GB2537120A

GB2537120A - Radiotherapy apparatus

Info

Publication number: GB2537120A
Application number: GB1505877.9A
Authority: GB
Inventors: Lundin Anders; Carlsson Per
Original assignee: Elekta AB
Current assignee: Elekta AB
Priority date: 2015-04-07
Filing date: 2015-04-07
Publication date: 2016-10-12
Also published as: GB201505877D0

Abstract

A radiotherapy apparatus comprising a gantry 126 rotatable around a rotation axis; a radiation source supported by the gantry, emitting a conical beam of therapeutic radiation 116 centred on a beam axis from a point offset from the rotation axis and directed towards the rotation axis; and where the source is fixedly supported within the gantry and the beam axis approaches the rotation axis at an acute angle. The acute angle may be between 30 & 80 degrees. Preferably the sum of the acute angle and half the aperture angle of the conical beam is no greater than 90 degrees. The source may emit the beam from an arm 110 extending from the gantry and the arm may also carry a source of diagnostic radiation 118. The apparatus may further comprise a detector supported on an arm extending from a location on the gantry that is opposite the source. The radiation source may include a multi-leaf collimator 132 for limiting the cross-section of the conical beam to a desired shape.

Description

Radiotherapy Apparatus

FIELD OF THE INVENTION

The present invention relates to radiotherapy apparatus.

BACKGROUND ART

External-beam radiotherapy apparatus usually comprises a source of high-energy x-radiation which is mounted on a gantry that is rotatable around an axis, usually generally horizontal. The radiation source is mounted at a location which is offset from the axis, and oriented to emit a beam of radiation perpendicularly towards the axis. Thus, as the gantry rotates, the convergence point of the beam and the axis (the so-called "isocentre") remains within the beam at all times. The beam approaches the isocentre from (potentially) every direction within a generally vertical plane perpendicular to the axis.

In this way, the dose delivered to the isocentre will be much higher than the dose delivered to the surrounding volume, as the isocentre remains in the beam for (in principle) the entire duration of the treatment whereas surrounding volumes are only in the beam at particular gantry angles. A therapeutic dose can thus be delivered to a tumour or other lesion placed at the isocentre, whilst minimising the dose delivered to surrounding healthy tissue.

Usually, the radiation source is provided with collimators to limit the transverse width of the beam and thus shape the radiation dose that is delivered. Such collimators include block collimators which have a straight edge, and multi-leaf collimators (such as that shown in EP-A-314,214) which can define a chosen shape or conical collimators that are fixed and usually shape the beam to a circle.

Radiotherapy systems with heads that tilt in and out of the generally vertical plane have been proposed, in order to provide specific treatment patterns. Our application W099/34866 described a system with a tilting head that could be set to a selected angle out of the generally vertical plane, as required. Subsequently, we proposed an apparatus in W02005/041774 for neurological use which allowed the source to rotate on two axes, thus one axis defined the angle of a cone whose vertex lay on the isocentre and along the surface of which the beam was directed, and the other axis rotated the beam around the surface of the cone.

US 7,564,945 discloses a radiotherapy apparatus with a rotating gantry supporting a therapeutic source, flanked on either side by rotating gantries carrying a diagnostic source and a diagnostic detector. The source and detector are angled out of the plane of their rotation so that they view the isocentre illuminated by the therapeutic source. In this way, the diagnostic source and detector can operate independently of the therapeutic source and provide continuous cone-beam CT scanning during treatment.

SUMMARY OF THE INVENTION

The beam geometry of conventional systems is thus in a circular shape, with the incoming ray from the source at one orientation coinciding with the outgoing ray when the source is at an orientation that is 180° apart. Both beams occupy the same line in the transverse (xy) plane in which the beam lies, and thus the dose fall-off around the target volume within the xy plane is not so rapid. The dose fall-off in z direction perpedicular to the xy plane is higher, as there are no overlapping beams in this direction.

The low dose fall off in the xy plane is not satisfactory in some circumstances, such as treating multiple metastasis, especially in the brain, where it is important to further lower the doses delivered to non-target areas. Previously, this has been done by making the beams come from more directions, for example by rotating the patient couch during treatment or by employing a more complex apparatus offering rotation of the beam in multiple axes. These solutions, however, require a more cumbersome workflow and/or greater expense.

The present invention therefore provides a radiotherapy apparatus, comprising a gantry that is rotatable around a rotation axis, a radiation source supported by the gantry and emitting a conical beam of therapeutic radiation, centred on a beam axis, from a point offset from the rotation axis and directed towards the rotation axis, wherein the source is fixedly supported within the gantry and the beam axis approaches the rotation axis at an acute angle.

