Abstract:
Nested ionization chambers provide independent measurements of a radiation beam that does not fully irradiate the volume of one or both chambers. By mathematically combining these independent measurements, partial volume effects caused by a change in ionization detector calibrations when the full detector volume is not irradiated by the radiation beam, may be decreased, providing more accurate measurement of extremely small radiation beams.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from previously filed U.S. Provisional Patent Application Ser. No. 61/089,751 filed on Aug. 18, 2008 hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    External beam radiation therapy systems provide beams of high-energy directed into a patient to treat tumors or the like. The size, location, angle and intensity of the beams are determined by a treatment plan based upon the precise measurement of the dose to be delivered to the patient to provide for precise control of the dose to the patient. 
         [0003]    Quantitative accuracy is ensured by periodic calibration of the machine using radiation detectors and phantoms to determine the relationship between the control settings of the machine and actual dose. One type of radiation detector is an ionization chamber in which electrodes are placed on opposite sides of a volume of gas. The gas is ionized by the radiation passing through the chamber volume and the ions are collected on one collector electrode under the influence of a voltage applied across the electrodes. 
         [0004]    Ideally the ionization chamber volume is small compared to the beam size to limit “partial volume” errors that affect the ionization chamber measurement when the volume is not fully irradiated by the measured radiation beam. 
         [0005]    Certain radiation therapy systems, for example, the Gamma Knife® or a linear accelerator configured for stereotactic radiotherapy or radiosurgery, provide extremely small radiation beams, for example, as small as  4  mm. It is difficult to construct ionization detectors that are small enough to avoid partial volume effects while providing desired sensitivity. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention provides an ionization chamber having two measurement volumes, one substantially surrounding the other, either or both of which may be larger than the desired measured radiation field. By using the readings from the two chambers of different sizes, it is possible to detect and or correct for the partial volume effects. 
         [0007]    Specifically then, the present invention provides a nested radiation detector system having a first radiation detector being an ionization detector defining a first volume and providing for the detection of ionized gas in the first volume at a first collection electrode and a second radiation detector positioned within the first volume and defining a second volume within the first volume and providing for the detection of radiation in the second volume to produce a signal at a second collection electrode, the first volume being electrically separated from the second volume. Radiation passing through the second volume also passes through the first volume providing current at the first and second collection electrodes from the ionization of gas in the first and second volumes. The second radiation detector may be an ionization detector or a diode detector, a diamond detector, a scintillation detector or the like. 
         [0008]    It is thus a feature of at least one embodiment of the invention to provide a radiation detector system that may be used to detect and correct for partial volume effects where the radiation beam is smaller than one of the ionization chambers or beam misalignment. It is further a feature of at least one embodiment of the invention to provide for the measurement of dose from narrow radiation beams where partial volume effects for reasonably sized ionization detectors will be present. 
         [0009]    At least one of the first and second radiation detectors may provide an outer chamber wall constructed of an air equivalent material. 
         [0010]    It is thus a feature of at least one embodiment of the invention to permit the construction of nested ionization detectors without adversely changing the energy spectrum of the radiation beam such as may unacceptably change the calibration of either detector. 
         [0011]    The outer chamber wall of the first and/or second chamber may be a conductive polymer material or a non-conductive material having an internally applied conductive coating. 
         [0012]    It is thus a feature of at least one embodiment of the invention to provide for simple fabrication of the outer chamber such as from easily machined or molded polymer materials. 
         [0013]    The second volume may be substantially centered within the first volume. 
         [0014]    It is thus a feature of at least one embodiment of the invention to minimize the effects of the angle of the measured beam on the relationship between the measurements of the ionization chambers used for the partial volume correction. 
         [0015]    The first radiation detector may provide an outer chamber wall that is substantially spherical. 
         [0016]    It is thus a feature of at least one embodiment of the invention to minimize the effect of the angle of the radiation beam on the measurements. 
         [0017]    Each of the first and second ionization chambers includes two electrically independent electrodes forming the outer surfaces of the first and second volumes respectively. 
         [0018]    It is thus a feature of at least one embodiment of the invention to permit fabrication of the device using a pre-existing ionization chamber. 
