Abstract:
An instrument for checking quality of therapeutic x-ray and electron radiation provides modes optimized for both electrons and for photons obtained by physically flipping the unit to interpose the necessary build-up material between the radiation beam and contained detectors. The invention provides an improved method of constructing ionization detectors for improved energy discrimination using such detectors and wire-free operation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     - -  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     - -  
       BACKGROUND OF THE INVENTION  
       [0003]     The present invention relates to quality assurance instruments for medical radiotherapy equipment used for radiation treatment of tumors or the like.  
         [0004]     Cancerous tumors may be treated by irradiating the tumor with high-energy photons or electrons (henceforth both termed “radiation”).  
         [0005]     Such radiotherapy relies in part on the fact that tumor tissue is more sensitive than normal tissue to such high-energy radiation. Nevertheless, the radiation dose must be carefully controlled to limit the exposure of healthy tissue while ensuring sufficient radiation is received by the tumor.  
         [0006]     Radiation dose may be controlled by a variety of means including shutters for collimating the radiation beam to the area of the tumor, filters for varying the intensity of radiation within the area of the tumor, and control of the exposure duration. An accurate understanding of the energy, flux, and alignment of the radiation beam is essential for such control. Generally, as is understood in the art, radiation energy describes the average energy of the individual photons or electrons whereas radiation flux is number of electrons or photons per unit area per unit time.  
         [0007]     Radiation energy may be determined by calculating changes in flux at two depths within a homogenous medium, for example, a water phantom.  
         [0008]     Radiation flux is normally determined using an ionization chamber or semiconductor detector placed in the radiation beam at a fixed distance from the radiation source. A “build-up” material such as a plastic block may be placed in front of the flux-detector to improve its sensitivity. For the purposes of periodic quality assurance of a radiotherapy machine, the output of the flux-detector may be compared to a base line for the same detector. In this way, precise calibration of the detector to a standard is not required.  
         [0009]     Radiation alignment is normally determined with respect to a visible light field projected along with the radiation showing, for example, an illuminated rectangular area and/or cross-hair pattern. Alignment may be verified by exposing a film marked to show the location of the light field or crosshairs and comparing the exposed film to the markings. Alternatively, as shown in U.S. Pat. No. 4,988,866, a fixture having multiple ionization detectors and multiple light detectors (also called edge detectors) may be used, and the signals from the ionization detectors and light detectors may be compared.  
         [0010]     It is desirable that the radiation therapy machine be checked on a frequent, periodic basis at each of its settings. Such quality assurance checks can be cumbersome and time consuming particularly when multiple pieces of test equipment must be used, for example, as would be required to calibrate a radiotherapy machine that provides both electron beams and photon beams at a variety of energy levels. It is difficult to construct a quality assurance instrument that works for a wide variety of different radiation energies and different radiation modes, e.g. electrons or photons, equally well.  
       BRIEF SUMMARY OF THE INVENTION  
       [0011]     The present invention provides a radiation beam checker that may verify flux profiles and constancy for a wide variety of radiation energies and modes. Several features contribute to this versatility. First, the beam checker may receive radiation from either of two directions, flipping to receive electrons through one side and photons through the other. In this way, a single set of detectors may be optimized for either of two different radiation modes. Second, rather than relying on a single filtered and unfiltered detector to determine energy level, the present invention may use multiple detectors, each having a different filtration to provide data for a more sophisticated energy discrimination function accurate over a wide energy range. Visual fiducia on both surfaces of the beam checker allow alignment to be determined by flux measurements from the multiple flux-detectors without the need for photosensitive edge detectors that would be required on both surfaces.  
         [0012]     One embodiment of the present invention permits wire-free operation simplifying manipulation of the beam checker without requiring radio communication that may be difficult to establish in the environment of the radiotherapy machine. An additional feature of one embodiment of the present invention is automatic linkage of data to energy levels to minimize necessary operator input. In one embodiment, the present invention may employ a new construction technique for ionization detectors simplifying the manufacture and improving the consistency of multi-detector systems.  
