US3582650A

US3582650A - Support structure for electron accelerator with deflecting means and target and cooperating patient support

Info

Publication number: US3582650A
Application number: US852958A
Authority: US
Inventors: Robert F Avery
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1960-08-01
Filing date: 1969-07-14
Publication date: 1971-06-01
Anticipated expiration: 1988-06-01
Also published as: US3360647A

Abstract

Improved linear accelerator apparatus for directing ionizing radiation on an object is realized by utilization of a horizontal axis of rotation about which the linear accelerator axis rotates while the accelerator axis maintains a horizontal spatial orientation at different circumferential points of rotation in conjunction with beam deflection means disposed at the downstream end portion of the accelerator which produce beam deflection along an axis different than the accelerator axis and preferably substantially 90* with respect thereto.

Description

United States Patent Robert F. Avery Moraga, Calif.

July 14, 1969 June 1 1971 Varian Associates Palo Alto, Calif.

Continuation of application Ser. No. 645,916, June 14, 1967, now abandoned which is a continuation of application Ser. No. 411,170, Sept. 14, 1964, now abandoned which is a continuation of application Ser. No. 46,432, Aug. 1, 1960, now abandoned.

Inventor Appl. No. Filed Patented Assignee SUPPORT STRUCTURE FOR ELECTRON ACCELERATOR WITH DEFLECTING MEANS AND TARGET AND COOPERATING PATIENT SUPPORT 3 Claims, 16 Drawing Figs.

US. Cl 250/54, 250/49.5, 250/52, 250/61.5 Int. Cl HOlj 37/00 Field of Search 250/49.5 O, 52, 54,55, 56,57,58,61.5, 91, 106 S References Cited UNITED STATES PATENTS 1,573,571 2/1926 Pohl 250/57 2,731,454 2/1957 Green et al. 250/91 2,890,349 6/1959 Huszar 250/91 2,913,619 11/1959 Geisler, Jr. 250/52X 3,360,647 12/1967 Avery 250/49.5 FOREIG N PATENTS 896,700 11/1953 Germany 250/6l.5

OTHER REFERENCES An 8-mev. Linear Accelerator for X-ray Therapy" by C. W. Miller from The Metropolitan Vickers Gazette," Vol. 25, No. 424, Nov. 1954, Pages 433- 447 Linear Electron Accelerators for Deep X-Ray Therapy by T. R. Chippendale from Philips Technical Review, Vol. 17, No.1, Jul. 1955, pages 3l- 33 Primary ExaminerWilliam F. Lindquist Attorneys-Stanley Z. Cole and Leon F. Herbert ABSTRACT: Improved linear accelerator apparatus for directing ionizing radiation on an object is realized by utilization of a horizontal axis of rotation about which the linear accelerator axis rotates while the accelerator axis maintains a horizontal spatial orientation at different circumferential points of rotation in conjunction with beam deflection means disposed at the downstream end portion of the accelerator which produce beam deflection along an axis different than the accelerator axis and preferably substantially 90 with respect thereto.

PATENTEDJUN Hen 3.582.650

sum 2 UF 7 WV/ 1. 5 2 16 1. .40., 2 4/ v SUPPORT STRUCTURE FOR ELECTRON ACCELERATOR WITH DEFLEC'IING MEANS AND TARGET AND COOPERATING PATIENT SUPPORT CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation of US. Pat. application Ser. No. 645,916 filed June 14, 1967 by Robert T. Avery, which application is in turn a continuation of US. Pat. application Ser. No. 411,170 filed Sept. 14, 1964, in turn a continuation of US. Pat. application Ser. No. 46,432 filed Aug. 1, 1960, all of which applications are now abandoned and applications assigned to the same assignee as the present application.

The present invention relates in general to particle accelerators and in particular to methods and means for irradiating objects with ionizing radiation for research, therapy, sterilization, polymerization, radiography and the like.

In the field of ionizing radiation, particle acceleration machines of large size such as Van de Graaff generators, resonant transformers and linear accelerators are used for producing very energetic radiation. In using these machines .it is often desirable to direct the high energy particles or the rays produced by high energy particles from many different directions at the object being irradiated. The conventional approach in designing high energy irradiation machines is to have the particle beam strike the object or irradiation target in the forward direction. The machines just referred to are very long and are not easily moved about and therefore, they require a large mounting fork or gantry. In this manner, these machines require considerable head room for overhead shots and require a large room to accommodate the rear of the machine for distant horizontal shots. Furthermore, this construction does not readily permit lateral internal shots of large hollow structures.