The result is that the coincidence of opposite-directed beams is avoided, thus delivering a greater dose fall-off within the xy plane albeit with a reduced fall-off in the z direction. Overall, however, the risk of side-effects due to dose fall-off can be expected to be reduced.

The acute angle B (theta) in figure 1, is preferably between 30° and 80°. We also prefer that the sum of the acute angle and half the aperture angle of the conical beam is less than or equal to 90° in total; this will have the effect that there is no part of the therapeutic beam that approaches the rotation axis beyond the perpendicular.

The source ideally emits the beam from an arm that extends from the gantry. The same arm can also carry a source of diagnostic radiation, which can be located on the arm at a point spaced further from the gantry and/or at slightly different angle in the rotation direction than the location of the therapeutic source. The source of diagnostic radiation preferably emits a diagnostic beam in a direction towards an intersection of the therapeutic beam and the rotation axis. This arrangement allows the diagnostic beam to view the patient at the same instantaneous angle as the therapeutic source, unlike existing devices which typically offset the diagnostic beam within the xy plane by a convenient angle such as 90°.

In embodiments of the invention that allow continuous and/or high-speed rotation around the patient, it may be preferable to offset the diagnostic source by a small rotational angle (as noted above) of up to (for example) about 10 or 20 degrees ahead of the therapeutic source. Thus, a small time delay is allowed for between capturing the diagnostic image from a specific rotational angle and position and delivery of the therapeutic beam along that angle and direction, to permit detection of displacements and adjustment of the therapeutic beam accordingly.

The gantry can also carry a detector, which can be supported on an arm extending from a location on the gantry that is opposite the source. This can be positioned in view of both the beam of therapeutic radiation and the beam of diagnostic radiation (if one is employed), thereby allowing a single detector to be used for both beams.

The detector can be supported on one end of a C-arm attached to the gantry, with the other end of the C-arm providing support for the diagnostic source and/or the source of therapeutic radiation.

The radiation source can include a multi-leaf collimator, such as that illustrated in EP-A-314214, for limiting the cross-section of the conical beam to a desired shape.

Ideally, the beam axis will intersect with the rotation axis at a specific point, which can then be identified as the isocentre of the apparatus. Having an identified isocentre assists with treatment planning and patient setup. Also, the rotation axis is often generally horizontal as that permits treatment of most patients in a supine position.

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 illustrates the beam geometry of a device according to the present invention; Figure 2 illustrates angular relationships within the treatment beam; Figure 3 illustrates the device from one side; Figure 4 illustrates the device of figure 2 with cosmetic covers removed; Figures 5 illustrates the device of figure 3 in an alternative disposition; and Figure 6 illustrates the device of figures 2 and 3 from above, in a still further disposition.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Figure 1 shows the beam geometry of the preferred form of the present invention. A source of therapeutic radiation 10 is mounted on a gantry (not shown in figure 1) so as to be rotatable around a horizontal rotation axis 12. The source 10 is offset from the rotation axis 12, and is oriented so that the beam 14 that it emits is directed towards the rotation axis at an angle 0 which is less than 90°, meeting the rotation axis at an isocentre 16. As the gantry rotates, it carries with it the source 10 which maintains its orientation relative to the rotation axis 12; thus after a 180° rotation the source 10 is in the position shown in dotted lines at 10', emitting a beam 14'.

The rotated beam 14' crosses the rotation axis at the same isocentre 16, approaching at the same angle 8, and thus a lesion located at the isocentre 16 will remain in the beam during the process of rotation. However, as a result of 0 being less than 90°, the incoming part 18 of the beam 14 does not coincide with the outgoing part 20 of the rotated beam 14'. Instead, they irradiate different spatial volumes, as opposed to a conventional apparatus where 0=90° and the two volumes would overlap in the vertical xy plane, i.e. one perpendicular to the rotation (z) axis 12 and passing through the isocentre 16. Thus, the dose delivered to these surrounding tissues is distributed more widely; approximately twice the volume of tissue is irradiated, but each unit volume only receives about half the dose.

Figure 2 illustrates the angular relationships within the treatment beam. The source 10, the rotation axis 12, and the beam 14 are shown, with the angles amplified relative to figure 1 for the purpose of clarity. As in figure 1, the beam centreline 16 approaches the rotation axis 12 at an angle 0 which is between 0 and 90°. The beam itself has a divergence angle of a, which corresponds to a maximum usable aperture of the beam. It will be appreciated by those skilled in the art that the actual divergence of the beam at any chosen moment during a treatment will likely be less than a, as the beam will be collimated by block collimators, and/or multi-leaf collimators, and/or other colllimation apparatus such as cones for example, to an angle which ensures the delivery of the desired radiation dose needed by the patient in question. However, the beam will have a range of potential apertures, up to a maximum angle of a. This maximum value may be dictated by one or more factors within the apparatus, such as the angle of the primary collimator adjacent the x-ray target, or a maximum aperture of the various adjustable (or other) collimators within the source 10, or by limits placed on those collimators through its controlling software, or user operating procedures, or the like.