         [0019]    The first radiation detector may provide a hollow shaft leading to the first volume and the second ionization chamber may be removable and slidably received within the hollow shaft against a stop locating the second volume in a predetermined position within the first volume. 
         [0020]    It is thus a feature of at least one embodiment of the invention to permit removal and independent use of the first or second ionization detector when partial volume effects are not at issue. 
         [0021]    The ionization chamber system may further include an electrometer receiving ionization signals from the first and second ionization chambers to provide a correction for at least one of the ionization chambers to accommodate partial volume effects caused by radiation beams having an axial cross-section smaller than a corresponding cross-section of the second volume along the axis. 
         [0022]    It is thus a feature of at least one embodiment of the invention to provide automatic correction of partial volume effects. 
         [0023]    The nested ionization chamber system may further include a third ionization chamber surrounded by the first volume and providing for the detection of ionized gas in a third volume at a third collection electrode, the third volume being isolated from the second volume wherein radiation passing through the first volume also passes through the third volume providing current at the first and third collection electrodes from the ionization of gas in the first and third volumes. 
         [0024]    It is thus a feature of at least one embodiment of the invention to provide for improved correction for partial volume effects through the use of an additional chamber. 
         [0025]    Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims and drawings in which like numerals are used to designate like features. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  is a simplified perspective view of an embodiment of an ion chamber system according to the present invention as positioned within a radiation beam; 
           [0027]      FIG. 2  is a schematic representation of one embodiment of the ion chamber system of  FIG. 1  showing nested radiation detectors; 
           [0028]      FIG. 3  is a fragmentary, detailed cross-sectional view of the ion chamber system of  FIG. 2 ; 
           [0029]      FIG. 4  is a block diagram of an electrometer suitable for use in the present invention; 
           [0030]      FIG. 5  is a plot showing example functional relationships between the calibrated outputs of the nested radiation detectors for different beam widths as may be used to correct for partial volume effects; and 
           [0031]      FIG. 6  is a simplied diagram of an alternative embodiment of the present invention using three radiation detectors. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0032]    Referring now to  FIG. 1 , the present invention provides an ion chamber system  10  including a detector assembly  12  connected by electrical cables  14  to an electrometer unit  16 . In use, a detector head  18  of the detector assembly  12  may be positioned within a radiation beam  20  to be exposed to the beam  20 . As will be described below, the detector head  18  contains two independent ionization detectors one within the other and each attached to a separate cable  14 . In one embodiment, the electrometer unit  16  may provide for two displays  15  each outputting a dose measurement received from one of the ionization detectors. 
         [0033]    Generally, the radiation beam  20  will have a cross-sectional area  22  taken perpendicular to an axis  25  of the beam  20  along which the radiation propagates. For beams  20  with cross-sectional areas  22 , a beam width  24  may be defined related to the cross-sectional area  22 . 
         [0034]    Referring now to  FIG. 2 , the detector head  18  of the detector assembly  12  may provide a substantially spherical outer electrode  26 , for example, enclosing an approximately 100 cm 3  volume. The outer electrode  26  may be constructed of an air equivalent conductive plastic providing radiation absorption characteristics approximating that of air. Such plastic material is commercially available, for example, under the trade name Shonka C552. Alternatively, the outer electrode  26  may be constructed of an air equivalent polymer or a non-air equivalent material having an internal conductive coating. This outer electrode  26  may be connected to ground potential at an internal power supply to the electrometer unit  16 . 
         [0035]    The outer electrode  26  may fit around a second electrode  28  providing a cylindrical tube  30  terminating with a hemispherical portion  32 . The hemispherical portion  32  preferably describes a portion of a sphere where the sphere has a common center with a sphere describing the outer electrode  26 . The second electrode  28  may be a conductive material or conductive layer on the outer surface of an insulating support. The conductive element of this second electrode  28  will be connected through a current sensor  40  to a power source  42  of high voltage electricity, for example, at  300  volts. Together the outer electrode  26  and second electrode  28  provide a first ionization detector  53  sensitive to the volume  52  between the outer electrode  26  and second electrode  28 . 
