LINEAR ACCELERATOR

LINEAR ACCELERATOR

FIELD OF THE INVENTION

The present invention relates to a linear accelerator.

BACKGROUND ART

In the use of radiotherapy to treat cancer and other ailments, a powerful beam

of the appropriate radiation is directed at the area of the patient which is affected.

This beam is apt to kill living cells in its path, hence its use against cancerous cells,

and therefore it is highly desirable to ensure that the beam is correctly aimed. Failure

to do so may result in the unnecessary destruction of healthy cells of the patient.

Several methods are used to check this, and an important check is the use of a so-

called "portal image". This is an image produced by placing a photographic plate or

electronic imaging plate beneath the patient during a brief period of irradiation. The beam is attenuated by the patient's internal organs and structures, leaving an image

in the plate. This can then be checked either before complete treatment or after a

dose, to ensure that the aim was correct.

Portal images are however extremely difficult to interpret. The energy of the

beam which is necessary to have a useful therapeutic effect is very much greater than

that used for medical imaging. At these higher energies there is smaller ratio in the

relative attenuation between bony and tissue structure, which results in portal images

with poor contrast. Structures within the patient are difficult to discern.

Some existing radiotherapy devices include a second radiation source which is

adapted to produce a lower energy beam for producing a portal image. This second

source is usually placed either alongside the principal accelerator and parallel thereto,

or is mounted at an angle such that the entire unit is rotated about the patient to bring

the second source into line for the portal image, following which the unit is rotated

back for treatment. Both arrangements present difficulties in ensuring adequate

alignment between the principal accelerator and the second source.

It has not hitherto been possible simply to reduce the energy of the principal

(therapeutic) accelerator, since this must operate in a relativistic mode in order to

maintain beam quality. If the final beam energy is too low, then the beam will be non-

relativistic at earlier parts of the accelerator, preventing satisfactory operation. SUMMARY OF THE INVENTION

The present invention therefore provides an accelerator comprising a plurality

of accelerating cells arranged to convey a beam, adjacent cells being linked by a

coupling cell, the coupling cells being arranged to dictate the ratio of electric field in

the respective adjacent accelerating cells, at least one coupling cell being switchable

between a positive ratio and a negative ratio.

Such an accelerator is eminently suitable for therapeutic use as part of a

radiotherapy apparatus as a phase change is in effect inserted into the E field by

imposing a negative ratio meaning that the beam will meet a reversed electric field in

subsequent cells and will in fact be decelerated. As a result, the beam can be

developed and bunched in early cells while accelerating to and/or at relativistic

energies, and then bled of energy in later cells to bring the beam energy down to (say)

between 1 00 and 300 KeV. Despite this low output energy, the beam is relativistic

over substantially the same length of the accelerator, as previously. Energies of this

magnitude are comparable to diagnostic X-rays, where much higher contrast of bony

structures exists. Hence the accelerator can be used to take kilovoltage portal images.

It is preferred that the switchable coupling cell comprises a cavity containing a

conductive element rotatable about an axis transverse to the beam axis. This is more

preferably as set out in our earlier application PCT/GB99/001 87, to which specific reference is made and the contents of which are hereby incorporated by reference.

Protection may be sought for features set out in this application in combination with

features set out in that application.

The application likewise relates to the use of an accelerator in which a plurality

of accelerating cells arranged to convey a beam, and adjacent cells are linked by a

coupling cell, the coupling cells being arranged to dictate the ratio of electric field in

the respective adjacent accelerating cells, wherein at least one coupling cell is

switched between a positive ratio and a negative ratio.

Further, the application relates to an operating method for an accelerator in

which a plurality of accelerating cells arranged to convey a beam, and adjacent cells

are linked by a coupling cell, the coupling cells being arranged to dictate the ratio of

electric field in the respective adjacent accelerating cells, wherein at least one coupling

cell is switched between a positive ratio and a negative ratio.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will now be described by way of example, with

reference to the accompanying figures, in which;

Figure 1 is a schematic illustration of a conventional linear accelerator; Figure 2 shows a desirable electric field in the accelerator of figure 1 ;

Figure 3 shows a typical electric field as "observed" by an electron being

accelerated;

Figure 4 shows a linear accelerator according to the present invention;

Figure 5 shows the variations of the individual coupling coefficients between cell

108 of figure 4 and the two adjacent coupling cells, and shows the variation of the

ration of these coefficients as the conductive element (the vane) is rotated;

Figures 5a and 5b proposes an explanation of figure 5;

Figure 6 shows an electric field seen by an electron for the accelerator of figure

4 with the rotatable element set to step down the E-field;

Figure 7 shows a similar electric field with the rotatable element set to step up

the E-field; and

Figure 8 shows a still further electric field with the rotatable element set to

reverse the E-field.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring to figure 1 , a conventional accelerator 100 has a series of accelerating

cells such as 1 02. These are arranged in a linear array and communicate via an

aperture 104 on the centreline of each. An accelerating beam of electrons passes

along that path through each accelerating cell. Coupling cells such as 106 are

arranged between adjacent accelerating cells and provide a degree of rf coupling between accelerating cells. This coupling regulates an rf standing wave which is

established in the accelerator by an external means (not shown).

Conventionally, the cells are numbered starting at the first accelerating cell and

sequentially for each cell of whatever type. Thus the first coupling cell, between the

first and second accelerating cells, is cell 2. The second accelerating cell is then cell

3. This is illustrated in figure 1 , and results in accelerating cells being odd-numbered

and coupling cells being even-numbered.