It will be appreciated that the application of straightforward geometrical principles shows that if 0 + a/2 90° then the innermost edge of the radiation beam cannot extend beyond the vertical. At this point where 0 + a/2 = 90°, the beam segment 18 (figure 1) will be just adjacent the beam segment 20, i.e. the closest they can reach without overlapping. For small fields, a can be likewise very small in which case 0 may be close to 90°). This therefore provides a limit point to the geometry of preferred embodiments of the invention. In general, provided that the approach angle B and the divergence angle a are controlled so that B + a/2 90° then overlapping will be prevented.

Figure 3 shows the layout of a practical radiotherapy apparatus implementing this geometry. A patient 100 is supported on a patient table 102 in a generally horizontal manner; this allows the patient 100 to recline during the treatment and maintain a substantially constant pose. The table 102 is mounted on an adjustment mechanism 104 which allows the table 102 to be raised and lowered, translated forwards and backwards, from side to side, and rotated within limits in three axes. This allows the patient 100 to lie on the table 102 and be moved into a desired position so that the tumour (or the like) is located substantialy at the isocentre 16. Such adjustment mechanisms are generally known and used in this field and will be familiar to those of skill in the art.

The treatment apparatus 106 is concealed behind covers 108, which ensure the apparatus is cosmetically acceptable and provide a degree of health & safety protection by concealing some moving parts within the apparatus that could otherwise present a hazard. Two main operating arms extend from the main part of the building, a source arm 110 and an imaging arm 112. Both arms are mounted on a rotating gantry (not visible in figure 3) which is rotatable around a horizontal axis so as to contribute to the geometry described in relation to figure 1 above.

The source arm 110 includes a source of therapeutic radiation 114, which emits a beam 116 of high-energy radiation (typically in excess of 1MeV). The beam 116 is directed towards the isocentre 16 and the rotation axis, and approaches it at the acute angle described in relation to figure 1. The beam has a maximum aperture defined by a primary collimator (not shown) which satisfies the relationship described with reference to figure 2. In addition, adjustable or different fixed collimators are provided to further limit the beam width as required for a specific treatment plan. A separate diagnostic x-ray source 118 is also mounted in the source arm 110, located at an end of the arm 110 and thus further from the gantry than the therapeutic source 114. It emits a low-energy (i.e. 1 to 150keV) diagnostic beam 120, again directed towards the isocentre 16 at which point it crosses the rotation axis and the therapeutic beam 116. In this embodiment, the diagnostic source 118 is spaced sufficiently further from the gantry that the diagnostic beam is angled towards the gantry in order to cross the isocentre, but this need not be the case in general. Indeed, the very much lower dose imparted by the diagnostic beam means that this beam need not be at an acute angle to the rotation axis and could approach it perpendicularly. However, our preferred arrangement is that illustrated, in which the diagnostic beam approaches the rotation axis at an acute angle (which could also be the same as theta but at a different rotation angle, as noted earlier).

The imaging arm 112 carries a flat panel imager 122, located diametrically opposite the two radiation sources 114, 118. To facilitate this, the imaging arm 112 and the source arm 110 are located on the gantry at positions that are 180° apart relative to the rotation axis of the gantry. This places the two arms on opposite sides of the rotation axis, allowing the imaging arm 112 to position the flat panel imager 122 in the path of the two beams 116, 120 after they have exited the patient 100. As the beams cross at the isocentre 16 on the rotation axis, they will have diverged to an extent by the time they reach the flat panel imager 122, so the panel can either be made large enough to accommodate both, or can be made up of a number of sub-panels, or can be movable between the required positions. As an alternative, the panel 122 could just monitor the diagnostic source, but the portal images obtainable by imaging the therapeutic beam after attenuation by the patient are often useful and we prefer to capture them.

Figure 4 shows the apparatus with the cosmetic covers 108 removed. A patient 100 is shown for reference purposes, although the apparatus would not normally be used for delivery of a treatment in the absence of the covers 108. A supporting chassis 124 is firmly mounted on the floor 125 and carries the rotatable gantry 126 via suitable bearings (not visible). A drive motor (not visible) controls rotation of the gantry 126 and can rotate it to a desired angle at a desired speed, or enable continuous rotation; each will be dictated by the treatment plan being put into effect at the time.