         [0036]    The second electrode  28  may in turn fit around a third electrode  44  conforming in profile to the second electrode  28  but with smaller dimensions to fit therein. The third electrode  44  may thus also provide a tubular portion  46  capped by a hemispherical portion  48  also describing a portion of a sphere centered on the center of the sphere describing the outer electrode  26 . The third electrode  44  encloses, for example, a volume of approximately 0.007 cm 3 . The third electrode  44  may also be a conductive material or a conductive layer on an insulating support, and in either case is connected to ground. 
         [0037]    Coaxially centered within the tubular portion  46  of the third electrode  44  is a conductive wire electrode  49  terminating substantially at a center of the sphere describing the outer electrode  26 . This wire electrode  49  is connected through a second current sensor  50  to a source of high voltage from power source  42 . Together the third electrode  44  and wire electrode  49  provide a second ionization detector  55  sensitive to the volume  54  between the third electrode and wire electrode  49  and within the volume  52  but isolated therefrom to prevent exchange of electrons therebetween. 
         [0038]    It will be understood that electrons formed in a gaseous volume  52  inside the outer electrode  26  and outside the second electrode  28  will migrate to be collected by the second electrode  28  to be measured by current sensor  40 . Similarly, electrons formed in a volume  54  within the third electrode  44  will be collected by the wire electrode  49  and measured by current sensor  50 . 
         [0039]    The current readings by current sensors  40  and  50  provide an indication of the dose of radiation passing through the respective volumes  52  and  54  according to principles well understood in the art. 
         [0040]    Referring now to  FIG. 3 , in construction, the outer electrode  26  may be formed from two interfacing hemispherical half shells  51  and  56  may be adhered together with conductive adhesive, or mechanically and conductively joined. Shell  51  may be threadably attached to a stem  57  extending radially therefrom and providing support for the detector assembly  12 , as shown in  FIG. 1 , by means of a stand  58  or the like. Attached to the stem is an interdetector wall  60  having an outer dimension conforming generally to that of second electrode  28  and in the preferred embodiment being an insulating material with an outer conductive coating or conductive material  62 . 
         [0041]    A triaxial cable  64 , forming one of the cables  14 , may be received through an offset bore in the stem  57  with its innermost conductor  66  electrically attached to the conductive coating or conductive material  62  and its outer shield braid  68  attached to a conductive portion of shell  51  and its inner shield braid (not shown) attached to a conductive inner electrode  27 . 
         [0042]    A separate ionization probe  70  may have its stem  72  inserted in a central bore in stem  57  to slide therein to stop against the inner surface of the interdetector wall  60  to be properly positioned within the volume described by outer electrode  26 . This probe  70 , for example, may be a standard ionization probe such as the model A16 Exradin Microchamber ionization chamber manufactured by Standard Imaging, Inc. of Middleton, Wis. or its equivalent. 
         [0043]    The probe  70  provides the wire electrode  49  centered within a housing  74  nesting within the interdetector wall  60 . The housing  74  have a conductive inner surface forming the third electrode  44  and attached to an outer shield braid  76  of a triaxial cable  78  forming a second of the cables  14 . The center conductor of the triaxial cable  78  may provide for the wire electrode  49  and an inner shield braid  77  may attached to an electrode  79  similar in function to electrode  27  described above. 
         [0044]    Referring now to  FIG. 5 , the detector assembly  12  may provide for accurate dose measurements of radiation beams  20  of various beam widths  24   a - 24   c  including those (beam width  24   b  and  24   c ) that do not fully irradiate the volumes  52  and/or  54 . Such beams would be expected to cause partial volume effects that occur when the volume of an ionization detector is not fully irradiated by the radiation beam. Partial volume effects are caused by either or both of a tailing off of the intensity  21  of the beam  20  at the edges of the beam  20  and a decrease in the volume of gas within the detector that may interact with radiation so as to generate ionized current. These partial volume effects may be compensated for mathematically through the measurements made by the detector assembly  12 . 
         [0045]    The process of correction for partial volume effects may begin with an empirical or theoretical modeling of the operation of the detector assembly  12  with a range of widths  24  of beams  20  including, for example, beam width  24   a  that fully irradiates the entire volumes  52  and  54  of the ionization detectors  53  and  55  of detector assembly  12 , a beam width  24   b  that fully irradiates all of volume  54  but only partially irradiates volume  52 , and beam width  24   c  that partially eliminates both volumes  52  and  54 . In each of these situations, two readings may be collected from the detector assembly  12  using electrometer unit  16 . 