         [0013]     Specifically then, the present invention may provide a test apparatus for both photon and electron radiation, the test apparatus having a housing providing opposed first and second faces holding a set of detectors between the first and second faces. In this embodiment, a first calibrating material for electrons is positioned to intercept electrons passing through the first face to the detectors, and a second calibrating material for photons is positioned to intercept photons passing through the second face to the detectors.  
         [0014]     It is thus one object of the invention to provide a single test unit that may be tailored to two modes of radiation by placing different build-up or filter materials on the opposite faces of the housing and flipping the housing according to the radiation mode.  
         [0015]     The test apparatus may include a quantitative radiation measurement display on a third face of the housing visible when either the second or first face is lying on the surface. The display may change orientation according to whether electrons or photons are being measured to be upright to an operator in either mode.  
         [0016]     Thus, it is another object of the invention to provide a device that retains ease of use in either orientation of the housing.  
         [0017]     One embodiment the invention provides a wire-free test apparatus for therapeutic radiation systems having a housing holding a set of radiation detectors for measuring radiation flux at predetermined locations and a solid state memory for receiving and storing the radiation flux measurements. A battery within the housing powers the radiation detectors and solid-state memories and a port is provided on the housing for downloading the stored radiation flux measurements to a remote computer.  
         [0018]     Thus, it is another object of the invention to provide an easily maneuverable test device unencumbered by connecting cables. It is another object of the invention to provide wire-free operation without the need for radio transmission of data such as can be blocked by the shielding used around radiotherapy machines.  
         [0019]     The housing may include a light field guide on its surface, delineating a region of the housing containing the detectors that should be exposed to radiation. The processing circuitry and memory may be within the housing outside of the region.  
         [0020]     Thus, it is another object of the invention to provide a simple method of minimizing radiation exposure to solid-state memory which is normally sensitive to radiation damage or interference.  
         [0021]     The processing circuitry contained within the housing may communicate with at least some of the radiation detectors to detect the start of a new radiation measurement from signals produced by the radiation detectors and to automatically store the radiation measurements in the solid state memory.  
         [0022]     Thus, it is another object of the invention to provide for simplified data acquisition without the need for complex keyboard control or a permanently attached remote terminal.  
         [0023]     In one embodiment, the invention provides a beam checker for therapeutic radiation comprising a set of spaced radiation flux-detectors producing flux signals and at least one radiation energy-detector providing an energy signal and a storage system for storing a set of energy ranges. Processing circuitry compares at least one of the flux signals to benchmark flux values of an energy range corresponding to the energy signal to provide an indication of any improper operation of the measured radiation source. Generally, the benchmark flux values may indicate flatness, symmetry, or constancy over time.  
         [0024]     Thus, it is an object of the invention to use the energy signal to automatically relate flux measurements to proper benchmark measurements for different energy ranges.  
         [0025]     The radiation energy-detector may be a set of at least three detector elements having different filtrations to provide radiation signals and the energy signal may be derived from an algebraic combination of the radiation signals from the set of detector elements. Alternatively or in addition, the radiation detector may be a set of detector elements, at least one of which element has a “backscatter element” positioned behind it with respect to the measured radiation so that the detector element is sensitive to backscatter, and the energy signal may be derived from an algebraic combination of the radiation signals.  
         [0026]     Thus it is one object of the invention to provide an improved low-profile energy sensor that works over a wider range of energy values than can be achieved with a single filtered detector.  
         [0027]     One embodiment of the invention provides an ionization detector that includes a front and rear plate positioned on a front and rear side of a volume of ionizable gas or other fluid to receive a voltage thereacross to collect the charges resulting from radiation ionizing the gas. The rear plate may be formed of a printed circuit board providing a collector on its front surface and multiple layers, including a middle layer providing a signal trace and a first and second ground flanking the middle layer, where the signal trace may connect to the collector.  
         [0028]     Thus it is an object of the invention to provide improved manufacturability for ionization detectors by using the fabrication techniques associated with printed circuit boards while providing the shielding needed to protect the faint ionization signals.  
