ARRANGEMENT IN A GAMMA UNIT

Abstract: An arrangement in a gamma unit, comprising a large number of radiation sources (9) mounted within a radiation shield (2) and having beam channels (6, 19) directed radially from said radia­ tion sources toward a common focal point (F), said radiation shield comprising a space adapted to accommodate the head of a patient resting on a support. The novel matter resides in that the radiation sources (9) and the beam channels (6, 19) directed radially therefrom toward the focal point are located, in rela­ tion to the diametrical plane through the open­ ing to the space, within a zone extending to latitudes 30°-45°, as seen from said diametrical plane.

Description: The present invention relates to an arrangement in a gamma unit, comprising a plurality of radiation sources mounted within a part-spherical radiation shield, said radiation shield being divided into a first semispherical part and a second part by a vertical diametrical plane, said shield surrounding a central space adapted to accommodate the head of a patient lying on a support, the unit further comprising a semispherical central body within the first part of the radiation shield, said central body having beam channels directed radially from said radiation sources towards a common focal point which is in the centre of said diametrical plane. Gamma units of the above-mentioned type are previously known from, for example, ACTA RADIOLOGICA; Vol. 13:1974, "Irradiation of small structures through the intact skull", ACTA RADIOLOGICA; Vol. 14:1975, "Destruction of small intracranial tumours with ⁶⁰Co gamma radiation" and JOURNAL OF NEUROLOGY; NEUROSURGERY & PSYCHIATRY; Vol. 46:1983 "Stereotactic Radiosurgery". In the prior art arrangements, the centre axis of the central body makes an angle of 55° with the patient's longitudinal axis and the horizontal plane, and the beam channels have a varying distribution in respect of both position and number about this centre axis. In known and planned gamma units, the beam channel distribution has varied between ±30 to ±51° in one plane and about ±80° in the other. In these prior art arrangements, some beams will approach the longitudinal axis of the patient so that there is a certain risk that scattered radiation and an occasional primary beam will reach the surroundings when the door towards the central body is opened. The present invention aims at providing a novel type of gamma unit adapted to the mode of operation of CT (Computer Tomography) and NMR (Nuclear Magnetic Resonance) machines which give cross-sections of the examined part of the body and minimize the risk of scattered radiation. The novel matter of the arrangement according to the invention resides in that the radiation sources and beam channels are located, relative to said diametrical plane, within a zone extending up to latitudes 30°-45°, and the second part of the radiation shield extends beyond the diametrical plane to define the opening such that the radiation passing through the focal point impinges on said second part of the radiation shield. An embodiment of a gamma unit arranged in accordance with the invention will be described below, reference being made to the accompanying drawings in which Figs. 1, 2 and 3 are schematic lateral views of the outer construction of the arrangement, Fig. 4 is a vertical section on a larger scale of the portion around the central body and the helmet, and Fig. 5 is a section on an even larger scale of a cutout portion comprising a beam channel. A frame 1 supports the essentially spherical radiation shield 2 which is divided into two halves along a vertical diametrical plane 12. One half 3 accommodates, in a semispherical space, the central body 4 which has a semispherical outer surface and a semispherical inner surface 7. In the outer surface, and in an essentially annular portion adjacent the edge of the semipherical central body 4, a large number of seats 5 are arranged, from which radially extending beam channels 6 are extending inwardly towards the inner surface 7 of said central body 4. The beam channels 6 are slightly widened towards the inner surface 7 of the central body 4, and all channels 6 are so directed that their axes converge at the centre of said body, the focal point F being located in the same plane as the edge of said central body. At the outer end of each beam channel 6, a bushing 8 is provided in the seat 5, said bushing carrying the radiation source 9 which preferably is a cobalt 60 isotope. The beam channels are located within an annular portion adjacent the free edge of the central body and have an inward extent amounting to 30°-45°. In other words, the beam channels are located within an annular curved zone extending maximally up to latitudes 30°-45°, as seen from the vertical diametrical plane. In the other half 10 of the radiation shield, a through opening 11 is provided which extends up to the vertical diametrical plane 12 of intersection between the two halves and thus opens into the interior of said central body 4. From the lower edge at the outer end of the opening, a frame 13 provided with guides for a bed-like support 14 carrying the patient is extending. The two halves of the radiation shield 2 are carefully and precisely fitted together and have, on their opposing surfaces, complementary lands and grooves to make the connection radiation safe. The patient support 14 is mounted on wheels travelling in the guides of the frame 13 and comprises an elongate bed-like surface 16 on which the patient is lying. The helmet 17 is supported by a bracket 18 secured to the support 14. The helmet 17 has an outer shape carefully adapted to the shape of the inner surface 7 of the central body 4 and can be inserted therein with a clearance of some millimetres. Mounted in the helmet are a number of radial channels 19 corresponding to the number of beam channels 6 in the central body 4, said channels 19 being exactly aligned with the channels 6 in said central body 4 when the helmet 17 has been inserted therein. The channels 19 form nonadjustable apertures defining the dimension of the beams passing therethrough to the patient to be treated. Thus, an uninterrupted path of radiation extends from the respective radiation source radially toward the focal point F within the helmet 17 in the diametrical plane thereof. Mounted within the helmet are attachments for a so-called stereotactic instrument, i.e. a device defining the position of the area to be treated. During treatment, the patient's head is fixed in a position such that the radiation focal point F coincides with the said treatment area. The geometrical central axis c of both the central body 4 and the helmet 17 is horizontal and thus essentially parallel with the longitudinal axis of the patient. The central axis may also be said to form a normal towards the vertical diametrical plane through the opening 11 in the spherical radiation shield. As has been mentioned above, the beam channels 6 and 19, respectively, are arranged in an annular portion of the semispherical central body 4 adjacent the edge thereof so that geometrical axes of the beam channels converge in a point, the focal point F, located in the vertical diametrical plane through the edge of the central body 4 and the helmet and lie within a portion extending up to 30°-45° from the vertical diametrical plane. As a result, no beams will be directed outwardly through the opening 11 of the radiation shield 2, and all beams that have passed the focal point will impinge solely upon the radiation-shielding edges of the said opening. The arrangement according to the invention reduces the scattered radiation and, because of the reorientation of the central axis and the annular grouping of the beam channels around the outer portion of the central body 4, is suitable for use with prior art machines for computer tomography (CT) and NMR which illustrate cross-sections of the examined part of the body.

