GB2330997A

GB2330997A - Linear accelerator for radiotherapy

Info

Publication number: GB2330997A
Application number: GB9722358A
Authority: GB
Inventors: Geoffrey Harding
Original assignee: Elekta AB
Current assignee: Elekta AB
Priority date: 1997-10-24
Filing date: 1997-10-24
Publication date: 1999-05-05
Also published as: GB9722358D0

Abstract

An accelerator produces an electron beam 10 which may be directed to either of two exit ports 14,16. At the first port 14, the beam strikes the reverse face of a converter plate 12 and produces a high energy photon beam for therapy. When directed to the second port 16, the electron beam is deflected by a magnetic field 18 so that it strikes the front face of the plate 12 and produces a low energy photon beam which can be used for imaging and then positioning a patient correctly for subsequent therapy.

Description

LINEAR ACCELERATOR The present invention relates to a linear accelerator, especially to a linear accelerator for medical treatment.

Linear accelerators have been in use for a significant time for the treatment of severe medical conditions such as cancer. The principle of operation of these devices is essentially that electromagnetic radiation of sufficiently high energy (greater than 1MeV) causes the death and/or destruction of living cells in its path. If therefore a beam of high energy photons or electrons is directed to a cancerous area, the cancer will be destroyed.

It is clear that this treatment has the potential to cause harm to the patient if the radiation is incident on healthy tissue. It is therefore common practice to collimate the beam in order to limit its spatial extent and therefore limit the amount of tissue, healthy or otherwise, upon which it is incident. Collimators to this effect are known, and include collimators of variable size and shape which are capable of selectively blocking the radiation beam at its edges thereby to produce a beam whose shape corresponds to that of the tumour.

In this situation, it is clearly necessary to ensure that the positioning of the patient is accurate. This is generally done by shining one or more collimated light sources on the patient, such as low energy lasers. When the patient is first treated, ink marks are placed on the patient's skin and these are subsequently used to align the patient relative to the lasers. This is able to provide a positioning accuracy of approximately 2-5 mm.

As a result, the shaped radiation beam needs to be formed to the approximate outline shape of the tumour plus a margin in each direction to allow for positioning error. This results in a significant volume of tissue around the tumour which is needlessly irradiated. If the patient could be positioned more accurately beneath the beam, then this margin around the tumour could be reduced thereby limiting the damage caused to the patient's healthy tissue.

It is known to prepare a "portal image" derived from the radiation transmitted through the patient and incident on a photographic plate or other detector. However, the absorption co-efficients of bone, tissue etc. at the energies normally used for radiotherapy are very similar and therefore a portal image shows very little detail.

The present invention therefore provides an accelerator comprising a means for the production of at least one output radiation beam, a means for supporting a patient in the path of that beam, and a means for imaging the beam after passage through the patient, wherein the means for production of at least one radiation beam comprises a means for producing at least one electron beam, a converter plate for generating electromagnetic radiation from the electron beam when incident thereon, and a deflector arrangement to selectively direct the electron beam onto the converter plate in one of two orientations, the first orientation being generally aligned with the output beam path and the second orientation being generally opposed to the output beam path.

Thus, as in a normal accelerator, the accelerator of the invention can produce a high energy photon beam for patient treatment by directing the electron beam onto the converter plate in the first orientation. Thus, the photons which escape collimation are travelling in substantially the same direction as the incident electron beam and have a high energy. However, when the second orientation is adopted, the photons which escape collimation are those which are emitted at a large angle to the incident beam. These will have a much lower energy, one at which the absorption coefficients of tissue and bone differ significantly. A meaningful image can thereby be created, which can be used to position the patient so that the tumour is located correctly.

It is preferred if the first orientation is within 450 of the output beam path.

It is also preferred if the second orientation is within 1 35 and 1 800 of the output beam path.

A suitable arrangement for the present invention is for the beam to fall on a reverse face of the converter plate when in the first orientation and on the front face of the converter plate when in the second orientation, front and reverse being defined with respect to the patient.

The present invention is particularly, but not exclusively, applicable to a dual output linear accelerator. Such accelerators have two beam ports, designated an electron port and a photon port. Both ports are supplied with a beam of electrons, but the photon port is covered by a suitable converter plate. Thus, photons are produced at the photon port. In such a system, a preferred way of achieving the invention is to provide a deflector arrangement in association with the electron port to selectively deflect the output of the electron port to the front face of the converter plate, thereby to produce low energy photons from the photon port.

The target, as referred to above may be a single homogenous block of a suitable material such as Tungsten. Alternatively, it may be a composite entity with different areas for production of high and low energy photons.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying Figures, in which Figure 1 is an illustration of the relationship between photon emission angle and photon energy; and Figure 2 illustrates a suitable deflector unit for use in the present invention.

In a standard radiotherapy system, a collimated beam 10 of high energy (e.g. 1 OMeV) electrons is incident on a thin ( 1 mm) converter plate 1 2 of e.g. Ta. The high energy electrons lose energy equally by inelastic collisions (leading to ionization of the converter atoms) and through the emission of bremsstrahlung radiation. In the former process they change their propagation direction slightly. After many collisions the electrons have lost a large fraction of their energy and diffuse more or less isotropically through the medium.

