Patent Abstract:
The present invention is intended to enable proper elimination of the remanent magnetization of the scanning magnet, which is used in a particle beam therapy system, in a short time. In the particle beam therapy system that irradiates an irradiation target with a particle beam  18  accelerated by an accelerator and scanned by scanning magnets  11  and  12 , power supplies  13  and  14  to operate the scanning magnets  11  and  12  output pattern currents for demagnetizing the scanning magnets  11  and  12 . The pattern current is controlled by a control circuit  15  that reads a demagnetization pattern  17  and controls the power supplies  13  and  14.

Full Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a particle beam therapy system that scans a charged particle beam accelerated by an accelerator with a scanning magnet and irradiates an irradiation object with the particle beam. 
       BACKGROUND ART 
       [0002]    Typically, a particle beam therapy system includes abeam generator that generates a charged particle beam, an accelerator that is connected to the beam generator and accelerates the generated charged particle beam, a beam transport system for transporting the charged particle beam emitted after being accelerated to the energy set by the accelerator, and a particle beam irradiation apparatus that is provided at the downstream side of the beam transport system and irradiates the affected area of the patient, who is an irradiation target, with the charged particle beam. 
         [0003]    In a particle beam irradiation apparatus that is of a scanning irradiation type to form a radiation field by scanning a thin pencil-shaped beam so as to match the shape of an irradiation target, high irradiation position accuracy is required in order to prevent particle beam irradiation to normal tissue other than the affected area. In order to satisfy this requirement, it is necessary to completely eliminate the remanent magnetization of the particle beam scanning magnet. The method of eliminating the remanent magnetization of the electromagnet is disclosed in PTL 1 and PTL 2, for example. 
       CITATION LIST 
     Patent Literature 
       [0000]    
       
