GB2598904A

GB2598904A - Beam shaping apparatus

Info

Publication number: GB2598904A
Application number: GB2014611.4A
Authority: GB
Inventors: Broad Martin
Original assignee: Elekta Ltd
Current assignee: Elekta Ltd
Priority date: 2020-09-16
Filing date: 2020-09-16
Publication date: 2022-03-23
Anticipated expiration: 2040-09-16
Also published as: GB202014611D0; GB2598904B

Abstract

A radiation head for a radiotherapy device has a source of radiation configured to emit a beam 20 of radiation; and a beam shaping device for collimating the beam of radiation. The beam shaping device comprises a diaphragm 16 with a diaphragm block movable along a curved path, the diaphragm block having a flat face focused on a focus point which is offset from the source of radiation. Preferably, the diaphragm is below the source of radiation and the focus point is above the source of radiation. A multi-leaf collimator 12 may be positioned below the diaphragm. A pair of diaphragm blocks may be slidably attached to a curved rail which defines the curved path. Offsetting the focal point of the beam from the source alters the characteristics of the penumbra.

Description

Beam shaping apparatus

FIELD

The present disclosure relates to a beam shaping apparatus for a radiotherapy device, and to a radiotherapy device comprising a beam shaping apparatus.

BACKGROUND

Radiotherapy involves the production of a beam of ionising radiation, usually x-rays or a beam of electrons or other sub-atomic particles. This is directed towards a cancerous region of a patient (e.g. a tumour), and adversely affects the cancerous cells, thereby reducing the prevalence thereof. The beam is delimited so that the radiation dose is maximised in the cancerous cells and minimised in healthy cells of the patient, as this improves the efficiency of treatment and reduces the side effects in a patient.

In a radiotherapy apparatus the beam can be delimited using a beam shaping apparatus which defines an aperture of variable shape to collimate the radiation beam to a chosen cross-sectional shape. A beam shaping apparatus can be formed by a combination of a diaphragm and a 'multi-leaf collimator' (MLC).

A multi-leaf collimator includes a plurality of leaves, each leaf being movable longitudinally so that its tip, or leading edge, can be extended into or withdrawn from the radiation beam. A multi-leaf collimator may include two opposing banks of leaves arranged face-to-face to narrow the aperture from opposing sides. The array of leaf tips can thus be positioned so as to define a variable edge to the collimator.

A diaphragm includes a solid block of radiopaque material such as tungsten, which has a front edge that spans the entire width of the device's aperture, and which can be advanced and/or withdrawn across the aperture in a direction transverse to the front edge. A diaphragm may include two opposing diaphragm blocks which narrow the aperture from opposing sides, with the effect of adjusting the aperture as needed.

Usually, an aperture will be collimated by a pair of opposed diaphragm blocks operating in one direction (e.g. the x direction) and a pair of opposed multi-leaf collimator banks operating in the transverse direction (y direction), both directions being transverse to that of the beam (the z direction).

SUMMARY

Aspects and features of the present invention are described in the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments are described below by way of example only and with reference to the accompanying drawings in which: Figure 1 illustrates a radiation beam shaped by a diaphragm and a multi-leaf collimator, viewed along the beam axis; Figure 2 illustrates a beam shaping apparatus according to the present disclosure; Figure 3 illustrates a diaphragm according to the present disclosure-and Figure 4 illustrates the penumbra created in a beam of radiation.

OVERVIEW

It is desirable to provide a compact radiation head to fit in existing medical bunkers. Positioning the diaphragm in the space between the multi-leaf collimator and the radiation source reduces the height of the radiation head. However, having the diaphragm positioned closer to the source of radiation increases the penumbra caused by the diaphragm.

To provide a consistent size of penumbra, the diaphragm moves on a curved path. To minimise the penumbra at wider angles the faces of the diaphragm are focussed on a point which is offset from the source of radiation. This increases the distance between the source of radiation and the collimating edge at larger angles, hence reducing the penumbra.

SPECIFIC DESCRIPTION OF CERTAIN EXAMPLE EMBODIMENTS

As explained in the background section, beam shaping apparatuses are used to define an aperture to collimate a beam of radiation to a defined shape. The aperture may be created by a multi-leaf collimator and a diaphragm.

