Abstract:
A radiation therapy device ( 2 ) includes a programmable power source ( 300 ) and controller ( 302 ) that monitor an injected current level and controls heater voltage in response thereto. The heater voltage is reduced in predetermined increments without affecting the beam profile.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to medical linear accelerators and, more particularly, to a system and method for reducing the effects of backheating on the electron gun in a medical linear accelerator.  
         DESCRIPTION OF THE RELATED ART  
         [0002]    Radiation emitting devices are generally known and used, for  15  instance, as radiation therapy devices for the treatment of patients. A radiation therapy device generally includes a gantry which can be swiveled around a horizontal axis of rotation in the course of a therapeutic treatment.  
           [0003]    A linear accelerator including an electron gun and a waveguide is located in the gantry for generating a high energy radiation beam for therapy. This high energy radiation beam can be an electron beam or photon (X-ray) beam. During treatment, this radiation beam is trained on one zone of a patient lying in the isocenter of the gantry rotation.  
           [0004]    An important consideration in delivering radiation treatment is control of the radiation beam. The electron gun is subject to a phenomenon known as “back heating.” Backheating causes an increase in the temperature of the electron gun resulting in an increase in the barium evaporation rate (Barium is impregnated into the gun cathode tungsten matrix to enhance electron emission.). This causes a deposition of barium in cavities of the linear accelerator adjacent the electron gun. This, in turn, results in an increase in dark current, which has a deleterious effect on the control of the radiation beam. The net result is that the linear accelerator&#39;s useful life is decreased, and therefore must be replaced sooner than would necessarily be desired.  
           [0005]    As such, there is a need for controlling the barium evaporation rate in an electron gun. There is a still further need for a medical linear accelerator having a relatively longer useful life.  
         SUMMARY OF THE INVENTION  
         [0006]    These and other problems in the prior art are overcome in large part by a system and method for control of a radiation therapy device according to the present invention.  
           [0007]    A radiation therapy device according to an embodiment of the present invention includes a programmable power source and controller that monitor an injected current level and controls heater voltage in response thereto. The heater voltage is reduced in predetermined increments without affecting the beam profile.  
           [0008]    A filament voltage controller according to an embodiment of the present invention includes an injector and a remotely programmable electron gun power source. A controller receives an injection trigger signal from the injector, and a measurement of the injection current. The injection current is measured during runup and when RAD ON commences, cuts back the filament voltage a predetermined amount, and measures the current again. So long as the injected current does not fall below the run up level, the heater voltage will be reduced incrementally. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    A better understanding of the present invention can be obtained when the following detailed description is considered in conjunction with the following drawings in which:  
         [0010]    [0010]FIG. 1 illustrates an exemplary radiation therapy system according to an implementation of the present invention;  
         [0011]    [0011]FIG. 2 is a block diagram of a radiation therapy system according to an embodiment of the present invention;  
         [0012]    [0012]FIG. 3 is a block diagram of an exemplary backheating compensation system for in accordance with an embodiment of the present invention; and  
         [0013]    [0013]FIG. 4 is a flowchart illustrating operation of an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]    Turning now to the drawings and, with particular attention to FIG. 1, a radiation treatment apparatus embodying the present invention is shown therein and generally identified by reference numeral  2 . The radiation therapy apparatus  2  may be a Mevatron or Primus linear accelerator available from Siemens Medical Systems, Inc., Concord, Calif. The radiation treatment apparatus  2  includes a beam shielding device (not shown) within a treatment head  4 , a control unit in a housing  9  and a treatment console  200  according to the present invention. The radiation treatment device  2  includes a gantry  6  which can be swiveled around a horizontal axis of rotation  8  in the course of a therapeutic treatment. The treatment head  4  is fastened to a projection of the gantry  6 . A linear accelerator is located in the gantry  6  to generate the high powered radiation required for the therapy. The axis of the radiation bundle emitted from the linear accelerator and the gantry  6  is designated by  10 . Electron, photon or any other detectable radiation can be used for the therapy.  
