation source 112 in order to collimate and shape the radiation beam. That is, a first collimation of the radiation (not illustrated) takes place close to the source 112. This collimates the radiation produced by the source into a beam, e.g. a cone-or fan-shaped beam diverging away from the source. A further collimator 114 then acts on this collimated beam in order to shape the radiation as required for therapy. An example of a suitable collimator for this aspect is a multi-leaf collimator (MLC). Such devices comprise one or more banks of parallel leaves, each of which can be moved in a direction transverse to the radiation beam axis. The leaves are moveable into and out of the path of the radiation beam to a greater or lesser extent, and thus the combination of leaf positions collectively defines a shaped aperture through which radiation passes. In one embodiment, the MLC comprises two banks of leaves positioned on opposite sides of the radiation beam, with each leaf able to take any position with a range from outside the radiation beam to crossing the radiation beam. In order to sufficiently attenuate (i.e. block) the high-energy radiation, the leaves have a significant depth in a direction parallel to the radiation beam axis, and are manufactured from high atomic number materials such as tungsten. Thus, the output of the radiation source 112 and the collimator 114 is a shaped radiation beam 124 directed generally inwards towards the axis of rotation of the gantry.

A patient 120 is positioned on a support 118 for treatment such that a treatment target 121 (e.g. a tumour) is placed at the isocentre of the system. The longitudinal axis of the support and, thus, the patient 120 usually but not necessarily lie substantially parallel to the rotation axis 122 of the gantry. Various processes and apparatus for positioning and locating the patient will be familiar to those skilled in the art. In one embodiment, the support allows linear translation of the patient in three dimensions.

It should be emphasised, however, that the invention is not limited to the above-described structures, which may be varied in many ways or replaced entirely with a different structure.

In theory, the patient is then placed on the patient support and the dose is applied by the radiotherapy apparatus. However, in practice there is often a delay between the CT scan being taken and the treatment commencing. Also, the CT scanning apparatus with which the scan was prepared may not be the same apparatus as is used for delivery of the radiotherapy. Thus, the patient will have left the apparatus on which the scan was taken and will have been re-positioned ready for treatment. Some difference in positioning as between the two episodes is likely or inevitable. As noted above, this is usually corrected by way of a moveable patient support that is able to adjust the position of the patient in six degrees of freedom in order to correct for positional errors. To determine the correction that is needed, a brief CT or x-ray image is taken of the patient immediately prior to treatment, or a visual determination is made of the patient position using (for example) illuminated markers on the patient's anatomy, or another means is used to determine the patient position. This is then compared to the position of the patient in the original CT scan used for treatment planning, and a displacement is obtained. The correction is then simply the reverse of that displacement so as to return the patient to the position for which the treatment was planned.

According to the invention, rotational errors around an axis that corresponds to a rotational axis of the radiotherapy apparatus are instead corrected by offsetting the radiotherapy apparatus during treatment. As is apparent from the above descriptions, in both cases the radiation source is arranged so that it can rotate freely around the patients Z axis 122. In practice there may be a maximum number of rotations that can be made in one direction at a time, but the ability to rotate at least 360 degrees means that (in terms of the control of the device) rotation is effectively free. The first embodiment (figures 1 to 5) can also rotate or move around the iso center in other axes, but rotation in those axes is limited.

Figure 8 shows the process sequence according to the present invention, applicable regardless of the type of radiotherapy apparatus to which it is applied. A reference image is taken of the patient (step 150), which might be via any suitable modality such as CT, MRI, ultrasound or the like, and may be performed using a standalone medical imaging apparatus or an imaging function integrated with the radiotherapy apparatus. This is used to prepare a treatment plan via a suitable treatment planning computer (step 152). A treatment plan of this type typically consists of a sequence of positions for the source 18, 112, defined as angular rotations relative to a datum point, and beam shapes and durations emitted while the source is at each of those sequential positions. The patient is then positioned on a patient support or couch of the radiotherapy apparatus (step 154).