         [0046]    These readings are then corrected or calibrated for temperature, pressure, a geometric calibration factor, and other factors known in the art, excluding partial volume effects. The calibrated readings may produce reading D 1  from the ionization detector  53  associated with volume  52  and reading D 2  from the ionization detector  55  associated with volume  54 . These readings may be displayed on displays  15  of electrometer unit  16 . 
         [0047]    For large beam widths  24   a,  shown in  FIG. 5  as those greater than  60  mm, a ratio of the two dose readings (i.e., D 1 /D 2 ) as a function  80  of beam width  24  will be constant and have a value of unity reflecting the fact that neither ionization detectors  53  and  55  are subject to partial volume effects and are fully and correctly calibrated. 
         [0048]    This ratio will start to drop as the beam width narrows to width  24   b  and thus fails to fully irradiate volume  52  causing a partial volume decrease in the value of D 1  with respect to dose D 2  as a function  82  of beam size. This region may, for example, extend between  8  mm and  60  mm of beam width  24   b.  The function  82  is related to the characterization of detector  53 . 
         [0049]    For small beam widths  24   c,  the ratio D 1 /D 2  will be a different function  84  of beam width  24  driven by partial volume decreases in the values of both D 1  and D 2 . The function  84  is related to the characterizations of detectors  53  and  55 . 
         [0050]    This collected data of functions  80 ,  82  and  84  may then be used to detect and/or correct for partial volume effects during calibration of the radiation therapy machine. In this process, the detector assembly  12  may be used to collect data of D 1  and D 2  for a known beam size W. In a first step, the ratio of the measured readings D 1  and D 2  may be taken and compared to the ratio indicated by the previously determined functions  80 ,  82  and  84  (depending on the beam size) using a preprepared chart or table similar to that described with respect to  FIG. 5 . If the indicated ratio for the beam size does not match, an alignment problem may exist in which the beam  20  is not centered on the volume  54 . In such cases the ratio will be smaller than suggested by  FIG. 5 . The process may end at this point if alignment is the only issue. 
         [0051]    If the alignment is correct, the values of D 1  and D 2  may be corrected. In a simple example of this correction process, the ratio of D 1 /D 2  taken from a chart similar to  FIG. 5  (based on known beam width) is simply multiplied by the reading of D 2  to obtain a corrected value of D 2  without or with reduced partial volume effects. 
         [0052]    Referring now to  FIG. 4 , the above calculations (and the data of a table per  FIG. 5  providing functions  80 ,  82  and  84 ) may be incorporated into a computerized electrometer unit  16 . Such an electrometer unit  16  receives signals over cable  14  from each of the ionization detectors  53  and  55  at the current sensors  40  and  50  as described before as powered by power source  42 . The measured currents may be provided to analog to digital converters  90  which provide data through a common bus  92  to a processor  94 . The processor  94  may communicate with a memory  96  holding a stored program  98  incorporating the calculations described above. Information about the measurement (beam width W) may be entered through a keyboard  104 , touchscreen or other method and the calculated actual dose corrected according to the equations described above may be presented on graphic display terminal  102 . Both the graphic display terminal  102  and the keyboard  104  may connect to the bus  92  via interface  100 . 
         [0053]    Referring now to  FIG. 6 , improved accuracy may be obtained by the use of more than one concentric ionization chamber, for example, by adding a third ionization detector  106  within ionization detector  53  and the addition of an additional current sensor  108  to the electrometer unit  16  this arrangement allows data from this detector  106  to be used to augment that collected by the other detectors  53  and  55 . 
         [0054]    It should be understood that the invention is not limited in its application to the details of construction and arrangements of the components set forth herein. The invention is capable of other embodiments and of being practiced or carried out in various ways. Variations and modifications of the foregoing are within the scope of the present invention. It also being understood that the invention disclosed and defined herein extends to all alternative combinations of two or more of the individual features mentioned or evident from the text and/or drawings. All of these different combinations constitute various alternative aspects of the present invention. The embodiments described herein explain the best modes known for practicing the invention and will enable others skilled in the art to utilize the invention.