A linear accelerator 128 is disposed within the gantry 126 and extends outwardly and forwardly of the gantry 126. This generates a relativistic beam of electrons (in a generally known manner) which it delivers to an x-ray target structure 130 to create the therapeutic beam 116. Collimators 132 including block collimators and a multi-leaf collimator and possibly cones serve to delimit the transverse width of the beam as required by the treatment plan; figure 4 shows the collimators 132 but (for clarity) not their supporting structures.

The diagnostic source 118 is also positioned on the source arm 110, beyond the bulky x-ray target and collimation structure, to produce the diagnostic beam 120 as described above.

The imaging arm 112 carries the flat panel imager 122, as described above. In addition, radiopaque material 134 is provided behind the flat panel imager in order to absorb the therapeutic beam 116 and the diagnostic beam 120.

Figure 5 shows the apparatus of figure 4 after rotation of the gantry 126 through 180°, thus taking up the orientation shown in figure 3 in which the sources 114, 118 are beneath the patient 100 and the two beams 116, 120 are emitted upwardly and obliquely towards the patient 100. Figure 6 illustrates, from above, the apparatus of figures 4 and 5 after a rotation of 90° to one in which the sources 114, 118 are to one side of and substantially level with the patient 100. It will be appreciated that the gantry 126 can occupy any rotational position, allowing free rotation around the patient 100.

Thus, by tilting the linac head by (for example) 30 to 40 degrees and still allowing rotation around the Z-axis, the isocentre is maintained but the inlet ray and outlet ray do not coincide. The beam geometry is then shaping a cone on both the beam inlet and beam outlet side, and thereby spreads the ray out over space while still aiming at the same point, the isocentre. The mechanical arrangements and controls can be kept at the same level as a standard linac, ie stationary with just one rotational axis, but with much better low dose levels and dosimetry performance.

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. Radiotherapy apparatus, comprising: a gantry, rotatable around a rotation axis, a radiation source supported by the gantry, emitting a conical beam of therapeutic radiation centred on a beam axis, from a point offset from the rotation axis and directed towards the rotation axis, wherein the source is fixedly supported within the gantry and the beam axis approaches the rotation axis at an acute angle.
2. Radiotherapy apparatus according to claim 1 in which the acute angle is between 30° and 80°.
3. Radiotherapy apparatus according to claim 1 or claim 2 in which the sum of the acute angle and half the aperture angle of the conical beam is no greater than 90° in total.
4. Radiotherapy apparatus according to any one of the preceding claims in which the source emits the beam from an arm that extends from the gantry.
5. Radiotherapy apparatus according to claim 4 in which the arm also carries a source of diagnostic radiation.
6. Radiotherapy apparatus according to claim 5 in which the diagnostic source is located along the arm at a point spaced further from the gantry than the location of the therapeutic source.
7. Radiotherapy apparatus according to claim 5 or claim 6 in which the diagnostic source is located on the arm at a point that is displaced rotationally around the rotation axis relative to the location of the therapeutic source.
8. Radiotherapy apparatus according to any one of claims 5 to 7 in which the source of diagnostic radiation emits a diagnostic beam in a direction towards an intersection of the therapeutic beam and the rotation axis.
9. Radiotherapy apparatus according to any one of the preceding claims in which the gantry also carries a detector.
10. Radiotherapy apparatus according to claim 8 in which the detector is supported on an arm extending from a location on the gantry that is opposite the source.
11. Radiotherapy apparatus according to any one of claims 5 to 7 in which the gantry also carries a detector, supported on an arm extending from a location on the gantry that is opposite the source, positioned in view of both the beam of therapeutic radiation and the beam of diagnostic radiation.
12. Radiotherapy apparatus according to any one of the preceding claims in which the detector is supported on one end of a C-arm attached to the gantry, and the other end of the C-arm provides support for the diagnostic source.
13. Radiotherapy apparatus according to claim 11 in which the other end also provides support for the source of therapeutic radiation.
14. Radiotherapy apparatus according to any one of the preceding claims in which the radiation source includes a multi-leaf collimator for limiting the cross-section of the conical beam to a desired shape.
15. Radiotherapy apparatus according to any one of the preceding claims in which the beam axis intersects with the rotation axis.
16. Radiotherapy apparatus according to any one of the preceding claims in which the rotation axis is generally horizontal.
17. Radiotherapy apparatus substantially as herein described with reference to and/or as illustrated in the accompanying figures.