         [0015]    During the treatment, the radiation beam is trained on a zone  12  of an object  13 , for example, a patient who is to be treated and who lies at 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  intersect in the isocenter.  
         [0016]    The plates or leaves of the beam shielding device within the treatment head  4  are substantially impervious to the emitted radiation. The collimator leaves or plates are mounted between the radiation source and the patient in order to delimit the field. Areas of the body, for example, healthy tissue, are therefore subject to as little radiation as possible and preferably to none at all. The plates or leaves are movable such that the distribution of radiation over the field need not be uniform (one region can be given a higher dose than another). Furthermore, the gantry can be rotated so as to allow different beam angles and radiation distributions without having to move the patient.  
         [0017]    The radiation treatment device  2  also includes a central treatment console  200  which is typically located apart from the radiation treatment device  2 . The radiation treatment device  2  is normally located in a different room to protect the therapist from radiation. The treatment console  200  includes output devices such as at least one visual display unit or monitor  70  and an input device such as a keyboard  19 . Data can be input also through data carriers such as data storage devices or a verification and recording or automatic setup system.  
         [0018]    The treatment console  200  is typically operated by the therapist who administers actual delivery of radiation treatment as prescribed by an oncologist by using the keyboard  19  or other input device. The therapist enters into the control unit of the treatment console  200  the data that defines the radiation dose to be delivered to the patient, for example, according to the prescription of the oncologist. The program can also be input via another input device, such a data storage device. Various data can be displayed before and during the treatment on the screen of the monitor  70 .  
         [0019]    A block diagram of the radiation treatment device  2  and portions of the treatment console  200  are, according to the present invention, illustrated in greater detail in FIG. 2. An electron beam  1  is generated in an electron accelerator  20 . The electron accelerator  20  includes an electron gun  21 , a wave guide  22  and an evacuated envelope or guide magnet  23 . A trigger system  3  generates injector trigger signals and supplies them to the injector  5 . Based on these injector trigger signals, the injector  5  generates injector pulses which are fed to the electron gun  21  in the accelerator  20  for generating electron beam  1 . The electron beam  1  is accelerated and guided by the wave guide  22 . For this purpose, a high frequency source, such as a magnetron or klystron (not shown), is provided, which supplies radio frequency signals for the generation of an electromagnetic field supplied to the waveguide  22 . The electrons injected by the injector  5  and emitted by the electron gun  21  are accelerated by this electromagnetic field in the waveguide  22  and exit at the end opposite to electron gun  21  in electron beam  1 .  
         [0020]    The electron beam  1  enters a guide magnet  23  and from there is guided through a window  7  along axis  10 . After passing through a first scattering foil  15 , the beam goes through a passageway  51  of a shield block  50  and encounters a flattening filter  17 . Next, it is sent through a measuring chamber  60  in which the dose is ascertained. If the scattering foil is replaced by a target, the radiation beam is an X-ray beam; in this case, the flattening filter  17  may be absent, but it is typically present.  
         [0021]    Finally, a beam shielding device  401  is provided in the path of radiation beam  1 , by which the irradiated field of the subject of investigation is determined. As illustrated, the beam shielding device  401  may include a plurality of opposing plates  41  and  42 , only two of which are illustrated for convenience. In one embodiment, additional pairs of plates (not shown) are arranged perpendicular to plates  41  and  42 . The plates  41 ,  42  are moved with respect to axis  10  by a drive unit  43  (which is indicated in FIG. 1 only with respect to plate  41 ) to change the size of the irradiated field. The drive unit  43  includes an electric motor which is coupled to the plates  41  and  42  and which is controlled by a motor controller  40 . Position sensors  44  and  45  are also coupled to the plates  41  and  42 , respectively for sensing their positions. The plate arrangement  401  may alternatively or additionally include a multi-leaf collimator having many radiation blocking leaves. The leaves of such a multi-leaf collimator include a plurality of opposing leaf or rod pairs, each driven by a motor or drive unit. The drive units move the leaves in and out of the treatment field, thus creating the desired field shape. The rods, or leaves, are relatively narrow, and cast a shadow of about 0.5 to 1. cm at isocenter.  