         [0029]     These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]      FIG. 1  is a perspective view of the beam checker of the present invention as held in a cradle prior to use showing a first side for receiving photon radiation;  
         [0031]      FIG. 2  is a perspective view of the beam checker and cradle (in partial phantom) together with various cables, a remote computer, and charging unit as may be used with the beam checker; and showing a second side for receiving electron radiation;  
         [0032]      FIG. 3  is a front elevational view of a third wall of the beam checker of  FIGS. 1 and 2  as supports a display of radiation energy such as may flip in orientation, depending on the particular radiation mode being detected;  
         [0033]      FIG. 4  is a fragmentary view of a printed circuit board positioned beneath the target markings of the units of  FIGS. 1 and 2 , the printed circuit board providing a number of detectors for measuring radiation flux and/or energy;  
         [0034]      FIG. 5  is a cross-sectional view through several detectors of  FIG. 4  and front and rear buildup materials of the housing showing passage of electron radiation and photon radiation through different build-up materials (for all detectors) and different filtration materials (for particular detectors) and a backscatter material (for one detector);  
         [0035]      FIG. 6  is an exploded perspective view of one of the detectors of  FIG. 4  showing its assembly from a cap placed on exposed traces of a printed circuit board;  
         [0036]      FIG. 7  is a perspective cross-sectional view of the assembled ionization detector  FIG. 6  showing the multiple layers of the printed circuit board used to provide shielding of the detected signals;  
         [0037]      FIG. 8  is a block diagram of the circuitry of the detector of  FIG. 1 , such as may be placed on the circuit board of  FIG. 4  and which employs a microprocessor based processing system to store data within an associated memory for later communication through a port;  
         [0038]      FIG. 9  is a flow diagram showing the calculation of energy for electron radiation;  
         [0039]      FIG. 10  is a figure similar to that of  FIG. 9  showing the calculation of energy for photon radiation; and  
         [0040]      FIG. 11  is a flow chart of a program executed by the microprocessor of the circuit of  FIG. 8  in implementing the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0041]     Referring now to  FIG. 1 , the beam checker  10  of the present invention provides a mobile detecting unit  12  having a generally rectangular, box-shaped housing  20  providing a first photon-receiving face  16  opposed to a second electron-receiving face  18 .  
         [0042]     Referring also  FIG. 2 , a portion of the photon-receiving face  16  and electron-receiving face  18  is marked with a target  22  defining an area of radiation exposure and thus providing a means for aligning the housing  20  of the mobile detecting unit  12  with a light field or laser crosshair provided by standard radiotherapy machines, as will be described. Typically, the target  22  or  22 ′ is 20 cm by 20 cm and includes a crosshair dividing the target area into equal quadrants.  
         [0043]     Referring to  FIGS. 2 and 3 , a first side wall  24  of the housing  20  holds a three-character, 1.2 inch tall, seventeen-segment light emitting diode (LED) alphanumeric display  26 , a reset button  28  and a separate bicolor LED  29 , radiation mode captions and lamps  30 , and a mode select button  32 . During operation, side wall  24  remains visible when either of the photon-receiving face  16  or electron-receiving face  18  of the housing  20  are supported by structure of the radiation therapy machine, for example, a horizontal patient support  34 .  
         [0044]     When photon-receiving face  16  is against the patient support  34 , electron-receiving face  18  is upward facing a source of radiation along axis  36 . The operator may then press the mode select button  32  to illuminate a lamp next to the radiation mode caption denoting “electron” and to indicate to the beam checker  10  that this is the type of radiation being measured. The caption “electron” will be right side up when the housing  20  is appropriately oriented for receiving electron radiation.  
         [0045]     Upon exposure of the beam checker  10  to electron radiation, the alphanumeric display  26  will display a detected energy range using both alphabetic and numeric characters. Typically, the energy ranges will include 6E, 9E, 12E, 16E, and 22E. The number being a measure of energy in MeV and the “E” suffix indicates that the beam checker  10  is checking for electron radiation. The alphanumeric display will be automatically oriented to be right side up, based on the mode selected, when the beam checker is correctly positioned to receive electron radiation as described. This reorientation requires simply a change in the mapping of segments of the alphanumeric display  26  and can be accomplished electronically by the contained processor described below.  