A view along the beam axis of an aperture 10 formed by a beam shaping device is shown in Figure 1. The beam shaping device allows transmission of a beam which has a desired cross-section and provides complete shielding across the remainder of the beam field. The beam field is the maximum extent of the beam's cross section at any point along the beam axis. A multi-leaf collimator (MLC) 12 comprises a series of individually movable leaves of a radiopaque material such as tungsten, arranged side-by-side and movable relative to each other, in two opposing arrays 12a, 12b. The leaves are movable in the y direction to provide shaping of the beam.

The lower array 12a extends into the beam field in the y direction from one side of the field, and the upper array 12b extends into the beam field in the y direction from the opposing side of the field. The leaves can each be moved independently to define a chosen shape 10 between the tips of the opposing leaf banks 12a, 12b. Each leaf is thin in its transverse (x) direction to provide good resolution, is deep in the (z) direction to provide adequate absorption, and long in its longitudinal (y) direction to allow it to extend across the field to a desired position.

Generally, the longitudinal length of the leaf will be greater than its depth, and both will be much greater than its transverse thickness.

Movable diaphragm blocks, 16a and 16b adjust the width of the aperture. That is, the diaphragm blocks define the aperture in the x direction. The leaves of the MLC can be fully extended such that directly opposing leaves meet. Solely using the MLC to define the beam width would constrain the width of the aperture is to integer numbers of the width of the MLC leaves. The diaphragm blocks 16a, 16b can be moved in the x-direction as desired, and therefore provide an unconstrained dimension of the beam width. Further, the tips of the leaves of the MLC are curved and there may be some degree of leakage between the tips of directly opposing MLC leaves 12 from opposing banks 12a, 12b when fully extended to close off parts of the field. The diaphragm blocks 16a, 16b absorb radiation outside the desired width of the aperture to reduce leakage of the beam in locations outside the aperture.

The diaphragm includes a pair of solid blocks 16a and 16b of radiopaque material such as tungsten, which extend inwards in the x direction from the two opposing sides of the beam field. They have a front edge that spans the entire width of the field, and the entire width of the aperture, and which is straight (in the y-direction), and which can each be advanced and/or withdrawn independently across the field in a direction transverse to the front edge. Thus, the block collimators provide additional shielding in locations spaced from the field shape along the x direction, limiting inter-leaf leakage between the tips of opposing leaves and between adjacent leaves.

Each of the diaphragm blocks 16a, extends across the entire width of the aperture. The lead edge of the diaphragm is straight in the y direction in the plane of the aperture.

Figure 2 shows a radiation head having a beam shaping apparatus of the present disclosure. The beam shaping apparatus (along with the radiation source) forms part of the radiation head. In many radiotherapy systems the radiation head is rotated on a gantry around the patient, so that radiation can be delivered from different angles to minimise the radiation dose to healthy tissue. It is desirable to have a compact radiation head with a reduced height (also known as the 'stack height'), since this reduces the volume needed to house the radiotherapy device. This means a smaller treatment room is required, and/or the radiotherapy device can fit into an existing treatment room.

In the radiation head of Figure 2, a radiation source 18 emits a beam of therapeutic radiation 20. In some implementations the radiation source 18 is a linear accelerator. The beam of therapeutic radiation is high energy x-rays, although in other implementation may be electrons or protons. The beam of radiation travels in the z direction and is collimated by a primary collimator and passes through an ion chamber below the primary collimator. The beam is then collimated by the beam shaping device 14. The beam shaping device 14 defines an aperture having a length in the y direction and a width in the x direction. The beam is collimated in the x-direction by a diaphragm 16, which has two diaphragms blocks 16a and 16b. The diaphragm blocks will be discussed in more detail below. The beam is collimated in the y-direction by a multi-leaf collimator 12. The leaves of the MLC travel in the y-direction, which is oriented out of the page of Figure 2. The leaves provide a variable edge to the aperture.

The direction of travel of the leaves of the MLC 12 is perpendicular to the direction of travel of the diaphragm blocks 16. Together, the opposing leaf banks and opposing diaphragm blocks define an aperture with each of the four edges being defined by a leaf bank or a diaphragm block. The position of the block or leaves of the leaf bank defines the respective edge of the aperture, and resultingly the edge of the beam of radiation. Each diaphragm block extends across the entire edge of the aperture.