         [0022]    The motor controller  40  is coupled to a dose control unit  61  which includes a dosimetry controller and which is coupled to the processing unit  18  for providing set values for the radiation beam for achieving given isodose curves. The output of the radiation beam is measured by the does measuring chamber  60 . In response to deviation between set values and actual values, the dose control unit  61  supplies signals to trigger system  3 , which changes in a known manner the pulse repetition frequency.  
         [0023]    In addition, as will be explained in greater detail below, the trigger system  3  includes a programmable power source  300  and a signal processing control unit  302  for monitoring and adjusting the injector voltage. The signal processing control unit  302  reads the injected current level and provides a DC representation of the level to the programmable power source  300 . The programmable power source  300  then cuts back the heater voltage.  
         [0024]    More particularly, FIG. 3 is a block diagram showing in greater detail the backheating compensation system of an embodiment of the present invention. The system includes a programmable power source  300  and a signal processing control unit  302  in the trigger system  3 , the console treatment console  200 , the injector  5 , and a current measurement unit  301  at the electron gun  21 . The treatment console  200  provides Vfil softpot  306  and Inj I softpot  308  control signals to the programmable power source  300 . These may be known control signals used to set the filament voltage and the injector current, respectively. In addition, a backheat comp on/off control signal  304  is used by the treatment console  200  to activate or deactivate the backheating compensation functionality according to embodiments of the present invention. In turn, the programmable power source  300  provides a Vfil prog signal  310  to the injector  5 . A current measurement unit  301 , which may be implemented as a current transformer, commonly known as a toroid, provides an Inj I signal  314  of the measured injector current to the signal processing unit  302 . The signal processing unit  302  may also receive an Inj Trig signal  312  from the injector  5 , identifying the triggered pulses. For example, the signal processing unit  302  may include a sample-and-hold circuit which causes a sampling of the Inj I signal  314  each time the Inj Trig signal  312  is received. The signal processing unit  302  provides a DC representation of the injector current to the programmable power source  300 . The programmable power source  300  communicates the injector current level to the treatment console  200  as the signal  318 .  
         [0025]    In operation, the console  200  provides the preprogrammed filament voltage and injector current (FIG. 3) to the programmable power source  300  and turns on the Backheat Comp signal  304  (FIG. 3). The filament voltage is then provided to the injector  5 . The current measurement unit  301  provides the measured injector current  314  to the signal processing unit  302 . The signal processing unit  302  includes a sample and hold circuit (not shown) to produce a DC representation of the injected current  316  in response to the trigger  312 . Measurement of the injector current begins during an initialization phase such as the system RUNUP phase. The programmable power source  300  sends a signal Inj I level  318  to the console  200 . When RAD ON commences, the programmable power source  300  cuts back on the filament voltage a preset amount (e.g., 200 mV) and waits a few seconds (e.g., 10) to determine the change in the injected current. As long as the injected current does not fall below a threshold, (as determined, e.g., by the console using the Inj I level signal  318 ), such as the RUNUP level, the programmable power source  300  will continue to reduce the heater voltage incrementally. A lower threshold may be preprogrammed into the programmable power source  300 .  
         [0026]    [0026]FIG. 4 is a flowchart depicting operation of an embodiment of the present invention. In a step  502 , the treatment processor  18  of the treatment console  200  sets the filament voltage and the injector current levels. In a step  504 , the system measures actual injector current, typically during an initialization or test phase, such as RUNUP. At RAD ON, in step  506 , the system decrements the injector voltage by a predetermined amount. At step  508 , the system measures the injector current again, typically after a predetermined wait period. In  510 , the system then checks to determine if the injector current is greater than a predetermined threshold. For example, the threshold may be the RUNUP current level. If it is, then the system cycles back to decrease the filament voltage in  506 . Otherwise, the system will continue treatment using the current injector current level, in step  512 .  
         [0027]    The invention described in the above detailed description is not intended to be limited to the specific form set forth herein, but is intended to cover such alternatives, modifications and equivalents as can reasonably be included within the spirit and scope of the appended claims.