Also the particle beam emanating from the particle accelerating machines contains particles of different energies many of which it is desired to direct in the same direction or focus on a small spot. Furthermore in certain irradiation machines, such as for use in therapy, a change in energy of the particle beam emanating from the machine over a period of time must not effect the ultimate direction on the ionizing radiation or serious harm might come to the patient.

The present invention relates to accelerating a particle beam along a substantially horizontal axis and directing the beam at an angle of 90 with respect to its original axis to irradiate an object from many directions. With this scheme in mind other criterion govern the manner in which this can be accomplished since invariably a wide range of particle energies (momentum) emanate from the particle accelerator. In an industrial radiographic irradiation machine, for example, it is important that the particle spot size on the X-ray target be as small as possible in order to achieve a sharp X-ray image. On the other hand in an irradiation machine which produces X-rays for therapy it is more important that the X-ray beam be uniform over the area being irradiated since small changes in the energy of the particles striking the X-ray target are apt to cause a dangerous overdose in a portion of the area being irradiated.

The object of the present invention is to provide a novel method and means for irradiating objects with large particle accelerating machines from many different directions within a relatively limited space.

One feature of the present invention is the provision of a novel irradiation machine including a horizontal particle accelerating means, means for deflecting the particle beam emanating from the particle accelerator means at an angle of 90 with respect to its original direction, and means for moving the accelerator in both horizontal and vertical directions for irradiating an object from many sides.

Another feature of the present invention is the provision of an irradiation machine of the first mentioned feature including means for rotating the deflecting means about the axis of the accelerator means whereby ionizing radiation may be directed on the inteiior surface of a cylindrical object.

Another feature of the present invention is a provision of an irradiation machine of the first mentioned feature including means for rotatably supporting the accelerator means about a horizontal axis spaced from the axis of the accelerating means.

Another feature of the present invention is the provision of a novel irradiation machine of the last mentioned feature including means for positioning the portion of the object being irradiated on the horizontal axis of the supporting means and rotating the portion of the object being irradiated about a vertical axis through the portion of the object being irradiated.

Another feature of the present invention is the provision of an irradiation machine of the first mentioned feature wherein the deflecting means includes first magnet means adapted to deflect the accelerated particle beam a small angle from its axis and second magnet means adapted to deflect the beam so that it crosses the axis of theparticle accelerator substantially perpendicular thereto.

Still another feature of the present invention is the provision of an irradiation machine of the first mentioned feature wherein the deflecting means includes magnet means adapted to deflect the beam through substantially 270 whereby the particle beam crosses the axis of the particle accelerator means substantially perpendicular thereto.

Still another feature of the present invention is the provision of an irradiation machine of the first mentioned feature wherein the deflecting means include first magnet means for deflecting the particle beam a small angle from its original direction, second magnet means for deflecting the particle beam the remainder of the and a target for producing ionization irradiation positioned in the second deflecting magnet means out of the path of the particle beam deflected by said first magnet means but positioned such that the particle beam entering said second deflecting magnet means along the axis of said particle accelerator means will be deflected through substantially 90 and strike said target.

Still another feature of the present invention is the provision of a novel method for irradiating objects with ionizing irradiation including the steps of accelerating a particle beam along a horizontal axis, deflecting the accelerated particle beam substantially 90 with respect to the accelerator axis and providing horizontal and vertical motion for the accelerating means and rotational motion about a vertical and horizontal axis whereby an object can be irradiated from many directions within a relatively small area.

FIG. I is a diagrammatic view of a radiographic irradiation machine embodying features of the present invention,

FIG. 2 is a diagrammatic view of the structure shown in FIG. I taken along line 2-2 in the direction of the arrows,

FIGS. 3 and 4 are additional views of the radiographic irradiation machine of FIGS. 1 and 2 with the machine posi tioned so as to direct X-rays at different portions of a cylindrical member from within the cylinder,

FIG. 5 is a side view partially broken away of the irradiation head on the radiographic irradiation machine as shown in FIGS. 1-4,

FIG. 6 is a side prospective view of an irradiation machine embodying features of the present invention and useful for therapy,