The radiation head comprises a head chassis (not shown) to which the components are attached in a fixed relationship.

As can be seen, the diaphragm 16 is positioned above the multi-leaf collimator 12. This can reduce the "stack height" of the radiation head as discussed above. That is, the diaphragm 16 is positioned between the multi-leaf collimator 12 and the source of radiation 18, such that the beam of radiation 20 travelling downwards in the z direction is first collimated by the primary collimator, then by the diaphragm 16 and is then collimated by the multi-leaf collimator 12.

The diaphragm blocks move on curved path so that the diaphragm blocks remain a fixed distance from the radiation source throughout their travel along the path. The provides a more consistent penumbra across the field. In use the movement of the diaphragm is linear in the x-direction to delineate the beam. From the point of view of the beam (such as the beam view shown in Figure 1) the movement of the diaphragm block 16 is linear along the x-direction. The path is curved in the z-direction. Any penumbra created by the diaphragm blocks is larger at large field sizes. This is because the penumbra is proportional to the distance between the collimating edge and the treatment region. The treatment region lies in the x-y plane. Therefore, since the collimator moves on a curved path in the z-direction, at larger field angles the diaphragm blocks are further from the treatment region, meaning the collimating edge to treatment region distance is increased, as is the penumbra.

Having the diaphragm blocks above the MLC 12, rather than below, means that the diaphragm is closer to the source of radiation. The beam diverges along its axis and therefore with the diaphtagm closer to the source, the required size and the length of travel of the diaphragm block is reduced. This is beneficial since tungsten is heavy and translating a tungsten block requires significant energy and robust components. Further, using less tungsten means that a reduced volume of mounting hardware is required, as well as considerably reduced cost and environmental impact.

Defocussed diaphragm As explained above, each diaphragm block defines an opposing edge of the aperture. A drive means moves the diaphragm block along its curved path into the beam to a greater and lesser extent to adjust the width of the aperture.

Each diaphragm block produces a penumbra at the edge of the collimated beam of radiation. The size of the penumbra at the treatment region is proportional to the distance between the collimating edge (the part of the diaphragm block which defines the edge of the aperture) and the treatment region, and is inversely proportional to the distance between the source and the collimating edge. The collimating edge is the section or point of the diaphragm block which extends furthest into the cross section of the beam.

It is desirable to minimise the penumbra at the treatment region. A small penumbra is critical for the shielding of vital organs near the tumour being irradiated. By moving the diaphragm closer to the source of radiation, the distance between the collimating edge and the source decreases, which increases the penumbra.

Each diaphragm block has a flat inner face, which remains facing or directed towards a focus point throughout the travel of the diaphragm block along its curved path. In known systems the focus point is the centre of the source of radiation, meaning that the flat face is directed towards the centre of the radiation source at all field angles. Physics simulations of the penumbra for the focused diaphragm blocks have concluded that the width of the penumbra remains unsatisfactory, particularly in large field sizes.

In the present invention the diaphragm is defocussed from the source of radiation. That is, the flat faces of the diaphragm blocks are focussed on a point which is offset from the source of radiation.

By focussing the diaphragm blocks on a point which is removed from the source of radiation, the inventors in the present application have realised the penumbra caused by the diaphragm in delivery of radiotherapy is reduced at non-zero field angles.

Figure 3 Figure 3 shows the relationship between the diaphragm and the source of radiation according to an aspect of the present disclosure.

A source of radiation 18 emits a beam of radiation 20 with a beam axis along the z direction. The beam has a cross section which is collimated in the x-direction by the diaphragm as explained above in relation to Figure 2. The diaphragm 30 comprises two diaphragm blocks 32 (corresponding to the diaphragm blocks 16 in Figure 2) which move along a curved path 36. In the implementation in Figure 3 the curved path 36 is a rail which defines the path along which the diaphragm blocks move. The diaphragm blocks are moved along the curved path in order to define the width of the aperture.

Each diaphragm block has a flat inner face 34. The inner face 34 is focussed on (i.e. directed or pointed towards) a focus point 38 throughout the travel of the diaphragm block along the curved path 36. The flat face moves radially around the focus point. As the position of the diaphragm block changes, the face tilts relative to the z axis in the radiation head, yet remains directed towards the focus point at all stages of travel along the curved path.