         [0046]     The housing  20  may be flipped as shown by arrow  38  so that photon-receiving face  16  is upward to receive a beam of photons along axis  40 . The operator may then press the mode select button  32 , but this time to illuminate a lamp next to the radiation mode caption denoting “photon” and to indicate to the beam checker  10  that this is the type of radiation being measured. The caption “photon” is inverted with respect to the caption “electron” to be right side up when the housing  20  is appropriately oriented for receiving photon radiation.  
         [0047]     Upon exposure of the beam checker  10  to photon radiation, the alphanumeric display  26  will display a detected energy range using both alphabetic and numbers typically 6X, 18X or 23X with the number being a measure of energy in MV and the “X” suffix indicates that the beam checker  10  is checking for x-ray photons. As before, the alphanumeric captions will be automatically oriented to be right side up based on the mode selected. Thus it will be understood that the alphanumeric display  26 , reset button  28 , LED  29 , radiation mode captions and lamps  30 , and a mode select button  32  may be readily used with either orientation of the housing  20 .  
         [0048]     Referring again to  FIGS. 1 and 2 , a second side wall  42  of housing  20  of the beam checker  10 , spanning electron-receiving face  18  and photon-receiving face  16 , provides on its surface a data/power connector  44 , a data-only connector  45 , and power connector  46 . When the mobile detecting unit  12  is placed within the cradle  14 , the side wall  42  abuts an upper face  48  of the cradle  14  so that data/power connector  44  connects with a corresponding data/power connector  52  and power connector  54  on the upper face  48  of the cradle  14 . In this way, data may be communicated to and from the mobile detecting unit  12  and power may be provided to the mobile detecting unit  12 .  
         [0049]     The mobile detecting unit  12  is held in position on the cradle  14  by two guiding pylons  50  extending upward from the cradle  14  and abutting the electron-receiving face  18  and photon-receiving face  16 . Note that in  FIG. 2  one guiding pylon  50  is removed for clarity.  
         [0050]     The cradle  14  includes provisions to receive a power cord  58  which may provide line power from a standard wall transformer  59 , or the like, to the mobile detecting unit  12  through power connector  54 . Alternatively, the power cord  58  may be received directly by the mobile detecting unit  12  at power connector  46 .  
         [0051]     Cradle  14  also incorporates two RS-232 connectors  60  and  62  which electrically communicate with data/power connector  52  (connector  60  is opposite connector  62  and not visible in  FIG. 2 ). Connectors  60  and  62  allow the mobile detecting unit  12  to be connected to the cradle  14  by means of a standard RS-232 cable  64  connecting between RS-232 connector  60  and data/power connector  44  on the mobile detecting unit  12  when mobile detecting unit  12  is not sitting on the cradle  14 . Connector  62  allows a second cable  66  to connect the cradle  14  (via connector  62 ) to an independent programming and data-logging computer  68 , or the like and thereby connect through data/power connector  52  with the mobile detecting unit  12 . Alternatively, the computer  68  may communicate directly with the mobile detecting unit  12  via cable  66  attaching to data connector  45 .  
         [0052]     Generally, a direct connection between the computer  68  and mobile detecting unit  12  will be used only during an initial calibration procedure when constant reference to the computer  68  will be required. For all other times, the mobile detecting unit  12  will communicate with the computer  68  (for example during periodic downloading of data) via the cradle  14  and the joining of data/power connectors  44  and  52 . While mobile detecting unit  12  is in the cradle  14 , the mobile detecting unit  12  may exchange data with the computer  68  and may receive power for operation and for charging internal batteries as will be described.  
         [0053]     Referring now to  FIGS. 1 and 4 , positioned within the housing  20  parallel to, and centered between, photon-receiving face  16  and electron-receiving face  18  is a printed circuit board  70  having a detector zone  72  located beneath the targets  22  and  22 ′. Positioned on the printed circuit board  70  and centered in the detector zone  72  is a central detector  74   a . Positioned on either side of detector  74   a  along a longitudinal axis of the printed circuit board  70  are detectors  74 b and  74   c  whereas positioned on either side of detector  74   a  along a lateral axis of the printed circuit board  70  are detectors  74   d  and  74   e . Detector  74   b ,  74   c ,  74   d , and  74   e  are located at midpoints between detector  74   a  and the edge of the radiation field as defined by the targets  22  and  22 ′.  