FIG. 7 is an end view of the structure of FIG. 6 taken along the line 7-7 in the direction of the arrows,

FIG.'8 is a side view partially broken away of a possible particle beam bending apparatus as utilized in the head of the irradiation machine shown in FIGS. 6 and 7,

FIG. 9 is a schematic view showing one manner in which the particle beam can be deflected through a 90 angle from the original direction and focused on a small spot,

FIG. 10 is a schematic diagram showing other apparatus whereby the particle beam can be deflected so that it is directed in a direction at 90 from its original direction,

FIG. 11 is a longitudinal view taken of the particle beam shown in FIG. 10 along the line 10I0 in the direction of the arrows with the electron beam laid out in a straight line,

FIG. 12 is an alternative embodiment of the structure shown in FIG. 10,

FIG. 13 is a cross-sectional view of the particle beam of FIG. 12 taken along line 1313 in the direction of the arrows with the electron beam laid out in a straight line,

FIG. 14 is a schematic view of other means for deflecting the particle beam in the irradiation machines embodying the present invention,

FIG. 15 is a longitudinal view of the particle beam shown in FIG. 14 taken along the line 15-15 in the direction of the arrows, and

FIG. 16 is a longitudinal view of the particle and X-ray beams shown in FIG. 14 taken along line 15-15 in the direction of the arrows.

The particular irradiation machines depicted in the drawing and described in the following specification are especially designed for directing a beam of electrons at a target or object to be irradiated. However, the features of the present invention are equally applicable to irradiating machines for irradiating objects with other particles such as, for example, protons, deuterons and neutrons. Also, the novel irradiating machines are adaptable for use with both pulsed and continuous beams.

Referring to FIGS. 1-5 of the drawings, an irradiating head 21 is rotatably mounted on the end of a particle accelerating device 22 such as, for example, a linear accelerator. This particle accelerator could be a Van de Graaff generator or a resonant transformer accelerator. The particle accelerator 22 is mounted horizontally with its cooperating electronic components housed in a cabinet 23 on the end of a vertically rotatable telescoping boom 24 the upper end of which is supported from an overhead crane 25 which can be moved the length and width ofthe room in which the particle accelerator 22 is housed.

With the irradiating head 21 rotatably mounted on the end of the horizontally positioned particle accelerator 22 an irradiating beam can be directed substantially anywhere within the room housing the particle accelerator 22. Thus, when using the irradiating head 21 to generate an X-ray beam for use in industrial radiography the X-ray source can be located closer to the floor, ceiling, or any walls with this configuration. Furthermore, the irradiating head 21 can be positioned within a cylinder as shown in FIGS. 3 and 4 and internal radiography achieved by rotating the irradiating head 21 about the horizontal axis of the particle accelerator 22.

Referring now to FIG. 5 a typical irradiation head 21 for use in industrial radiographic applications would include first and

second bending magnets

26 and 27 respectively for first deflecting an electron beam 28 emanating from the particle accelerator 22 through a small angle and then bending the particle beam back toward the accelerator axis so that when it crosses the axis it is substantially perpendicular thereto. The manner in which this particular scheme for bending the particle beam focuses the beam into a small spot even when the beam includes particles of different energies will be described in detail below with reference to FIG. 9. At the position at which the bent particle beam crosses the axis of the accelerator an X-ray target 28' is positioned such as by means of a motor driven rotating rod 29 whereby the X-ray target 28 can be rotated to aid in long life. The X-rays emanating from the X-ray target 28' will pass through a beam flattener 31 as of, for example, aluminum which provides a greater thickness of the center portion of the beam of X-rays to provide a uniform intensity across the X-ray beam. The size of the X-ray beam is limited by a collimator 32 as such as lead and the remainder of the area around the second bending magnet and the X-ray target 28' is surrounded by lead shielding 33 for preventing excessive irradiation in directions other than through the collimator 32.

Referring now to FIGS. 6-8, there is shown apparatus for adaption of the present invention for use in therapy. An irradiation head 41 is mounted at the end of a substantially horizontal particle accelerator 42 which is supported on a counterweighted gantry 43 rotatable about a horizontal axis 55 which passes through the spot 40 where the portion of the patient which is being irradiated is positioned. The X-ray beam will always be directed perpendicularly on the same spot 40 on the axis of the gantry, and the gantry can make a complete rotation about spot 40 within a room and require very little headroom. A typical machine fits within a room 8 feet high.