The centreline 42 is the line between bottom-most point of the curved path (referred to herein as the centre point) and the focus point. The source of radiation 18 is centered on the centreline 42, such that the centre point is aligned with the centre of the beam.

The diaphragm blocks have a closed position, in which the faces of the diaphragm blocks meet. This creates an aperture with zero width -i.e. substantially all of the radiation is blocked out. In the closed position usually the diaphragm blocks meet at the centreline 42. It is, however, conceivable that the diaphragm blocks meet at a different point on the curved path.

Field angle refers to the position of the diaphragm block along the curved path, specifically to the degree to which the block is positioned away from the centreline. Zero field angle is the inner face of the diaphragm block positioned on the centreline, and maximum angle corresponds to the diaphragm block located at the end point of the curved path. A wide field corresponds to the blocks positioned away from the centreline near to the edge of the maximum beam field.

As the field angle of both blocks increases and the blocks 32 move out from the centreline, the width of the aperture formed by the diaphragm blocks increases. The maximum aperture size is provided when both diaphragm blocks are located at their respective "end points" of the curved path. This is the position shown in Figure 3.

The focus point 38 is offset from the source of radiation 18 by a distance d. The focus point 38 is positioned above the source of radiation 18, and the diaphragm is positioned below the source of radiation. The effect this offset d has on the beam of radiation is illustrated in Figure 4.

Figure 4 Figure 4 illustrates the penumbra created in a beam of radiation 20 from a source of radiation 18 collimated by diaphragm blocks in an open position away from the centreline. The source of radiation 18 and the diaphragm 30 are comprised in a radiation head. The diaphragm block 32A on the right-hand side is focussed on a point at the centre of the source of radiation. This arrangement would result if the source is modelled as a point source. The flat face 34A of the block 32A is directed towards point at the centre of the radiation source at all field angles. The diaphragm block 32B on the left-hand side is defocussed from the source of radiation, meaning that the focus point of the diaphragm block 32B is offset from the source of radiation. The source of radiation is modelled as a disc, and the focus point of the diaphragm block is located a distance d above the disc. In radiotherapeutic radiation produced, for example, by a linear accelerator, source can modelled as a flat disc, or 'spot' of multiple sources, the disc having a diameter Si.

The spot size (diameter) is determined by calculation. The spot position is normally on the inside face of the waveguide output window. The radiation spot size and position can change as the energy is increased.

The radiation source 18 emits a beam of radiation 20 comprising x-rays which travel away from the source. The radiation beam is illustrated by dashed lines.

On the right-hand side of Figure 4, radiation from the far side (i.e. left side) of the disc is collimated by the upper most part of the face of the diaphragm block 32A. Radiation from the near side (i.e. right side) of the disc is collimated by the lower-most part of the face of the diaphragm block 32A. The width of the penumbra created by the diaphragm block is illustrated as Pl. Both the uppermost part and the lowermost part of the inner face define the aperture. The collimating edge therefore can be taken as a point between the uppermost and lowermost parts (i.e. where the two dashed lines cross).

The diaphragm block 32B is defocussed from the source of radiation 18, as in an embodiment of the present disclosure. The focus point 38 of the diaphragm block 32B is located above the source of radiation 18, offset from the source by a distance d. The focus point 38 is positioned directly above the source of radiation 18 such that the source of radiation lies on the centreline. When the diaphragm block 32B is positioned with it's inner face on the centreline, the face 34B is directed towards the centre of the source.

When a diaphragm block is positioned away from the centreline, as shown in Figure 4, the inner flat face 34B is directed to a point which is offset from the source, and not towards the centre of the source.

The face is directed to a point above the source of radiation 18, and therefore the lower portion of the diaphragm block extends into the beam by a greater amount than the upper portion. The diaphragm block presents its lowermost edge to the radiation source. As can be seen in Figure 4, the uppermost edge is no longer defining an edge of the penumbra the overall width of the penumbra is reduced. Radiation from both the near side (left side) and the far side (right side) of the disc is collimated by the lowermost part of the inner face 34B, creating a penumbra P2.

