Patent Number: 051480328
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION FIG. 1 shows a part of a radiation therapy device 2 of common design, in which plates 4 and a control unit constructed in accordance with the principles of the invention are used. The radiation therapy device 2 comprises a gantry 6 which can be swiveled around a horizontal axis of rotation 8 in the course of a therapeutic treatment. The plates 4 are fastened to a projection of the gantry 6. To generate the high-powered radiation required for the therapy, a linear accelerator is located in the radiation therapy device 2. The axis of the radiation bundle emitted from the linear accelerator and the radiation therapy device 2 is designated by 10. Either electron radiation or photon radiation (X-ray radiation) can be used for the therapy. During the treatment, the radiation beam is trained on a zone 12 of a patient 13 which is to be treated and which lies in the isocenter of the gantry rotation. The rotational axis 8 of the gantry 6, the rotational axis 14 of a treatment table 16 and the beam axis 10 all intersect in the isocenter. The construction of such a radiation therapy device is described in detail in a publication "A Primer on Theory and Operation of Linear Accelerators in Radiation Therapy", U.S. Department of Health and Human Services, Rockville, MD, December 1981. FIG. 2 illustrates portions of the control unit and of the beam generation system in the radiation therapy device 2 according to FIG. 1. An electron beam 1 is generated in an electron accelerator 20. Accelerator 20 comprises an electron gun 21, a waveguide 22 and an evacuated envelope or guide magnet 23. A trigger system 3 generates injector trigger signals and supplies them to injector 5. Based on these injector trigger signals, injector 5 generates injector pulses which are fed to electron gun 21 in accelerator 20 for generating the electron beam 1. The electron beam 1 is accelerated and guided by waveguide 22. For this purpose, a HF source (not shown) is provided which supplies RF signals for the generation of an electromagnetic field supplied to waveguide 22. The electrons injected by injector 5 and emitted by electron gun 21 are accelerated by this electromagnetic field in waveguide 22 and exit at the end opposite to electron gun 21 as electron beam 1. Electron beam 1 then enters the guide magnet 23 which bends electron beam 1 by 270 degrees. Electron beam 1 then leaves guide magnet 23 through a window 7 along axis 10 and then encounters a first scattering foil 15, goes through a passage way 51 of a shield block 50 and encounters a second scattering foil 17. Next, it is sent through a measuring chamber 60, in which the dose is ascertained. If the scattering foils are replaced by a target, the radiation beam is a X-ray beam. Finally, aperture plate arrangement 4 is provided in the path of radiation beam 1, with which the irradiated field of the subject of investigation is determined. Aperture plate arrangement 4 comprises a pair of plates 41 and 42 which are moveable in a direction substantially perpendicular to axis 10 of radiation beam 1. An additional pair of aperture plates can be provided being moveable in a direction perpendicular to axis 10 and to the moving direction of plates 41 and 42. It is also possible that only one plate of said pair is moveable during radiation. Plates 41 and 42 are moved by a drive unit 43 which is indicated only with respect to plate 41 in FIG. 2. Drive unit 43 comprises an electric motor which is coupled to plate 41 and which is controlled by a motor controller 40. A position sensor 44 is also coupled to plate 41 for sensing its position. Motor controller 40 is coupled to a dose control unit which includes a dosimetry controller 61 for providing set values for the radiation beam dose rate in correlation with the position of plate 41 for achieving a given isodose curve. The dose rate of the radiation beam is measured by measuring chamber 60. In response to the deviation between the set values and the actual values, dosimetry controller 61 supplies signals to trigger system 3 which change the pulse repetition frequency so that the deviation between the set values and the actual values of the radiation beam dose rate is minimized. Thus, the dose control unit controls the dose rate of the radiation beam in correlation with the movement of plate 41 in order to achieve the given isodose curve. The ability to change the dose rate is generally known and it is particularly advantageous to use a digital dosimetry system. FIG. 3 shows a graph of dose rate and the accumulated dose for the radiation therapy device of FIGS. 1 and 2 with the moveable plate arrangement. During movement of the plate, the dose rate is changed in a way so that an accumulated dose is achieved which corresponds to a given rigid filter. The mathematical algorithm for a dynamic wedge filter is as follows: If one wants to generate a wedge-shaped isodose contour of an angle .alpha. at 10 cm depth, the dose profile at depth d can be expressed as: ##EQU1## wherein: D(d) is the dose at depth d; and .mu. is the attenuation coefficient of the beam. PA1 x is the jaw position; PA1 k is a scaling factor; PA1 D.sub.10 is the dose at the central axis 10 cm from the surface; PA1 .alpha. is the desired wedge angle; PA1 I.sub.1 is the machine's constant intensity during the idle time; PA1 t.sub.1 is the idle time; and PA1 D total is the total dose. Assume we have a moving jaw at constant speed v along a radiation field of length S.sub.0. Under the above conditions, it can be derived from equation (1) that the machine's intensity during the jaw movement should follow the function: ##EQU2## and the total dose is: ##EQU3## wherein: I is the machine's intensity; There has thus been shown and described a novel radiation therapy device which fulfills all the objects and advantages sought for. Many changes, modifications, variations and other uses and applications of the subject invention will, however, become apparent to those skilled in the art after considering the specification and the accompanying drawings which disclose an embodiment thereof. For example, the variation of the radiation dose rate does not have to be done simultaneously with the movement of the plates. The radiation could be interrupted or intermittently kept constant during the movement of the plates. All such changes, modifications, variations and other uses and applications which do not depart from the spirit and scope of the invention are deemed to be covered by the invention which is limited only by the claims which follow.