Patent Number: 046722122
Section: summary

CROSS REFERENCES TO RELATED PATENTS AND PATENT APPLICATIONS U.S. Pat. No. 4,463,266 to A. Brahme entitled "Neutron Collimator" issued July 31, 1984 and assigned to the same assignee as the present invention. BACKGROUND OF THE INVENTION This invention pertains to a multi leaf collimator for a beam of radiation the particles of which are selected from the group comprising high energy photons, electrons, protons and heavy ions to the extent not already covered by said U.S. Pat. No. 4,463,266. Modern beam radiation technique makes use of high energy particles to irradiate deep seated tumours. High energy photons and electrons with energies in the order of 1 to 50 MeV are considered for radiation therapy. In the neutron collimator described in said patent wedge-shaped slabs are arranged in opposed pairs such that the inner opposed edges of each pair of slabs are always directed towards the effective radiation source. This will prevent the formation of an undesired penumbra, that is a half shaded area extending in the longitudinal direction of motion behind each slab. Due to the wedge like shape of each slab also the plane main side surfaces of each slab will be directed towards the radiation source thereby also preventing the formation of an undesired penumbra in the direction perpendicular to the said longitudinal direction. If high energy particles of the kind referred to above should be used the slabs of said neutron collimator are much to thick to absorb the particles and will generate undesirable scatter. Since the slabs are of solid tungsten and low carbon soft iron the known neutron collimator is heavy and requires a sturdy frame for supporting the slabs. This is a drawback from constructional point of view. Moreover, if high energy particles of the kind referred to above in particular electrons are to be used in a collimator the air molecules within the collimator will interact with the particles of the radiation beam thereby giving rise to an effect appearing as if the effective radiation source is no longer, as desired, a point but rather an area of a certain extension. This is undesired since the particles emitted from the periphery of said area will cause an undesired scatter on the inner opposed edges and an increased penumbra resulting from the fact that neither the said inner edges nor the plane main side surfaces of the slab will point to the effective radiation source. Should the known collimator be used for treatment with high energy photons said photons will collide with the atoms of the air giving rise to Compton interactions and secondary electrons, which will give rise to a substantial, non-desired increase of the dose to the skin of the patient. SUMMARY OF THE INVENTION The present invention concerns a multi leaf collimator avoiding the drawbacks of the neutron collimator in accordance with the prior art in order to produce high quality variable field shapes and to maintain a point type radiation source particularly when high energy electrons and photons are used for radiation therapy. Moreover, the weight of the collimator will be reduced. In one aspect of the present invention there is the provision of a multi leaf collimator for a beam of radiation the particles of which ar selected from the group comprising high energy photons, electrons, protons and heavy ions which are emitted from a point type effective radiation source, comprising a protective radiation casing, a frame surrounded by the casing, a plurality of pairs of opposed, elongated, curved in cross section wedge-shaped leaves, wherein adjacent leaves are placed side by side such that a fan-shaped configuration converging towards an apex at the effective radiation source is achieved, each wedge-shaped leaf being mounted for a combined rotational and translational movement on a support structure such that the leaves of each pair are mounted for motion towards and away from each other along a path which intersects the edge of the radiation field from the radiation source at right angle, the inner edge surface as well as the main surfaces of each leaf always being directed generally towards the radiation source setting means for setting each leaf in a predetermined position along each individual path and read out means to determine the position of the leaves. In another aspect of the invention the protective casing is filled with helium gas at atmospheric pressure.