A METHOD AND A DEVICE RELATING TO A TRANSMISSION ION CHAMBER

Abstract: A method and a device relating to a transmission ion chamber (11). The transmission ion chamber includes four flat inner ion trapping electrodes (15 to 18) and four outer flat ion trapping electrodes (19 to 22). The transmission ion chamber is inserted in a beam of therapeutic rays. The inner electrodes are completely exposed to the beam, and the ion currents from these electrodes are used to cause the beam to be sym­ metric with reference to the centre line (14) of the primary collimator inserted in the beam. The characteristic feature of the invention is that the outer electrodes are exposed to the beam, on one hand, and are struck by the shadow of the primary collimator inserted in the beam, on the other hand. The ion currents from the outer electrodes are used to center the beam in relation to the centre line (14) after the beam first being caused to be symmetric in relation to the centre line (14).

Description: A METHOD AND A DEVICE RELATING TO A TRANSMISSION ION CHAMBER The present invention refers to a method of centering a beam of therapeutic rays by means of a transmission ion chamber and of causing the beam to be symmetric with reference to each one of a number of inner electrodes which are included in the transmission ion chamber and are located near its centre. The invention also refers to a transmission ion chamber for carrying out this method. A transmission ion chamber conventionally consists of a housing which is filled with an ionizable gas (usually air) where one or more high voltage electrodes are arranged in parallel with and spaced from one or more ion trapping electrodes. The transmission ion chamber is placed in a beam of ionizing radiation with its electrodes orientated generally at right angles to the central axis of the beam. If the high voltage electrodes are connected to a suitable voltage source and the ion trapping electrodes are connected to a suitable measuring system one may measure electric currents which pass from the iron trapping electrodes to earth. These currents are generally proportional to the mean value of the intensity of the beam across the iron trapping electrodes. By designing the ion trapping electrodes in a particular way and by arranging these ion trapping electrodes when designed in such particular way in special patterns it is also possible to use the ion currents of the electrodes for controlling the centering and symmetry of the beam. Examples thereof are known from the French patent specification 7302839 and from the U.S. patent specification 3 838 284. The said French patent specification shows a transmission ion chamber, the principal object of which is to study the homogeneity of the beam, and for thins purpose two different sets of electrodes are used both of which are entirely within the beam. The U.S. patent specification describes a linear accelerator having a transmission ion chamber, the object of which is to measure and correct the centering and symmetry of a beam of photons. In radiotherapy with photons a so called equalizing filter is used to produce treatment fields with uniform dose distribution. The invention described in the U.S. patent specification utilizes properties of this equalizing filter and, therefore, can be utilized only when equalizing filters are used. In a modern radiotherapy apparatus, along with radition of photons, radiation of a different kind, for example electrons, is also used. When utilizing electron radiation no equalizing filter is used. According to the U.S. patent specification the form and placing of the transmission ion chamber will be dependent on properties of the equalizing filter which may be specific to a particular filter. In a modern radiotherapy apparatus there are often several equalizing filters. The utilization of the invention described in the U.S. patent specification together with several different equalizing filters would be possible in certain cases only if the transmission ion chamber is exchanged or has its position changed and the equalizing filter is exchanged. The present invention aims at eliminating the limitations mentioned above and refers to a method and a device for measuring and correcting the centering and symmetry of a beam of ionizing radiation of any kind. The invention is not dependent on the presence of any equalizing filter. The characteristic features of the invention appear from the attached claims.. The basic idea of the invention lies in the fact that the outer electrodes are located so that these are both exposed to the beam and lie in the shadow of a primary collimator inserted in the beam. The primary collimator is a stationary, non-exchangeable structure element which is included in the radiotherapy device, for example a linear accelerator or microtron. A preferred embodiment of the invention will be described more closely below in connection with the attached drawings, in which: Figure 1 diagrammatically shows a linear accelerator in which the