The patient is placed on a couch 44 with the malignancy to be irradiated positioned at the spot 40 on the gantry axis 55. The couch is supported by a vertically movable post 44 placed a distance away from a vertical axis 56 passing through the spot 40 so that the irradiation head 41 can pass beneath the spot 40. The post 44 is supported on a floor level horizon tal platform 45 which is rotatable about the vertical axis 56 passing through the spot 40. In this manner the gantry can rotate through 360 in a vertical plane which passes through the spot 40 and the patient can be moved through almost 360 in a horizontal plane so that the malignancy can be irradiated from almost every direction. It is obvious that a chair could be used in place of the couch 44.

The irradiating head 41 includes magnet means such as that shown in FIGS. 1013 and described in more detail below whereby the electron beam 46 emanating from the particle accelerator 42 is bent through substantially 270 and directed vertically downward onto an X-ray target 47. The amount of X-rays emanating from the X-ray target 47 is limited to a small spherical angle by a collimator 48 and then further limited to the desired spot or field size by two pairs of movable shielding jaws 4-9. As is apparent from the drawing, since the rotational axis 55 of the gantry 43 passes through the spot 40, the gantry 43 will maintain the irradiation head 41 and, hence, the target 47 at a fixed distance from spot 40 at all positions of rotation of the head about the axis 55. Also, a light 51 can be directed on the area being irradiated by reflecting it off a mirror 52 which lies in the path of the X-ray beam. Thus the light 51 will follow the path of the beam and, as is apparent from the drawing, indicate visibly the size of the area irradiated. A wedge filter 53 can be positioned in the path of the X-ray beam when the patient is lying on his side so that the X-ray intensity across a malignancy is relatively uniform even though some X-rays have passed through a greater distance of body tissue.

Referring now to FIG. 9, there is shown the manner in which the particle beam emanating from a particle accelerator with an energy spread of, for example, 10 percent is deflected a small angle away from the axis of the particle accelerator upon passing between the pole pieces ofa first deflecting magnet 26 and is then bent back toward the axis of the particle accelerator upon passing between the pole pieces of a second magnet 27. In this manner the beam is focused such that it crosses the axis of the accelerator at right angles thereto and focuses into a small spot thereon. The deflection caused by the first deflecting magnet 26 causes rays of varying energies to diverge and causes rays of the same energy to converge horizontally. The second bending magnet 27 bends the beam in the reverse direction and causes rays of different energies in an original horizontal position to focus down to a small spot.

The location of the energy focus" is primarily determined by the entrance angle of the second bending magnet 27. The location of the horizontal focus can be shifted by varying the entrance or exit angles or the radius of curvature of the first bending magnet 26. By suitable adjustment of these perimeters these two foci can be made coincident. The location of the vertical focus can be changed by varying the gap radius ratio of the second magnet which changes the amount of bending in the magnet fringe field. By suitably shifting the vertical focus, it can be made coincident with the other two foci which produce a very small focal spot size. Industrial radiographic applications require an extremely small electron beam spot size on the X-ray target to create a point source" in order to produce a sharp image on the X-ray films surrounding the particle being X-rayed. By utilizing the first and

second bending magnets

26 and 27 it is possible to focus a 10 mev. electron beam with a 10 percent energy spread from a beam with a 5 mm. diameter to a 1 mm. diameter on the target.

Furthermore, this arrangement allows the target to lie on the axis of rotation of the irradiation head. In this manner when irradiating a cylinder from within as shown in FIGS. 3 and 4 with a sheet of X-ray film surrounding the cylinder the source for irradiating in the 360 directions will be the same point on the axis of rotation to the irradiation head.

Referring now to FIGS. and lll there are shown vertical and horizontal sections through a particle beam which is bent through an angle of substantially 270 upon passing between the pole pieces of a bending magnet 50. The output rays from the bending magnet 50 are substantially parallel to each other on the plane of bending, and the beam is of slightly larger size than the input beam. The vertical focusing characteristics can be varied by changing the pole gap thereby changing the effective width of the fringe field.

Alternatively as shown in FIGS. 12 and 13 the particle beam can be bent through an angle of substantially 270 and also provided with an energy focus at some distance from the magnet by means of a bending magnet 50' similar to the bending magnet 50 described above but with slightly concave input pole faces. This arrangement will focus the beam to a smaller spot size and the target can be located on the accelerator axis if desired.