In general, clinical staff seek to position the patient accurately on the couch, and various technologies exist to assist in the regard. Examples include mouldable cushions, visual markers, bite posts and the like. However, it is impossible to rule out any error in the precise positioning of the patient, and (depending on the manner in which the patient is positioned) some error is likely. In addition, the target structure within the patient may have moved, for example due to movement of the internal organs of the patient; such movement will be referred to herein as movement of "the patient", which should be read as including either or both such types of movement.

Therefore, the actual position of the patient is determined (step 156), which may be via an imaging function integrated into the radiotherapy apparatus or otherwise. This position is compared with the position of the patient in the reference image on which the treatment plan was based (step 158) and a six-dimensional displacement of the patient relative to the reference image is determined (step 160). This six-dimensional displacement consists of three translational components d, d, d and three rotational components r, ry, r, defining the x, y and z axes as shown in figures 6 and 7 such that the z axis is horizontally along the cranio-cordal axis of the patient, y is vertically upwards, and x is horizontal, transverse to the cranio-cordal axis and orthogonal to y and z. Thus, a translational displacement of d is a translation along the x axis, a rotational displacement of r is a rotation around the x axis, and mutatis mutandis for dy, d7 and r r7.

It is then a straightforward matter to translate the couch (and hence the patient) in order to correct for d, dy and d7 (step 162). It is also straightforward to rotate the couch about the y (vertical) axis in order to correct ry (step 164), as movement in this plane does not affect the plane of the couch and is therefore relatively easy to engineer into the couch.

The r displacement may be correctable via the present invention or by other means, dependent on the geometry of the radiotherapy apparatus in question.

The r displacement (and/or any rotational displacement around an axis for which the radiotherapy apparatus is able to rotate freely) is then corrected at step 166 by adding the value of r7 to each of the rotational positions around that axis of the source (and hence the beam). Thus, instead of trying to correct the patient back toward the apparatus, the apparatus is adjusted to take account of the actual position of the patient. The treatment plan can then be delivered (step 168).

The value of r can be added to each of the rotational positions of the source by recalculating the positions in the treatment plan and then delivering the revised plan, or by delivering the rotational error as an input to the control system 126 and correcting the values locally. This adjustment is much cheaper to achieve than to engineer the corresponding 6 degrees of freedom into the couch, such as via a hexapod.

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.

Claims

CLAIMS1. A radiotherapy apparatus comprising a source able to rotate about at least one axis, and an associated imaging device, both under the control of a control means and wherein the control means is adapted to detect the patient position using the imaging device, calculate a rotation of the patient around the axis relative to a predetermined position, and offset the source by an equal rotation.
2. A radiotherapy apparatus according to claim 1 in which the source can rotate by at least 360 degrees.
3. the apparatus essentially determines a rotational difference and inserts a corresponding offset into the source rotation.
4. A radiotherapy apparatus according to any one of the preceding claims, further comprising a treatment planning computer that accepts an image of the patient and creates a treatment plan based thereon, and wherein the predetermined position is a position of the patient in the image.
5. Apparatus according to claim 4 in which the image of the patient is a one of a CT scan or NIRI scan.
6. Apparatus according to claim 5 in which the image of the patient is three-dimensional.
7. Apparatus according to claim 5 or claim 6 in which the image of a patient is obtained via the imaging device.
8. Apparatus according to any one of the preceding claims in which the imaging device is integrated with the source.
9. Apparatus according to any one of the preceding claims in which the source and the imaging device are both mounted on a rotatable gantry.
10. Apparatus according to claim 9 in which the source and the imaging device are mounted on the same rotatable gantry.
11. Apparatus according to any one of the preceding claims in which the control means determines the rotation by obtaining a further image of the patient using the imaging device.
12. Apparatus according to claim 11 in which the further image of a patient is one of a CT, cone-beam CT, or MRI scan.
13. Apparatus according to claim 12 in which the further image of the patient is three-dimensional.
14. Apparatus according to any one of claims 11 to 13 in which the control means determines the rotation of the patient by comparing the image and the further image.
15. Apparatus for treatment by radiotherapy substantially as described herein with reference to the accompanying figures.