This situation can be investigated by means of a Monte Carlo electron photon transport code such as EGS4 (Electron Gamma Shower 4th version). In this code the path of each particle (electron, positron, photon etc.) through the (generally inhomogeneous) medium is simulated using distribution functions based on random number generators to determine such collisional parameters as the type of interaction (Compton, pair production, photoeffect etc.), the polar and azimuthal angle of the scattered particle (photon or Compton electron) and the track length to the next interaction.

Further details of the code are described by Nelson et al in "The EGS Code System: Computer Programs for the Monte Carlo Simulation of Electromagnetic Cascade Showers (Version 4)", Stanford Linear Accelerator Center Report Number SLAC-256, (1985).

Figure 1 shows the output of EGS4 when a 6 MeV beam of electrons 10 falls perpendicularly onto a 0.8mm thick Ta target 12. The emergent photons are labelled according to their energy E and the angle 8 relative to the direction of the primary electrons. Figure 1 indicates that the average photon energy decreases rapidly as the angle increases.

Each point in Figure 1 represents one photon which has escaped from the target. The absence of any photons emitted at an angle 8 around 900 is explained by the high self-absorption of these photons in the converter plate (bremsstrahlung target). Table 1 shows the quantitative relationship between photons emitted within a certain angular range and the mean energy of the photon spectrum.

Table 1

Angular range Average photon energy 0 # ### # 45 2.3 MeV 450 < : 181 # 900 1.1 MeV 90 # ### # 135 0.65 MeV 135 # ### # 180 0.164 MeV Table 1 shows that back-scatter photons have only a very low energy.

They arise from electrons which have suffered many collisions and thus lost much of their primary energy. This relationship shows that a simple way to derive low energy photons from a megavoltage source is to deflect the electrons through a large (at least approximately 800) angle so that the patient is irradiated with back-scatter photons from the target.

It is known that if an electron beam of kinetic energy T (MeV) enters a magnetic field of B (Tesla) such that the magnetic field is perpendicular to the direction of travel, then the electrons will execute a circular path of radius R (cm), where R is given by the following equation: R = [0.334/B1(2m,c-T+ TZ)H where me c2 is the rest mass energy of the electron (0.511 MeV). For T = 10 MeV and B = 1 T the orbital radius R is 3.5cm.

To arrange for the electrons to change their direction by 1800 they have to rotate around a semicircle. It is not necessary for the electrons to travel the full 1 800 since, as Table 1 indicates, they have already lost an appreciable amount of energy after 1350 scatter. The device for realising the deflection is indicated in Figure 2.

In many accelerators, such as that illustrated in Figure 2, there are two exit ports 14, 1 6 to which the electron beam can be directed by switching a magnet. The first 14 leads to the converter target 12 for producing photons 20, the second one 16 allows the use of the direct electron beam for radiotherapy purposes. The electron beam emerging from the second port 16 enters the magnetic field region 18 and begins to move in a circular path whose radius is given by equation 1. The magnetic field of e.g. 1 T is produced by a small electromagnet with a bore of - 8cm. The gap between the two poles can be quite small e.g. 5mm. The electrons are deflected through practically 1800 and then impact on the target 12 which is a block, cooled if necessary, of a high atomic number converter. A secondary target can be arranged to be as close as possible to the photon converter: it may even be the rear side of the converter for the photon port.

In this case the focus of the high energy (radiotherapy) beam and the low energy (imaging) beam are at the same position. This is advantageous for aligning the patient properly.

In those accelerators which have only one beam exit port, an additional magnetic deflection (not shown) can be provided to steer the electron beam prior to its use for low energy X-ray production.

Claims

CLAIMS 1. An accelerator comprising a means for the production of at least one output radiation beam, a means for supporting a patient in the path of that beam, and a means for imaging the beam after passage through the patient, wherein the means for production of at least one radiation beam comprises a means for producing at least one electron beam, a converter plate for generating electromagnetic radiation from the electron beam when incident thereon, and a deflector arrangement to selectively direct the electron beam onto the converter plate in one of two orientations, the first orientation being generally aligned with the output beam path and the second orientation being generally opposed to the output beam path.
2. An accelerator according to claim 1 in which the first orientation is within 450 of the output beam path.
3. An accelerator according to claim 1 or claim 2 wherein the second orientation is between 135 and 1800 of the output beam path.
4. An accelerator according to any preceding claim arranged such that the beam falls on a reverse face of the converter plate when in the first orientation and on the front face of the converter plate when in the second orientation, front and reverse being defined with respect to the patient.
5. An accelerator according to any preceding claim being a dual output linear accelerator, ie having two beam ports, one adapted to emit electrons and another adapted to emit photons.
6. An accelerator according to claim 6 in which both ports are supplied with a beam of electrons, the photon port being covered by a suitable converter plate.
7. An accelerator according to claim 6 including a deflector arrangement in association with the electron port, arranged to selectively deflect the output of the electron port to the front face of the converter plate, thereby to produce low energy photons from the photon port.
8. An accelerator according to any preceding claim wherein the target is a single homogenous block.
9. An accelerator according to claim 8 wherein the block is of Tungsten.
1 0. An accelerator according to any one of claims 1 to 7 wherein the target is a composite entity with different areas for production of high and low energy photons.
11. An accelerator substantially as described herein with reference to and/or as illustrated in the accompanying drawings.