Also, in therapy where a particle beam such as an electron beam is directed onto a target to produce X-rays for irradiating a malignancy it is extremely important when slight variations occur in beam energy that the particle beam remain perpendicular to the target, or otherwise a greater dose of irradiation would be applied to one part of the malignancy than to the other. The present invention embodies a magnet system whereby slight changes in energy of the beam emanating from the particle accelerator will not significantly change the axis of the lobe of the X-rays generated from the X-ray target.

Thus, with respect to the magnet arrangements shown in FIGS. 10 13, higher energy particle rays will remain between the bending magnets 50, 50' for a greater length of path and will therefore be bent through substantially the same angle as lower energy rays with the result that the sum of all the deflecting forces applied to any given particle will be substantially proportionate to the energy of that particle. In this manner, rays of all energies will strike the X-ray target at substantially right angles thereto and thus the axis of the lobe of X-rays will be directed perpendicular to the X-ray target and directly at the area being radiated. This avoids the possibility that a change in particle energy will cause a greater dose of irradiation in any particular area of the irradiating spot.

Referring now to FIGS. l4-16, there is shown another embodiment of the present invention wherein either the particle beam can be bent through'an angle of 90 with respect to the axis of the accelerator and onto the object to be irradiated or can be directed upon a target to produce X-rays which are directed on the object to be irradiated.

For the direct irradiation the particle beam 60 emanating from the particle accelerator is passed between the pole pieces of a deflecting electromagnet 61 which deflects the particle beam 60 away from the axis of the particle accelerator from which it emanates. The particle beam then passes between the pole pieces of a second deflecting magnet 62. Between the pole pieces 62 of the second deflecting magnet the higher energy rays travel a further distance and are therefore subjected to a greater magnetic field so that they are deflected through substantially the same angle as lower energy rays. Upon emerging from between the pole pieces 62 of the second bending magnet the particle beam is slightly diverging for irradiation of a generally broad subject.

FIG. shows a top view of the particle beam path through the

magnets

61 and 62.

When it is desired to create X-rays with the apparatus of FIG. 14 the electromagnet 61 is turned off and the particle beam 60 is directed along the axis of the particle accelerator until it enters a gap between the pole pieces of the second deflecting magnet 62. With the input face of magnet 62 normal to the particle beam 60, horizontal foci are produced in a plane containing the X-ray target 63. This plane is approximately at 45 to the directions of both the input and output beam directions. The rays of lower than normal energy are bent through on a shorter radius and the rays of greater than nominal energy are bent through 90 on a longer radius. Thus, central rays of all energies over the chosen energy range are traveling parallel at the instant of impingement on the target. Rays of excessively low energy miss the target and are collected in a shielded cup. This design makes the direction of the axis of the X-ray lobe always in the same direction even when the machine energy drifts; consequently, changes in electron energy do not produce asymmetries in the X-ray field.

The input face of magnet 62 can be tipped slightly if desired to produce slight vertical convergence of the beam rays at the X-ray target, thereby producing slight enlargement of the beam horizontally. FIG. 16 shows a top view of the particle beam path with the input pole pieces of the magnet 62 tipped slightly to produce this slight vertical convergence. Also with. this arrangement the electron beam shown in FIG. 15 would not diverge as much upon passing through the magnet 62.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What I claim is:

1. Radiotherapy apparatus capable of directing a beam of ionizing radiation from a multitude of directions onto a portion of an object to be irradiated, said apparatus comprising a high energy linear accelerator for producing a beam of high energy particulate radiation including particles having different energy levels; particulate radiation deflecting means for deflecting particulate radiation emanating from said accelerator through an angle of approximately 270 in one direction back across the path of said particulate radiation emanating from said accelerator for focusing said particulate radiation onto a target reactive with said particulate radiation to produce ionizing radiation, the sum of all the deflecting forces applied to any given particle of said particulate radiation being substantially proportionate to the energy of that particle in order to deflect all of said particles within a predetermined range of energy levels through substantially the same angle; collimation means for directing said ionizing radiation along a beam axis; movable shielding means for defining the cross-sectional area of the beam of ionizing radiation; a light source; means for directing light from said source through said movable shielding means onto the area to be irradiated to indicate visibly the size of the area irradiated; said target, deflectingmeans, collimating means, movable shielding means, light source and said light directing means being carried by an irradiation head from which said ionizing radiation emanates along said beam axis; a gantry mounting said linear accelerator and said irradiation head for rotation in a generally vertical plane about a second axis which is spaced from said head and traverses the path of said beam axis at a preselected point, said gantry maintaining the point at which said particulate radiation is focused onto said target at a fixed distance from said preselected point at all positions of rotation of said head about said second axis; a holding member for positioning the portion of the object to be irradiated at said preselected point at which said second axis traverses said beam axis, said holding member being mounted by a support means for rotation with respect to said irradiation head on a generally vertical axis passing through said point, said support means for holding member including a base which is spaced a greater distance from said second axis than said irradiation head whereby said head is rotatable between said holding member'and said base, said support means further including a supporting column connecting said base and said holding member at a location outside the path of travel of said gantry and said irradiation head between said holding member and said base and means for adjusting the location'of said holding member in the direction of said axis about which said holding member is rotatable to enasecond axis is substantially horizontal. ble adjustment of the location of said holding member with 3. The radiotherapy apparatus of claim 1 wherein said base respect to said preselected point. of said support means is rotatable about a generally vertical 2. The radiotherapy apparatus of claim 1 in which said axis to provide said rotation ofsaid support means.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 582 650 Dated June 1 1971 Inventor(s) Robert T Avery identified patent It is certified that error appears in the aboveshown below:

and that said Letters Patent are hereby corrected as On the title page, left column, line 12, "abandoned" should read issued as Patent 3,360,647 Column 1, line 9, after "1964" insert upon which Patent 3,360,647 issued on December 26, 1967 line 11, after "which applications" insert except for application Ser.. No. 411,170, li 11 and 12, cancel "applications".

Signed and sealed this 22nd day of February 1972.

(SEAL) Attest:

ROBERT GOTTSCHALK EDWARD M.FLETCHER,JR. Commissioner of Patents FORM PO-1050 [10-69) USCOMM DC 603754359 9 u s, sovsnumzm PRINTING omcr lacs o-aco-au

Claims

1. Radiotherapy apparatus capable of directing a beam of ionizing radiation from a multitude of directions onto a portion of an object to be irradiated, said apparatus comprising a high energy linear accelerator for producing a beam of high energy particulate radiation including particles having different energy levels; particulate radiation deflecting means for deflecting particulate radiation emanating from said accelerator through an angle of approximately 270* in one direction back across the path of said particulate radiation emanating from said accelerator for focusing said particulate radiation onto a target reactive with said particulate radiation to produce ionizing radiation, the sum of all the deflecting forces applied to any given particle of said particulate radiation being substantially proportionate to the energy of that particle in order to deflect all of said particles within a predetermined range of energy levels through substantially the same angle; collimation means for directing said ionizing radiation along a beam axis; movable shielding means for defining the cross-sectional area of the beam of ionizing radiation; a light source; means for directing light from said source through said movable shielding means onto the area to be irradiated to indicate visibly the size of the area irradiated; said target, deflecting means, collimating means, movable shielding means, light source and said light directing means being carried by an irradiation head from which said ionizing radiation emanates along said beam axis; a gantry mounting said linear accelerator and said irradiation head for rotation in a generally vertical plane about a second axis which is spaced from said head and traverses the path of said beam axis at a preselected point, said gantry maintaining the point at which said particulate radiation is focused onto said target at a fixed distance from said preselected point at all positions of rotation of said head about said second axis; a holding member for positioning the portion of the object to be irradiated at said preselected point at which said second axis traverses said beam axis, said holding member being mounted by a support means for rotation with respect to said irradiation head on a generally vertical axis passing through said point, said support means for holding member including a base which is spaced a greater distance from said second axis than said irradiation head whereby said head is rotatable between said holding member and said base, said support means further including a supporting column connecting said base and said holding member at a location outside the path of travel of said gantry and said irradiation head between said holding member and said base and means for adjusting the location of said holding member in the direction of said axis about which said holding member is rotatable to enable adjustment of the location of said holding member with respect to said preselected point.

2. The radiotherapy apparatus of claim 1 in which said second axis is substantially horizontal.

3. The radiotherapy apparatus of claim 1 wherein said base of said support means is rotatable about a generally vertical axis to provide said rotation of said support means.