         [PTL 1x] JP-A-63-133506 
         [PTL 2] JP-A-10-229014 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0006]    In a particle beam therapy system including two scanning magnets, which control the scanning of a supplied charged particle beam in different directions so that the supplied charged particle beam is shaped into a three-dimensional irradiation shape based on the treatment plan, and a power supply that operate the electromagnet, three-dimensional irradiation is performed on the different affected area for each patient. Therefore, assuming that a patient B is treated after a certain patient A is treated, it is necessary to completely eliminate the influence of the remanent magnetization of the scanning magnet after completing the treatment of the patient A and then to start the treatment of the patient B. If remanent magnetization remains after the treatment of the patient A, an irradiation shape planned for the patient B becomes different from the affected area of the patient B. 
         [0007]    In addition, when eliminating remanent magnetization to treat the patient B after the treatment of the patient A or when resuming the treatment after the treatment is stopped by the interlock operation during the treatment of the patient A due to a certain problem, it is desirable to eliminate the influence of the remanent magnetization as quickly as possible. 
         [0008]    However, in the conventional excitation power supplies for elimination of remanent magnetization that are disclosed in PTL 1 and PTL 2, in order to realize relatively low-cost power supplies, demagnetization is performed by generating a damped oscillation current using an electromagnet and an external capacitor, or demagnetization is performed using a pulse magnetic field from the commercial power supply that is cut by the thyristor. Accordingly, when these are applied to the particle beam therapy system of the present invention, it has not been possible to completely eliminate the influence of remanent magnetization in a short time depending on the state of the remanent magnetization after the completion of demagnetization. 
         [0009]    The present invention has been made in view of the above, and it is an object of the present invention to enable proper elimination of the remanent magnetization of the scanning magnet, which is used in a particle beam therapy system, in a short time. 
       Solution to Problem 
       [0010]    A particle beam therapy system according to the present invention is a particle beam therapy system that irradiates an irradiation target with a particle beam accelerated by an accelerator and scanned by a scanning magnet. A power supply to operate the scanning magnet outputs a pattern current for demagnetizing the scanning magnet. 
       Advantageous Effects of Invention 
       [0011]    According to the present invention, using a power supply for a scanning magnet that performs scanning of a particle beam, a pattern current is made to flow from the power supply at the time of demagnetization, so that the scanning magnet is demagnetized. Therefore, it is easy to change the current pattern and to appropriately eliminate the remanent magnetization within the magnetic pole in a short time with little unevenness. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0012]      FIG. 1  is a block diagram showing a main part of a particle beam therapy system of the present invention. 
           [0013]      FIG. 2  is a flow chart showing the operation procedure of the particle beam therapy system. 
           [0014]      FIG. 3  is a cross-sectional view showing the scanning magnet of the particle beam therapy system of the present invention. 
           [0015]      FIG. 4  is a diagram showing the demagnetization current waveform of a first embodiment of the present invention. 
           [0016]      FIG. 5  is a diagram showing the demagnetization current waveform of a second embodiment of the present invention. 
           [0017]      FIG. 6  is a diagram showing the demagnetization current waveform of a third embodiment of the present invention. 
           [0018]      FIG. 7  is a diagram showing the comparison between the demagnetization performance of the present invention and the demagnetization performance in the related art. 
           [0019]      FIG. 8  is a diagram showing the frequency dependence of the typical B-H curve. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
       [0020]      FIG. 1  is a block diagram showing a main part of a particle beam therapy system according to the present invention. In this drawing, a particle beam therapy system includes a scanning magnet (x)  11  that performs scanning in an x-axis direction and a scanning magnet (y)  12  that performs scanning in a y-axis direction. The scanning magnets  11  and  12  are operated by a power supply (x)  13  and a power supply (y)  14  that are connected to a control circuit  15 . The control circuit  15  controls the power supplies  13  and  14  in response to commands from a scanning pattern for treatment  16  and a demagnetization pattern  17 . During the treatment operation, a particle beam  18  passes between the magnetic poles of the scanning magnets  11  and  12 . The control circuit  15  reads the pattern for treatment  16 , and this particle beam  18  is scanned by the scanning magnets  11  and  12 , which are operated by the pattern currents supplied from the power supplies  13  and  14  according to this pattern, and is irradiated to the affected area of the patient. 
         [0021]    On the other hand, during the demagnetizing operation of scanning magnets, the control circuit  15  reads the demagnetization pattern  17  before the start of treatment and (or) after the completion of treatment, and the scanning magnets  11  and  12  are demagnetized by the pattern currents supplied from the power supplies  13  and  14  according to this pattern. 
         [0022]    The treatment operation and the demagnetizing operation described above are performed as in the flow chart shown in  FIG. 2 , for example. A demagnetizing operation is performed before the start of the treatment operation to treat a patient (step S 1 ). Then, pre-irradiation for apparatus adjustment is performed on a phantom (step S 2 ). Then, in order to eliminate the influence of hysteresis of the electromagnet, error factors on the control, and the like, the amount of scanning pattern correction is determined by calculation on the basis of the result (step S 3 ). Then, it is determined whether or not the error after correction is within the acceptable value (step S 4 ). If the error after correction exceeds the acceptable value, the process returns to step S 1 . If the error after correction is within the acceptable value, the demagnetizing operation is performed (step S 5 ), and then body irradiation that is a treatment operation is performed (step S 6 ). In addition, the demagnetizing operation is performed when necessary (step S 7 ), and the process proceeds to treatment for another patient. 
         [0023]      FIG. 3  shows a cross-sectional view of one of the scanning magnets in  FIG. 1  (here, the scanning magnet  11 ). The scanning magnet  11  includes an outer magnetic pole  2  having a mouth-shaped cross-section, a pair of central magnetic poles  3 A and  3 B provided so as to protrude from the center of the outer magnetic pole and face each other, and coils  4 A and  4 B wound around the central magnetic poles  3 A and  3 B. The coils  4 A and  4 B are connected to the power supply  13  so that current is supplied thereto. The particle beam  18  passes through the opposite region of the central magnetic poles  3 A and  3 B of the scanning magnet  11  in a vertical direction with respect to the plane of the drawing. At this time, however, the particle beam  18  is deflected on the x axis due to the current flowing through the coils  4 A and  4 B and is irradiated toward the affected area of the patient. 
         [0024]    In order to generate a high-strength magnetic field with low current, iron or electromagnetic steel sheet is generally used as a core material of the scanning magnet  11 . For this reason, remanent magnetization remains after current interruption due to the current at the time of particle beam irradiation operation. Remanent magnetization within the magnetic pole has a bias in the magnetic pole depending on the situation of a scan. In the scanning magnet  11  shown in  FIG. 3 , an S portion has a high magnetic flux density and accordingly, remanent magnetization is likely to remain, and remanent magnetization is relatively difficult to remain in a W portion. Thus, since the strength of the remanent magnetization changes depending on a portion of the magnetic pole, it is necessary to study the demagnetization pattern in order to eliminate the remanent magnetization of each portion of the magnetic pole evenly in each of the coils  4 A and  4 B wound around the central magnetic poles  3 A and  3 B. 
         [0025]    If remanent magnetization occurs, it is difficult to accurately control a particle beam during the subsequent particle beam irradiation operation. As a result, erroneous irradiation of a particle beam to apart other than the affected area may be caused. In particular, in a scanning device for cancer treatment that is of a type to scan a particle beam using a scanning magnet, remanent magnetization needs to be controlled accurately since the remanent magnetization is a direct cause of the error in irradiation position to the affected area. 
         [0026]    Therefore, it is essential to the particle beam therapy system to demagnetize the scanning magnet  11  time-efficiently and completely evenly using the power supply  13 . In the power supply of the scanning magnet, the power supply performance is defined by the maximum voltage and the maximum current. Since the current is a current flowing through the electromagnet, it is an amount having a strong correlation with the maximum magnetic field. In addition, the induced voltage V satisfies V=L×dI/dt (L is the inductance of a magnet, and dI/dt is a current change over time). Therefore, in a certain power supply, it is necessary to reduce the frequency in order to make large current flow when the maximum voltage is regulated, and it is necessary to increase the frequency in order to reduce the current.  FIG. 8  shows an example of the B-H curve of the electromagnet. As a general trend, it is known that the hysteresis decreases as the frequency of excitation current increases. 
         [0027]    In the present embodiment, therefore, during the demagnetizing operation, current having an amplitude decreasing with time and a frequency increasing with time is used as the current supplied from the power supply  13  to the scanning magnet  11  (referred to as pattern current). This pattern current is generated when the demagnetization pattern  17  capable of setting the current pattern arbitrarily is read into the control circuit  15  and the control circuit  15  controls the power supply  13 . 
         [0028]      FIG. 4  shows the waveform of this pattern current. This current waveform is an AC waveform whose amplitude decreases with time and frequency increases with time. In  FIG. 4 , the horizontal axis indicates time t and the vertical axis indicates current amplitude I, and the waveform is expressed as in the following expression. 
         [0000]    
       