         [0054]     Detectors  74   a - 74   e  detect radiation flux and may be, for example, ionization detectors, solid-state detectors, or other detector types known in the art. Detector  74   a  provides a measurement of the central flux of the radiation beam and together with detectors  74   b - 74   e  provides indication of the variation in that flux over the area of the targets  22  and  22 ′ as may form the basis of a measure of flatness and symmetry. Multiple measurements from detector  74   a  over time provides a measure of flux constancy.  
         [0055]     Also positioned on the printed circuit board  70  in the detector zone  72  are energy-detectors  76   a ,  76   b , and  76   c . Energy detectors  76   a ,  76   b , and  76   c  may be located arbitrarily within the detector zone  72  but are preferably equidistant from the detector  74   a  to reduce the effects of variations of the beam profile on the their signals. These detectors directly measure radiation flux but include filters and other elements which allow the energy of the radiation beam to be determined from the flux signals. Detectors  74  and  76  will be described in more detail below.  
         [0056]     Referring to  FIGS. 4 and 5 , the printed circuit board  70  positions the detectors  74  and  76  between build-up material  80  on electron-receiving face  18  of the housing  20  and build-up material  82  on the photon-receiving face  16  of housing  20 . During use, therefore, electrons  84  will arrive at detectors  74  and  76  after passing through build-up material  80  and photons  87  will arrive at detectors  74  and  76  after passing through build-up material  82 . Each of build-up materials  80  and  82  is optimized for the particular type of radiation it is intended to receive. In the preferred embodiment, the build-up material  80  is a plastic material equivalent to 1.5 centimeters of water optimized for electrons and a build-up material  82  is a plastic material equivalent to 3.5 centimeters of water optimized for photons. More generally, the amount of build-up material  80  and  82  is selected to increase the sensitivity of the detectors  74  and  76  to the particular mode of radiation and to provide even sensitivity of the detectors  74  and  76  (ignoring for the moment any filtration) to the expected energy range of the particular radiation mode.  
         [0057]     Referring specifically to  FIG. 5 , detectors  74   a - 74   d  are intended to measure radiation flux directly and have no additional filtration. Energy-detectors  76   a ,  76   b , and  76   c , however, have additional filter and backscatter elements to allow them to distinguish among different energies of radiation. In the preferred embodiment, detector  76   a  has 10 mm of aluminum  86  on its side toward electron-receiving face  18  and energy-detector  76   b  has 1 mm of aluminum  88  on its side toward electron-receiving face  18 . Energy-detector  76   c , in contrast, provides no filtration material on its die toward electron-receiving face  18 , but on the side closest to photon-receiving face  16  provides 6 mm of lead. This lead provides backscatter of electrons coming through electron-receiving face  18 , which hit the lead  90  and scatter back into energy-detector  76 . The lead may alternatively be a brass disk.  
         [0058]     The filtration and backscatter element cause each of these energy detectors  76   a ,  76   b , and  76   c  to produce a slightly different signal. When combined, these signals provide a discrimination of different energies as will be described below.  
         [0059]     Referring now to  FIG. 6 , in the preferred embodiment, each of detectors  74 - 76  is an ionization detector of a type in which ionized gas provides a path of conduction between charged and separated plates, and are manufactured using printed circuit board techniques such as those employing a photoresist/etching process or the like.  
         [0060]     In particular, a manufacturing technique of the present invention provides circular disk-shaped collector  92  on the upper surface of the printed circuit board  70  to provide one charged plate. The collector is surrounded by a guard ring  94 , which in turn is surrounded by a high voltage ring  96  leading by trace  98  to a high voltage source. The remainder of the surface of the printed circuit board  70 , in near proximity to the detector  74  or  76 , may include a ground plane  100 .  