transmission ion chamber according to the invention is included, Figures 2, 3, 4 are diagrams which show various dose distributions, Figure 5 is a plan view of the -electrode configuration in the transmission ion chamber according to the invention, Figure 6 is a cross-sectional view along the line V-V in Figure.4 and control equipment coupled thereto, and Figure 7 is a cross-sectional view along the line VI-VI in Figure 4 and control equipment coupled thereto. A particle accelerator 1 emits a ray 2 of loaded particles to a ray transport system which includes four magnetic control elements 3, 4, 5 and 6. After possibly being subjected to a 900 deflection in a deflection magnet 7 the ray 2 strikes a target 8 which then emits a beam of therapeutic radiation, for example, photons, electrons, neutrons, protons or mesons. A primary collimator 9 confines the space angle occupied by the beam emitted by the target 8. The primary collimator 9 is made of a material which is substantially impervious to the beam from the target 8. An equalization filter 10 may be inserted under the primary collimator to change the properties of the beam, for example the dose distribution transversely across the beam. It is to be noted that such an equalization filter 10 is not required for a correct function of the transmission ion chamber according to the invention as described hereinafter. Such a transmission chamber 11 is placed between the primary collimator 9 and the surface 12 to be irradiated. The size of the surface to be irradiated is adjusted by means of a movable collimator 13. The transmission ion chamber ill is used in the first place for determining the dose rate in the treatment field. The means used for this purpose are conventional and therefore will not be described more in detail. By arranging the electrodes of the transmission ion chamber 11 in a particular way the centering and symmetry of the beam may be sensed, and the electric signals from the electrodes may be used to correct errors possibly occurring in the centering and symmetry, respectively. Figure 2 shows an example of a dose distribution transversely across the field of irradiation, a so called transverse distribution, which is obtained with a beam which is centered but unsymmetric with reference to a central line 14 which is the longitudinal symmetry axis (the centre line) of the primary collimator 9 and which ideally has to pass through the effective source of radiation in the target 8. The central ray of the beam is marked with the dashed line 2' in Figure 1. Figure 3 shows an example of a transverse distribution of a symmetric but uncentered beam corresponding to Figure 2. The effective source of radiation is offset in relation to the longitudinal symmetry axis of the primary collimator 9. The central ray of the beam is marked with the dot-and-dash line 2" in Figure 1. Figure 4 shows an example of a transverse distribution corresponding to Figure 2 and obtained with a correctly centered beam symmetric with the reference to the central line 14. The effective centre of radiation is here located on the central line 14 and the central ray of the beam coincides with the central line 14. Figure 5 shows an electrode arrangement in the transmission ion chamber according to the invention. Four flat inner electrodes 15, 16, 17, 18 are arranged in a circular area which is centered on the central line 14. This circular area has the radius R1. Furthermore, four flat outer electrodes 19, 20, 21, 22 are arranged beyond the said circular area with the radius R1 but within another circular area which has the radius R2 and which is centered on the centre line of the primary collimator. The primary collimator 9 collimates the beam so that its diameter in the plane of the transmission ion chamber has the radius RS. The radii of the electrodes 15 and 22 are chosen according to the invention so that R1 < RS( R2. Accordingly, the inner electrodes 15 to 18 will be completely exposed (i.e. will lie entirely within) the beam while the electrodes 19 to 22 are partially shaded by the primary collimator 9 which, as mentioned above, constitutes a non-exchangeable unit of the assembly. As will be seen from Figure 6 the electrodes 16 and 18 are connected to a differential amplifier 23 which supplies the magnetic control element 3 with a current which is proportional to the difference between the signals from the electrodes 16 and 18. Figure 7 shows that the electrodes 15 and 17 are connected to another differential amplifier 24 which supplies the magnetic control element 4 with a current which is proportional to the difference between the signals from the electrodes 15 and 17. The magnetic control elements 3 and 4 cause the beam to be symmetric with reference to the central line 14. A symmetrical beam is, as will be seen by comparing Figures 2, 3 and 4 with each other, a necessary but not sufficient condition that must be fulfilled if the dose distribution is to be also homogeneous or balanced across the surface 12 to be irradiated. A