         
           
             
               
                 
                   
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         [0029]    I 0 : maximum excitation current, ω 0 : initial angular frequency, α: frequency rate of increase, t: time, e: natural logarithm, τ: attenuation time constant 
         [0030]    By exciting the magnetic pole with the above-described pattern current flowing from the power supply for scanning  13  to the coils  4 A and  4 B of the scanning magnet  11 , the magnetic flux distribution in the magnetic pole of the scanning magnet  11  changes. Accordingly, both the region S where the remanent magnetization of the scanning magnet  11  is large and the region W where the remanent magnetization of the scanning magnet  11  is small can be evenly and quickly demagnetized by the coils  4 A and  4 B. 
         [0031]      FIG. 7  is a diagram showing the comparison between the conventional demagnetization characteristic C 2  using L-C damped oscillation and the frequency increase type demagnetization characteristic C 1  due to the pattern current in  FIG. 4 . As is apparent from the drawing, it can be seen that the demagnetization time in the frequency increase type has been shortened. 
         [0032]    Although the above explanation has been given for the scanning magnet  11 , the same demagnetizing operation is also required for the scanning magnet  12 , and the scanning magnet  12  is demagnetized by the pattern current supplied from the power supply  14  by the setting of the demagnetization pattern  17 . 
       Second Embodiment 
       [0033]    A second embodiment has the same apparatus configuration as the first embodiment. A difference between the first and second embodiments is the waveform of the pattern current for demagnetization that is supplied from the power supply to the scanning magnet.  FIG. 5  shows the waveform of the pattern current for demagnetization in a particle beam therapy system according to the second embodiment. 
         [0034]    In the present embodiment, the frequency is made to increase with an amplitude decrease so that V=L×dI/dt, which is the induced voltage of the scanning magnet  11 , is fixed. This pattern current setting is performed by the demagnetization pattern  17 . By performing such an operation, it is possible to perform demagnetization with V=L×dI/dt fixed. Therefore, in addition to the effect of the first embodiment, there is an effect that the performance of the power supplies  13  and  14  can be drawn as much as possible. In  FIG. 4 , the horizontal axis indicates time t and the vertical axis indicates current amplitude I, and the waveform is expressed as in the following expression. 
         [0000]    
       
         
           
             
               
                 
                   
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         [0035]    I 0 : maximum excitation current, t: time, e: natural logarithm, τ: attenuation time constant 
       Third Embodiment 
       [0036]    In a third embodiment, as shown in  FIG. 6 , a current whose frequency is not changed and only amplitude decreases is set as the waveform of the pattern current for demagnetization supplied from the power supply to the scanning magnet. The degree of decrease in the amplitude is set by the demagnetization pattern  17 . Effective demagnetization can be performed by performing the setting arbitrarily. In  FIG. 6 , the horizontal axis indicates time t, and the vertical axis indicates current amplitude I. 
         [0037]    As described above, according to the present invention, since demagnetization is performed by supplying the pattern current according to the demagnetization pattern to the scanning magnet from the power supply that scans a particle beam, it is easy to change the current pattern in the power supply for scanning. In addition, since the amplitude or frequency of the demagnetization current can be arbitrarily set, demagnetization can be efficiently performed quickly and with little unevenness. As a result, it is possible to obtain a reliable particle beam therapy system. 
       REFERENCE SIGNS LIST 
       [0000]    
       
         
           
               2 : outer magnetic pole 
               3 A,  3 B: central magnetic pole 
               4 A,  4 B: coil 
               11 ,  12 : scanning magnet 
               13 ,  14 : power supply 
               15 : control circuit 
               16 : scanning pattern for treatment 
               17 : demagnetization pattern

Technology Classification (CPC): 0