         [0061]     A brass cap  102  being a hollow cylinder with an open lower base may be attached at the edge of the lower base to the high voltage ring  96  by solder, or the like. The upper solid base of the brass cap is preferably approximately 0.25 mm thick. A vent port  104  is drilled through the printed circuit board  70  to provide pressure equalization to the inner surface of the brass cap  102 , which holds ionizing air at ambient pressure. Alternatively, the vent port  104  could be drilled through the brass cap  102 . Other materials than brass can be used for the cap as will be understood to those of ordinary skill in the art. A chamber created within the cap and upper surface of the printed circuit board encloses approximately 0.6 cubic cm of air.  
         [0062]     Referring to  FIG. 7 , the printed circuit board  70  may be a multi-layer printed circuit board having the upper layer shown in  FIG. 6  providing copper cladding forming the collector  92 , guard ring  94 , high voltage ring  96 , and ground plane  100 . Beneath and supporting this upper layer is an insulator  106  and then a ground plane  108 . Beneath the ground plane  108  is another insulator  110 , followed by a signal plane  112  another insulator  109  and finally an outer ground plane  114 . The printed circuit board  70  may be standard copper clad epoxy-impregnated fiberglass.  
         [0063]     Generally, the signal plane  112  includes multiple traces, one connecting to collector  92  by via  116 . The ground plane  100  may be joined by vias  118  and  120  to guard ring  94  and ground planes  108  and  114 , the former which may provides holes through which via  116  may pass. As apparent from  FIG. 7 , the traces of the signal plane  112  are thus always flanked on their upper, lower, right and left surfaces by ground planes  114  and  108  providing shielding to the signals detected signals.  
         [0064]     Referring now to  FIGS. 4 and 8 , the traces of the signal plane  112  and the ground planes  108 ,  100 , and  114  and high voltage traces  98  may pass out of the detector zone  72  to a circuit area  73  also on the circuit board  70  but displaced from the targets  22  and  22 ′ and positioned between radiation shields  124 . The circuit area  73  holds processing circuitry  122  including amplifiers  126  receiving signals from the signal plane  112 . The amplifiers  126  connect to a multiplexing A to D converter  128  providing digitized signals to a processor  130  via an internal bus  132 .  
         [0065]     The processor  130  accesses an internal clock and calendar and communicates with a temperature and barometric pressure sensor  134  to correct for changes of ionization detectors caused by changes in ambient atmospheric pressure and temperature (as is understood in the art), with an audible annunciator  133 , the RS-232 data/power connector  44 , the controls  137  of the first side wall  24  including: the alphanumeric display  26 , the reset button  28  the LED  29 , the radiation mode captions lamps  30 , and the mode select button  32 . The processing circuitry  122  of the circuit area  73  may also include power supply  136  communicating via a jack  138  with the power cord  58  shown in  FIG. 2 .  
         [0066]     The processor  130  executes a program stored in memory  140  to process the signals received from the detectors  74  and  76  and to store them in memory  140  as will be described. The program accepts inputs from the temperature and barometric pressure sensor  134 , the RS-232 data/power connector  44 , the reset button  28 , and the mode select button  32  and provides outputs to the LED  29 , RS-232 data/power connector  44 , the alphanumeric display  26 , and the radiation mode captions lamps  30  according to the inputs and the logic of the control program described herein.  
         [0067]     Referring now to  FIGS. 2 and 11 , the present invention is used in three distinct phases. In a first phase, the mobile detecting unit  12  is connected to computer  68  by cable  66  and instructed by software in the computer  68  to enter a calibrate mode as indicated by process block  150 . During this calibrate mode, the mobile detecting unit  12  is exposed to radiation from a radiotherapy machine (not shown) at each energy level and for each radiation mode. The energy of radiation is identified via the operator of the computer  68  and the mode identified by the mode select button  32  and matched to an energy signature determined from the measurements of the energy detectors  76   a - 76   c.    
         [0068]     Referring momentarily to  FIG. 9 , for electrons, the energy signature value is determined by taking the signal from detector  76   b  having 1 mm of aluminum and dividing it by the signal from detector  76   a  having 10 mm of aluminum. This fraction is multiplied by the backscatter signal from detector  76   c  having a backstop of 6 mm of lead to produce the electron energy signature value  156 .  
         [0069]     For photons as shown in  FIG. 10 , the signal from detector  76   c  having the 6 mm of lead as a backstop is divided by a signal from the center detector  74  to produce a photon energy signature value  156 .  