The lowermost part of the inner face is the collimating edge. Therefore by defocusing the diaphragm block from the centre of the source of radiation, effectively the distance between the source and the collimating edge is increased. The distance between the source and the collimating edge of block 32A is A, and the distance between the collimating edge of block 32B is distance B. By defocussing block 32B, B>A. It can be seen that the penumbra P2 is smaller than the penumbra P1.

When an angle is introduced between the face of the diaphragm block and the direction of the radiation, the collimating edge changes. If the face is focussed on a point positioned on the opposite side of the source to the diaphragm, the point which defines the edge of the aperture moves further away from the source.

As explained above, the penumbra caused by the diaphragm block is inversely proportional to the distance between the source of radiation and the collimating edge. In Figure 4 1 1 P1 ix - P2 -

A B

By defocussing the diaphragm from the source of radiation, the distance between the source of radiation and the collimating edge is increased at non-zero field angles. B > A

It therefore follows that P2 < P1 That is, by focussing the diaphragm blocks on a point above the source of radiation, the penumbra produced by the blocks at non-zero field angles is reduced.

In the present disclosure a simple offset between the source of radiation and the focus point of the diaphragm is introduced which reduces the penumbra at non-zero field angles. The effective distance between the source and the collimating edge increases without having to increase the distance between the diaphragm and the source. This means that the penumbra can be reduced without increasing the stack height of the radiation head.

The diaphragm blocks provide two opposing faces moving on the same curved path and focussed on the same focus point. In the closed position the diaphragm blocks meet with parallel faces and the beam is blocked by the full depth of the diaphragm blocks. This blocks substantially all of the radiation. Accordingly the diaphragm's ability to block radiation is not compromised. The radiation head provides a reduced penumbra without compromising the diaphragm's ability to block radiation.

In summary, by moving the focus point of the diaphragm above the source of radiation, the distance between the collimating edge and the source at non-zero field angles is increased, and therefore the penumbra caused by the diaphragm block is decreased.

Finding the optimum focus point The optimum location of the focus point can be determined by modelling the source of radiation as a disc. At the maximum field angle the face of each diaphragm block is focussed on the edge of a nominal flat disc. The intersection of the lines of focus of the diaphragm blocks in this position is the focus point for the diaphragm.

The source is modelled as a flat disc having a nominal diameter. In Figure 3 the source 18 is simulated as a flat disc of multiple sources, the disc having a diameter Si. The simulation estimates the diameter of the radiation source, which is dependent on beam energy. At the maximum field angle, i.e. with the diaphragm block positioned at the end point of the curved path, the inner face 34 of the diaphragm block is focussed on the edge of the nominal disc of the source. This is the 'end point focus line'. Each diaphragm block has an 'end point focus line' at its maximum field angle at its respective end of the curved path. The optimum focus point for the diaphragm blocks along the curved path is the intersection of the two end point focus lines. The end point might correspond to the maximum required field size of the treatment beam, rather than, for example, the physical end of the curved path.

The focus point 38 is displaced from the source 20 by a distance d. Using the above method: the larger the nominal disc of the source, the larger the distance between the focus point and the source, the greater the offset. If the source is modelled as a point source (as in known systems) the 'nominal diameter' of the source is zero, meaning that the focus point lies on the source.

The corrected focusing point is now behind the radiation source, the point is optimised for the radiation source and the required projected field size. An initial value for D can be calculated by trigonometry from the radiation spot diameter, distance from source to the isocentre and maximum projected field size. The distance could then be fine-tuned using monte-carlo simulation.

It is noted that, in a defocussed diaphragm the full depth of the block is not presented into the radiation beam, only its lower edge. Therefore the amount that each block of a defocussed diaphragm is extended into the radiation beam to attenuate the beam to a specific field size is required to be greater than that of the block focussed at the centre of the radiation source.

Variations In other examples the distance d between the source and the focus point is determined using other methods. In other examples the focus point is displaced form the source in a different direction -i.e. other than located on the direct opposite side of the source to the diaphragm. For example, the focus point may be positioned between the diaphragm and the source.

In Figure 3 the diaphragm block moves along a curved rail 36 which defines the curved path. However, the diaphragm block may move along any type of curved path.

In Figure 3 two diaphragm blocks 32 move along a single rail 36 to define opposing edges of an aperture. Moving two diaphragm blocks along a single rail reduces the number of components and means that the blocks 32 are aligned. In other examples the diaphragm blocks may move on separate rails.

