Radiation, in the form of electromagnetic radiation or particle radiation, is today used for many purposes, e.g. modification of properties of materials, sterilising purposes as well as medical diagnostics and treatment. The different purposes put different requirements on the methods and devices used, but there are also big similarities.
In the field of radiotherapy and radiosurgery treatment, a common general method is to arrange a patient on a couch. A radiation head is directed relative the patient in order to direct radiation to a certain treatment volume, e.g. a tumour. The general requirement in such applications is to maximise the dose within the treatment volume, while minimise the dose in tissue material outside the treatment volume. One way to accomplish this is to irradiate the treatment volume from different directions, whereby the treatment volume receives a radiation dose at all instances, while certain parts of the surrounding material only receives doses at some occasion.
There are different ways to accomplish multi-directional radiation. For small radiation sources, a multitude of sources may be used. This may e.g. be used for Co-60 sources, which normally are limited in size and weight. For other radiation sources, which are heavier and/or larger, this solution becomes unpractical. Another approach is instead to arrange the radiation head in a movable fashion, e.g. attached to a gantry, which is allowed to move. Since radiation heads often are very heavy, in some cases up to several thousand kilograms, the gantries have typically to be build extremely stable. The easiest way to move the gantry is in such cases to use a rotating movement. Since it in most applications is desirable to have the body to be irradiated positioned horizontally, the rotation axis is typically oriented horizontally, so that the gantry is allowed to move besides and under the body.
The gantry according to prior art is typically designed in a general L or C shape, and the rotational support is provided at one axial end of the body. However, the heavy weights may cause the gantry arms to elastically deform, which results in a modified rotational motion. Instead of a pure rotation, the gantry will precess, creating an inaccuracy in the positioning of the radiation head. Furthermore, the heavy loads will apply extreme forces on the rotation bearings, even if many attempts of balancing the gantries have been made. Due to these reasons, the gantry movements are limited in speed, since high accelerations and retardations increase the load even further.
The couches used together with this type of gantries is typically designed with a support portion, which is situated outside the area of the gantry movement, and a couch sticking into the gantry region substantially horizontally. The reason for this is that no couch supports can be placed in the path of the gantry rotation. Due to the asymmetric design, the couches are typically very sensitive for the positioning of the body on the couch. Placing a body on the couch will typically cause a displacement of up to 1 cm, and changing the location of the body on the couch may alter the displacement several millimeters. This implies that the motion of the couch can not be used during an accurate treatment of e.g. a tumour in the treatment volume.
Since the radiation treatment is intended to be performed also from below, it is important that the couch close to the treatment volume do not absorb the radiation and causes any production of secondary radiation. One way to solve this is to remove most of the solid material from the couch close to the treatment volume, and only leave e.g. a thin stretched foil of material with low cross section for the used radiation. However, in order to support body weight on both sides of the treatment volume, some stiff solid constructions, connecting solid parts of the couch, have to pass relatively close to the treatment volume, e.g. in the form of C-arms or beams. However, changing the irradiation direction, may lead to collisions with such arms, which has to be movable, depending on the position of the radiation head.
Using gantries with a single rotational movement allows for irradiation in different directions in one and the same plane, so called coplanar irradiation. If the ratio between the doses of the treatment volume and the tissues outside, respectively, is going to be increased further, radiation in directions outside this irradiation plane has to be accomplished, so called non-coplanar irradiation. A trivial solution could be to provide the gantry with several radiation heads, irradiating the treatment volume in different out-of-plane directions. However, expensive or heavy radiation heads are not suitable for such solutions.
There are several attempts for solutions giving non-coplanar irradiation in prior art. Some uses radiation deviation means, or uses the possibility to tilt the body to be irradiated. Deviation means are, however, often difficult to control in an exact way, in particular when constant radiation beam properties are concerned. Tilting of the body is unsuitable due to risks of displacements, between the body and the couch, of the couch itself as well as displacements within the body.
The British patent GB 1,129,653 discloses a device for radiation therapy, which is movable in several directions. It discloses the possibility to rotate a gantry around a horizontal axis and at the same time displace the radiation head along a perpendicular circular arch relative to the gantry. This solution provides non-coplanar irradiation, but still performs the problems with instability in the gantry and couch.
The American patent U.S. Pat. No. 5,577,094 discloses another solution, where the gantry is movable along two perpendicular circular paths. This is, besides the earlier described stability problems also restricted mainly to cranial treatments, since the available space around the treatment volume substantially is restricted to one side.
A common problem with most equipment according to the state of the art is that there is a risk for collision between the radiation head and the body, to be irradiated, or couch. When changing irradiation directions, a manual inspection is normally required during movements of the radiation head and/or gantry to ensure that the body is not jammed between the couch and the gantry. This problem makes any requested automation of the treatment difficult. There are some solutions of this problem, which are based on contact switches, which stops any movement, if any distances become too small. Normally, this is combined with using friction safety clutches, applied to the gantry movement. When using such solutions, the maximum allowed moment on the gantry is reduced, which further reduces the maximum retardation and acceleration. Furthermore, the contact switch solution has some reaction time, which put restrictions on the maximum speed of any motion.
The international patent application WO 89/08269 discloses an X-ray tomography apparatus. The apparatus is mounted for rotation around a patient within a circular frame. The apparatus can be displaced along the patient irradiating parallel sections of the body. However, facilities for non-coplanar irradiation are neither discussed nor easily provided for in this apparatus.