BACKGROUND ART Collimators are used (principally) in the field of radiotherapy. A beam of radiation is directed toward a patient and must be collimated to fit the shape of the area to be treated. It is important to ensure that the dose in the areas outside that shape is as low as possible, but also that the whole area is treated.

If areas are left untreated then the likelihood of recurrence is increased, whereas if healthy regions are irradiated then unnecessary damage will be caused, resulting in greater side effects and longer recovery times after treatment.

As the treatment area is rarely the same shape and size for different patients, a plurality of different types and shapes of collimators are employed.

These include multi-leaf collimators which comprise an array of finger-shaped leaves of a radiation-absorbing material, each disposed in a parallel relationship and each able to move longitudinally relative to the others. One example of such a collimator is described in our co-pending GB application number 0315909.2, the contents of which are hereby incorporated by reference. Other types of collimators are suitable, and may be (for example) simple masks of a circular, rectangular or other shape.

The plurality of collimators are generally mounted in a housing supported above the treatment area. Collimators can be stacked one on top of another in a direction parallel with the radiation beam. As the treatment area for a particular patient is determined, the relevant collimator is selected from the plurality of collimators and is slid into position so as to intercept the radiation beam and provide collimation.

Each collimator system may house up to 120 or more different collimators and approximately the same number or more of different filters, making the collimator system bulky. This may have an adverse effect on the patient as the collimator housing is suspended a very short distance from the treatment area.

The more collimators and filters that are housed in the collimator system, the more bulky the head and the lower it needs to hang down above the patient.

This is due to the fact that the radiation source is generally a fixed distance from the treatment area. The patient is usually able to see an aperture or window through which the radiation exits the housing, and this prompts many patients to experience further feelings of anxiety.

The manufacture of such collimating systems is also expensive and time consuming as each collimator and filter must be accurately aligned to the radiation beam. The set up time for the clinician is also long as the particular collimator and filter for a particular treatment area must be accurately and precisely set. In a complex system, there is also a heightened risk of error. The clinician or other suitable personnel may select the wrong collimator or wrong filter or both, leading to either an underdose or overdose of radiation in the case of a mistake in the filter. In the case of the wrong collimator being selected, this can lead to healthy tissue being subject to radiation.

SUMMARY OF THE INVENTION The present invention seeks to provide a simpler collimator system, which is able to alleviate the problems discussed above by being more efficient in use of space, less time consuming to set up for the clinician, less expensive and quicker to manufacture.

According to a first aspect of the invention there is provided a collimator system for a radiation beam comprising a cassette containing at least one of a collimator and a filter and a housing for the cassette adapted to retain the cassette in a fixed relation with the radiation beam.

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 known collimator system; Figure 2 shows a perspective view of the outside of a housing according to an embodiment of the present invention ; Figure 3 shows a perspective view of a housing and a cassette according an embodiment of the present invention ; Figure 4 shows a perspective view of the inside of a housing according to an embodiment of the present invention; Figure 5 shows a perspective view of the individual cassette according to an embodiment of the present invention; Figure 6 shows an exploded view of the collimator and filter shown in figure 5; and Figure 7 shows the collimator according to an embodiment of the present invention, fitted onto the kV source arm of the radiation generator.

DETAILED DESCRIPTION OF THE EMBODIMENTS Figure 1 shows a known oncology apparatus, 1, with a large head, 2, which holds the plurality of collimators and filters. The patient is positioned on the bed, 3, looking up at the head 2, if the treatment area is in the front of the body and with head towards the bed 3, if the treatment area is in the back of the body. A flat panel imager 4 captures the beam after attenuation by the patient.

As can be seen, the large head is suspended above the patient and can give arise to feelings of anxiety in patients as they look up at the head 2.

Figure 2 shows a receptacle or housing, 10, that replaces the large head 2 shown in figure 1. It is discreet in design and has a housing formed of a material that is easily permeable to the radiation used to treat the treatment area. Various polymeric materials are suitable for the manufacture of this housing. Thus it requires no aperture or exit hole for the radiation beam to exit the housing.

Figure 3 shows the housing, 10, with a slot or insertion hole 12 into which the individual cassette 11 may be inserted. Cassette 11 is one of a plurality of cassettes, each of which may be manufactured to have a pre-selected fixed field size (collimating mask) and a fixed filter. Alternatively the cassette may have recesses which allow the insertion of a collimating mask and filter as selected by the clinician for the patiant. It is envisaged that the most preferred embodiment of the present invention would include a plurality of sealed cassettes with different masks and different filters, such as to comprise a series of 120 or more of such cassettes, each of which may be selected by the clinician or other trained personnel with the minimum of ease. This eliminates the risk that the wrong filter may be accidentally used and the patient receives an overdose or underdose.

The individual cassettes may be selected by the clinician or other suitable personnel and inserted manually into the receptacle 12. The apparatus is then ready for use in patient imaging and/or treatment.

Figure 4 shows the inside of the housing 10. The cassette rack 20 is comprised of slots or grooves, 20a, 20b and 20c, of a suitably rigid material such as a steel section, aluminium extrusion, etc. Side sections 20a and 20b form the two sidewalls of the rack 20, and slot 20c forms the back or rear wall of the rack 20. The cassette 11 can be slid into the rack 20 and is guided into the correct position, in the x and z directions (illustrated by the arrows) by the grooves 20a and 20b, until it reaches the end groove or slot 20c. This ensures that the cassette is accurately positioned.

Also shown in Figure 4 is the input position 30, through which the radiation enters. This is accurately positioned to be aligned to the optical axis of the radiation.

Figure 5 shows the inside of the individual cassette 11. The individual cassette 11, comprises a rectangular or other suitably shaped frame 31, designed to receive the collimating mask 40 and a filter 41. The frame may be made of aluminium, stainless steel or other suitable material.

Figure 6 shows the cassette frame 31 and a collimating mask 40 which slots into the recess 45 in the cassette frame 31. A second recess 46 in the collimating mask 40 is suitably shaped to receive a filter 41. The cassette frame 31, as mentioned above, may be encased in a cover or otherwise sealed to form a sealed cassette, if it is desired to manufacture individual cassettes. There will then be no need for the clinician or other trained staff to make up the cassette 11 with a suitable collimating mask and/or filter.

Of course, the frame 31 is manufactured such that the clinician or other trained personnel may make up the cassette by the insertion of a collimating mask and filter as required for each treatment.

Figure 7 shows the collimator system 10, attached to the kV source arm of the radiation generator. The radiation beam exits the face 10a of the collimator system, below which is positioned a table or bed suitable for the patient.

As can be seen, the collimator system according to the present invention is more compact and discreet than the prior art collimator system. The set up of a required collimator and filter is faster and more accurate. It is less expensive and less time consuming to manufacture and can alleviate the adverse effects on a patient as they are being treated by the radiation.

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.