In other examples the disclosure relates to a single diaphragm block driven along a curved path which is defocussed, i.e. has a focus point which is offset from the source of radiation.

In the implementation described above the diaphragm is part of a beam shaping apparatus which includes a multi-leaf collimator. In other examples, the diaphragm may be provided alone without a multi-leaf collimator.

In some examples, the diaphragm is part of a beam shaping apparatus which comprises two sets of diaphragms. The first set of diaphragms is configured to delimit radiation in the X direction, the second set of diaphragms is configured to delimit the radiation beam in the Y direction. In this implementation both the first and second sets of diaphragms are driven along respective curved paths, each curved path having a focus point which is offset form the source of radiation. The focus point of the curved path of the first pair of diaphragms is may be at a separate location to the focus point of the curved path of the second pair of diaphragm blocks.

In another example the diaphragm is part of a primary collimator for collimating a radiation beam. There is provided a radiation head having a primary collimator comprising a set of diaphragm blocks driven along a curved path having a focus point which is offset from the source of radiation.

In the implementation described above the radiation source 18 is a linear accelerator. In other examples the radiation source 18 may be a radioactive material or an x-ray gun.

The diaphragm includes a diaphragm block movably attached to a chassis. When the diaphragm block is comprised in a beam shaping device, the chassis is part of a chassis which supports both the multi-leaf collimator and the diaphragm in a fixed relationship. When the beam shaping device is comprised in a radiation head, the chassis is part of a chassis which supports components of the radiation head (source of radiation, primary collimator, beam shaping device) in head is a fixed relationship.

In certain implementations a slide is affixed to each block to slideably attach the block to the curved rail. A channel in the curved rail is shaped to hold the slider and the rail together. An end stop on the rail prevents the slide from sliding off the end of the rail. A drive mechanism drives the diaphragm along the curved path. Each diaphragm block has a gear, such as a quadrant gear, fixedly attached to the block. The gear is driven via engagement with a worm screw which is rotated by a motor. The gear is fixedly attached to the block and therefore rotating the worm screw drives the diaphragm block along the curved rail.

There is provided a beam shaping device comprising a diaphragm block, wherein the diaphragm block is driven along a curved path which is defocussed from the source of radiation.

Features of the above aspects can be combined in any suitable manner. It will be understood that the above description is of specific embodiments by way of aspect only and that many modifications and alterations will be within the skilled person's reach and are intended to be covered by the scope of the appendant claims.

Claims

Claims 1 A radiation head for a radiotherapy device, the radiation head comprising: a source of radiation configured to emit a beam of radiation; and beam shaping device for collimating the beam of radiation, the beam shaping device comprising: a diaphragm comprising a diaphragm block movable along a curved path, the diaphragm block having a flat face focused on a focus point which is offset from the source of radiation.
2. A radiation head according to claim 1, wherein the diaphragm is below the source of radiation and the focus point is above the source of radiation
3. A radiation head according to claim 1 or 2 wherein the focus point is offset from the radiation source by a distance d.
4 A radiation head according to any preceding claim, wherein the source is configured to emit a beam of radiation that is directed along a beam axis and has a width transverse to the beam axis in a first direction and a second direction, wherein the diaphragm block selectively limits the width of the beam in the first direction.
5. A radiation head according to claim 4 further comprising a multi-leaf collimator for selectively limiting the width of the beam in the second direction.
6. A radiation head according to claim 5 wherein the diaphragm is positioned between the source and the multi-leaf collimator.
7. A radiation head according to any preceding claim wherein the diaphragm block is slidably attached to a curved rail which defines the curved path.
8 A radiation head according to any preceding claim, wherein the diaphragm comprises a first diaphragm block and a second diaphragm block defining opposing edges of an aperture to delimit the beam.
9 A radiation head according to claim 8 wherein the first diaphragm block and the second diaphragm block are movable along the same curved path and focussed on the same focus point.
10. A radiation head according to any preceding claim, wherein the source comprises a nominal disc, and at the maximum field angle the first block is focussed on a first edge of the nominal disc and the second block is focussed on a second edge of the nominal disc.
11. A radiotherapy apparatus comprising:a rotatable gantry;a radiation head according to any preceding claim, wherein the radiation head is fixedly attached to the rotatable gantry.