Claims: 1. Gamma unit, comprising a plurality of radiation sources (9) mounted within a part-spherical radiation shield (2), said radiation shield (2) being divided into a first, semispherical part and a second part by a vertical diametrical plane (12), said shield (2) surrounding a central space adapted to accommodate the head of a patient lying on a support (14), the unit further comprising a semispherical central body (4) within the first part of the radiation shield (2), said central body (4) having beam channels (6) directed radially from said radiation sources (9) towards a common focal point (F) which is in the centre of said diametrical plane (12), characterized in that the radiation sources (9) and the beam channels (6) are located, relative to said diametrical plane (12), within a zone extending up to latitudes 30°-45°, and the second part of the radiation shield (2) extends beyond the diametrical plane (12) to define an opening (11), such that the radiation passing through the focal point (F) impinges on said second part of the radiation shield (2). 2. Gamma unit as claimed in claim 1, characterized in that the radiation sources (9) are arranged, in per se known manner, in seats (5) adjacent the outer surface of an essentially semispherical central body (4) provided in the space of the spherical radiation shield (2), said beam channels (6) extending radially from the said seats (5) through the central body (4) towards the inner cavity thereof, in which a helmet (17) is insertable which has a number of radial trough channels (19) corresponding to the beam channels (6) within said central body (4) and is provided with means for essentially nondisplaceable holding the patient's head, and that the radial channels (6 and 19, respectively) within said central body (4) and said helmet (17), which channels extend radially from the seats (5) in the outer surface of said central body (4), are located within curved annular zones of said central body (4) and said helmet (17), said zones extending to maximally latitudes 30°-45°, as seen from the essentially vertical diametrical plane (12) through the spherical shield (2) and the opening (11) of said central body (4) and said helmet (17), respectively. 3. Gamma unit as claimed in claim 2, characterized in that the semispherical central body (4) provided with said radial beam channels (6) from said radiation sources (9) is so located in relation to the position of a patient lying essentially horizontally on the support (14, 16) that its geometrical central axis (c), which is perpendicular to the vertical diametrical plane (12), is essentially horizontal and parallel and/or coincident with the patient's longitudinal axis. 4. Gamma unit as claimed in claim 2 and 3, characterized in that the radiation shield (2) comprises two halves (3, 10) of heavy material with a vertical plane of division (12), that a semispherical space accommodating the semispherical central body (4) is provided within one radiation shield (3), while the other radiation shield (10) has a through opening (11) into the interior of said central body (4), that a bedlike support comprises a bed (16) supporting the patient and movable into said opening (11), and that the bed at its inner end is provided with a bracket (18) carrying the helmet (17) adapted to support the patient's head, said helmet being provided with channels (19) and adapted to be accommodated by the space within said central body (4). 5. Gamma unit as claimed in claim 4, characterized in that the channels (19) of the helmet (17) are formed as nonadjustable apertures determining the dimension of the beam penetrating therethrough.

Inventors: SUNDQVIST, HANS
Applicants: ELEKTA INSTRUMENT S.A.
Application year: 1987
Main classification: A61N5
IPC codes: G21K1, G21K5, A61N5, A61B6