Figure 2 shows the desired rf pattern in the cells. It should be remembered that

the pattern is that of a standing wave illustrated at an instant in time, so the actual E

field at a particular location oscillates between the maximum shown in figure 2 and the

reverse field. The field is ideally positive in cell 1 , zero in cell 2, negative in cell 3,

and zero in cell 4. It then repeats this pattern of being zero in the coupling cells and

alternating polarity in successive accelerating cells. The accelerator is sized in relation

to the frequency of the rf standing wave such that in the time that the accelerating

electron moves from one cell to another, for example from cell 23 to cell 25, the

standing wave will have completed one half cycle. As a result, the E field in cell 25

will, when the electron arrives, be the opposite of its value when the electron was in

cell 23. Thus, the E field will be positive, so far as the electron observes, in every

accelerating cell and the electron will steadily gain energy from the E field as it

progresses. In the later accelerating cells, the energy of the electron is such as to render its

movement relativistic. As it gains energy, therefore, its speed remains substantially

constant despite its rising kinetic energy. This allows the phase relationship between

the rf standing wave and the progressing electron to remain fixed. It is therefore

important that the beam remains relativistic, since it will otherwise fall out of

synchronisation with the rf standing wave. It is not therefore possible to reduce the

output energy of the beam by reducing the acceleration (ie the rf power) since

although the beam would in theory be relativistic when output, it would have been

non-relativistic for a substantial length of the accelerator and the beam would therefore

suffer loss of phase synchronism.

Figure 3 shows a plot of the likely actual E field as observed by the electron

during its passage through the accelerator. It can be seen that there are a number of

points corresponding to the centres of accelerating cavities where the E field is strong

and positive. Between these areas the field is small and can be ignored. Within cells,

the field approximates to that desired.

Figure 4 shows a linear accelerator according to the present invention. Cell 1 0

is replaced with a variable coupling cell 1 08 which comprises a substantially cylindrical

cavity 1 1 0 aligned transverse to the axis of the accelerator in which is placed a

rotateable vane 1 1 2. This is as described in our earlier application PCT/GB99/001 87,

to which the reader is referred. As described in that application, this arrangement allows a wide range of ratios of coupling coefficients to be obtained. However, it is

now further apparent that this arrangement can in fact generate a negative ratio, as

shown in figure 5. This shows the coupling coefficients and the ratio between them

as the vane is rotated through 360° . It will be seen in this figure that over some

ranges of vane angle, both coupling coefficients have the same sign and hence the

ratio between them is positive, but that over other ranges of vane angle the coupling

coefficients have different signs and hence the ratio in negative.

It is this ability of the arrangement to produce coupling coefficients that can

either eb of the same sign or be of opposite signs that can permit two portions of a

linear accelerator either to both provide acceleration of particles or for one portion to

accelerate whilst simultaneously for the other to decelerate.

In some regions, the ratio is very large indeed and the accelerator may well be

unstable in these regions. However, in other areas such as between 30° and 1 80°

on the scale as illustrated, the ratio can be varied smoothly between a moderate

positive value and a moderate negative value.

Figures 5a and 5b illustrate how this is believed to arise. Within the cavity, the

orientation of the entire EM field pattern is dictated by the position of the vane 1 1 2,

since (for instance) the E-field (1 1 4) lines must meet a conductive surface

perpendicularly. However, RF coupling between the accelerating cells and coupling cell is predominantly magnetic with the axial H-field indicated by arrow ends (x and * )

according to whether the field points into or out of the page).

Thus when the vane 1 1 2 is between ports 1 1 6, 1 1 8 (figure 5a) linking the

accelerating and coupling cells, each port will see an H-field of the same polarity (e.g.

both x ), giving rise to a positive coupling coefficient ratio and electron acceleration

both upstream and downstream of the coupling cell. In general, these accelerating

field strengths will differ according to the exact angular setting of the vane.

When the vane 1 1 2 is transverse to the ports 1 1 6, 1 1 8 (figure 5b), the polarity

of the H-fields seen by the ports will be opposite (eg x and • ) giving rise to a negative

coupling coefficient ratio and thus electron acceleration upstream and deceleration

downstream of the coupling cell.

Figures 6 and 7 show the effect on the accelerating cell E fields of a coupling

coefficient ratio greater than unity and less than unity respectively. In figure 6, after

cell 10, the electric field experienced by the accelerating beam drops, and the beam

will therefore gain less energy and the output energy will be less. In figure 7, after cell

10, the electric field experienced by the accelerating beam rises, and the beam will

therefore gain more energy and the output energy will be greater. This illustrates the

ability of the apparatus of PCT/GB99/001 87 to vary the output energy of the beam. Figure 8 shows the effect of a negative coupling coefficient ratio. The E field

from cell 9 to cell 1 1 is reversed, effectively a phase change in the rf standing wave.

Thus, from cell 1 1 onwards, the beam experiences an E field which acts to decelerate

it, ie it loses energy to the E field. Thus, the beam output can be of a very low energy

indeed. This enables a portal image to be taken with adequate contrast.

Attempts have previously been made to insert a phase change in the rf field by

separating it from the beam and inserting an additional half wavelength path, but this

raises severe difficulties in reuniting the rf and the beam. This arrangement avoids this

difficulty entirely.

It will of course be apparent to those skilled in the art that many variations could

be made to the above arrangements without departing from the scope of the present

invention.