         [0070]     It will be understood that other algebraic combinations of these multiple detectors can be used and that generally the energy may be fit to a polynomial function of the signals from detectors  76  and/or  74 .  
         [0071]     The computer  68  then compiles, per process block  152 , an energy table consisting of an entry for each energy and mode providing benchmark flux measurements from each of the detectors  74   a - 74   d  and the energy signature value  156 . The energy table is downloaded into memory  140 .  
         [0072]     Referring again to  FIG. 11 , in a second phase of operation, the mobile detecting unit  12  is armed automatically and the LED  29  turns green and the indicators display “RDY” for ready. Then unit  12  is placed on the radiotherapy machine in the path of the radiation with the light field of the radiotherapy machine aligned with targets  22  or  22 ′. The operator selects the desired radiation mode corresponding with the orientation of the mobile detecting unit  12  and begins the radiation exposure. Once armed, the processor  130  monitors the signals from the detectors  74  and  76  per decision block  158  until the signals exceed a predetermined threshold indicating radiation is present.  
         [0073]     Once this threshold is passed, at succeeding process block  160 , the energy of the radiation is determined by matching the readings from detectors  74   a  and  76   a - 76   c  (as appropriate) to the energy signature value  156  stored in the energy table in memory  140  plus and minus a predetermined range. If the energy readings do not match with any energy signature value  156  stored in the energy table, the alphanumeric display  26  shows an error message (“ERR”) and the LED  29  flashes red and there is an audible beep produced by an annunciator  133 . No further readings can be taken until the reset button  28  is pressed whereupon the LED  29  returns to its default color of green.  
         [0074]     Once the energy level of radiation has been determined, the processor  130  compares the benchmark flux measurements associated with the particular entry of the energy table, per decision block  162 , to the signals from the detectors  74   a  - 74   d . The flatness and the symmetry of the current flux of the radiation beam is compared to a predetermined threshold value based on the benchmark flux measurements and the constancy of the flux is compared to a predetermined acceptable range also based on the benchmark flux values. Flatness is generally determined by finding the maximum and minimum values of the detectors  74   a - 74   e  (values of detectors indicated in the following by the detector number). Then, flatness=(Max(detectors  74   a - 74   e )−Min(detectors  74   a - 74   e )/(Max(detectors  74   a - 74   e  )+Min(detectors  74   a - 74   e )). Symmetry is determined by axial=(top (detector  74   b )−bottom (detector  74   c ))/bottom (detector  74   c ) and transverse=(right (detector  74   d )−left (detector  74   e ))/left ( 74   e ). Constancy is determined by the center detector value over time: (detector  74   a  (at time x)−detector  74   a  (at benchmark time))/detector  74   a  (at benchmark time).  
         [0075]     If the current flatness, symmetry of constancy is outside of a predetermined range related to the benchmark value, an error signal is indicated per process block  164  and the alphanumeric display  26  shows an error message (“ERR”) alternating with the type of error (“SYM”, “FLT”, and “CST” for symmetry, flatness and constancy, respectively) and the LED  29  shows red and flashes together with an audible beep by the annunciator. No further readings can be taken until the reset button  26  is pressed whereupon LED  29  returns to its default color of green.  
         [0076]     If the flux measurements are within the acceptable predetermined range, the alphanumeric display  26  shows and indicates the deduced energy level, and the unit resets itself. At process block  166 , the flux values are stored in memory together with a date stamp maintained by the processor  130  as linked to the determined energy level.  
         [0077]     The operator may then proceed through energy ranges and modes stopping only as necessary to flip the mobile detecting unit  12  according to the mode. When the measurements have been made, the operator may install the detecting unit  12  back on the cradle  14  and download the data to the computer  68  for additional analysis or preparation of automatic reports. The memory  140  is sized to hold up to thirty days worth of data so that downloading may be postponed as desired on any given day.  
         [0078]     The third phase of operation is a wired version of phase two for real-time data collection. In this phase, the same functionality exists as is phase two, but beam checker  10  is hardwired either through the cradle  14  or directly to a remote computer  68  allowing real-time data collection and beam checker controls.  
         [0079]     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.