second condition which must be fulfilled is that the symmetric beam must also be centered on the central line 14. When the beam is caused to be symmetric with reference to the central line 14 in the way described above it is found that the relative difference between the ion currents from the electrodes 19 and 21 will be proportional to the offset of the beam, i.e. the offset of the effective centre of radiation in relation to the central line 14 counted along the line VI-VI (Figure 5) while the relative difference between the ion currents from the electrodes 20 and 22 will be proportional to the offset of the effective centre of radiation in relation to the central line 14 counted along the line V-V at right angles to the line VI-VI. To make correction for an uncentered but symmetric beam the outer electrodes 20 and 22 are connected to a differential amplifier 25 shown in Figure 6. The differential amplifier supplies the magnetic control element 5 which moves the beam 2 in a direction that tends to move the effective centre of radiation in the direction of the line V-V towards the central line 14. In a corresponding way the outer electrodes are connected to a differential amplifier 26 which in turn supplies the magnetic control element 6 with a current which moves the beam 2 in a direction that tends to move the effective centre of radiation towards the central line 14 in the direction of the line VI-VI. The differential amplifiers 25 and 26 are designed so that they operate more slowly than the differential amplifiers 23 and 24, whereby the beam is first caused to be symmetric with reference to the central line 14 and is thereupon centered with reference to this line. In the embodiment of the invention described above the magnetic control elements 3 to 6 have been shown to be separate elements which are grouped two and two in the path along which the ray 2 is transported from the particle accelerator 1 to the target 8. According to the invention one of the magnetic control elements in one or the other or in both of the groups may consist of a deflection magnet. For example, the element 6 may be omitted and replaced by a deflection magnet 7 which therefore, in addition to deflecting the ray, also corrects the position thereof. The embodiments of the invention described above may be modified and varied in many different ways within the scope of the basic concept of the invention.

Claims: Claims: 1. A method using a transmission ion chamber (11), which includes at least four flat inner ion trapping electrodes (15 to 18) located at the centre of the ion chamber and at least four flat outer ion trapping electrodes (19 to 22) located near the periphery of the transmission ion chamber, for centering a beam of rays and cause the beam to be symmetric with reference to the centre line (14) of a primary collimator (9) inserted in the beam by deriving electric signals from the electrodes, said signals via control elements (3 to 6) correcting any possibly occurring centering and symmetry errors in the beam, characterized by the fact that the outer electrodes (19 to 22) are located so that they are both exposed to the beam and struck by the shadow of the primary collimator (9), that the inner electrodes (15 to 18) are exposed completely to the beam, that the electric signals from the outer electrodes are utilized for the centering of the beam with reference to the centre line (14) and that the electric signals from the inner electrodes are utilized for causing the beam to be symmetric with reference to the centre line (14). 2. A transmission ion chamber for carrying out the method according to claim 1 and intended to be placed in a divergent beam of therapeutic rays spaced from the surface (12) to be irradiated, including at least four inner ion trapping electrodes (15 to 18) located at the centre of the ion chamber and completely within the beam and at least four outer ion trapping electrodes (19 to 22) located near the periphery of the transmission ion chamber, characterized by a primary collimator (9) inserted in the beam and casting its shadow on the outer electrodes so that these are both exposed to the beam and also lie in the shadow of the primary collimator, the said outer electrodes emitting electric signals which combined with each other represent the offset of the beam in relation to the centre line (14) of the primary collimator, and the inner electrodes emitting electric signals which combined with each other represent the symmetry of the beam with reference to the centre line (14). 3. A transmission ion chamber according to claim 2, characterized by the fact that it includes control members (3 to 6, 23 to 26) connected to the electrodes (15 to 22) for deriving electric signals which cause the central ray of the beam to coincide with the centre line (14) of the primary collimator (9).

Inventors: BRAHME, ANDERS, REISTAD, DAG
Applicants: INSTRUMENT AB SCANDITRONIX
Application year: 1981
Main classification: G01T1
IPC codes: G01T1, G21K5, H05H7, G21K1