Abstract:
A phantom for heavy ion radiation therapy provides characterization of an ion beam that may enter but not exit from the phantom. The phantom may include multiple materials and multiple spatially dispersed ion detectors to obtain signals that may be fit to known beam curves to accurately characterize the location and other parameters of Bragg peak of a given ion beam within a patient.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application 60/891,859, filed Feb. 27, 2007, the disclosure of which is incorporated herein by reference. 
    
    
     This invention was made with United States government support awarded by the following agency:
         NIH CA088960       

     The United States government has certain fights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to radiotherapy systems using ions for the treatment of cancer and the like and, in particular, to a phantom for such systems. 
     External beam radiation therapy may treat a tumor within the patient by directing high-energy radiation in one or more beams toward the tumor. Recent advance external beam radiation systems, for example, as manufactured by Tomotherapy, Inc., treat the tumor with multiple x-ray fan beams directed at the patient over an angular range of 360°. Each of the beams is comprised of individually modulated rays whose intensities can be controlled so that the combined affect of the rays over the range of angles provides an arbitrarily complex treatment area with minimized skin dose. 
     The benefit of the improved accuracy possible with such systems is ensured by careful characterization and monitoring of the x-ray beam geometry and intensity, for example, through portal imaging devices and entrance dose monitors. 
     X-rays expose tissue not only within the tumor but also along the path of each ray into and out of the patient. While judicious selection of the angles and intensities of the rays of x-ray radiation can limit radiation dose outside of the tumor, a desire to more closely conform the radiation dose to the tumor has raised interest in substituting ions such as protons for x-ray radiation. Unlike x-rays, the dose deposited by a proton beam is not uniform in homogenous tissue, but rises substantially, at the “Bragg peak” just before the proton stops within the tissue. Further, because the proton can be controlled to stop within the tissue, exit does from the proton beam can be substantially eliminated. These two features allow improved placement of dose within the tumor. 
     Unfortunately, unlike x-rays, ions are not easily characterized by entrance dose monitors and portal imaging device. In part, this is because such devices are sensitive largely to flux (photons per unit time) as opposed to radiation energy, and it is this latter characteristic which determines the important quality of proton range. Portal imaging systems, which rely on radiation exiting the patient, are of limited value in a proton imaging system in which the protons stop within the patient. 
     SUMMARY OF THE INVENTION 
     The present invention provides a phantom that may characterize the energy and hence the range of protons. The phantom uses a one or more tissue-mimicking material and an array of intensity detecting elements whose pattern of intensities may locate the Bragg peak. The location of the Bragg peak, in turn, reveals the proton energies. The phantom may be used to calibrate a proton or heavy ion therapy machine and/or may be used to better characterize a CT image used for treatment planning, by equating CT numbers to materials that have been characterized with respect to their interaction with protons. 
     Specifically then, the present invention provides a phantom including a tissue-mimicking support and a set of ion detectors spatially separated within the tissue-mimicking support to detect passage of ions through the tissue-mimicking support. A data processing system receiving signals from the ion detectors deduces a location within the phantom of a Bragg peak of ions passing through the tissue-mimicking support. 
     Thus it is one aspect of the invention to provide a measurement system suitable for next-generation ion radiation therapy machines to provide accurate characterization of the ion beam energies. 
     The tissue-mimicking support may include removable portions wherein the ion detectors are held within one removable portion to be repositioned within the tissue-mimicking support. The removable portions may provide materials representing different tissue types so that portions may mimic different tissue types. 
     It is thus an aspect of at least one embodiment of the invention to provide a phantom that can characterize the effect of different materials and material thicknesses on Bragg peak location. It is another aspect of at least one embodiment of the invention to allow the phantom to be configured to approximate the actual treatment volume. 
     The tissue-mimicking support may include fiducial markers allowing automated identification of the different materials in a CT image. 
     It is thus another object of the invention to allow the phantom to be used to calibrate planning CT images. 
     The ion detectors may be arrayed both along a direction of propagation of the ion beam and across the direction of propagation of the ion beam and the data processing system may fit a template multidimensional Bragg peak to the signals and spatial locations of the ion detectors to identify the Bragg peak. 
     It is thus an aspect of the present invention to provide for multidimensional characterization of the Bragg peak including range and beam widening. It is another aspect of the invention to allow characterization of the beam center line. 
     These particular features 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 
         FIG. 1  is a perspective view in partial phantom of an example proton therapy system suitable for use with the present invention having a synchrotron proton source providing protons to multiple gantry units; 
         FIG. 2  is a cross-section along line  2 - 2  of  FIG. 1  showing a proton beam modified by a beam control system before being directed into a patient; 
         FIG. 3  is a block diagram of the beam control system of  FIG. 2 ; 
         FIG. 4  is a perspective view of a phantom positionable in place of the patient of  FIG. 2 , the phantom having removable plugs of tissue-mimicking material held by a larger tissue-mimicking support, and further showing one plug as instrumented with ion detectors; 
         FIG. 5  is a cross-section along line  5 - 5  of  FIG. 4  showing a configuration of the plugs inserted into the phantom and showing a radio opaque fiducial marker on one plug for CT identification; 
         FIG. 6  is an elevational, cross section of the plug instrumented with ion detectors showing a centerline of an ion beam through detector elements, and a plot of the signals from the elements as fit to a Bragg peak of the ion beam; 
         FIG. 7  is a perspective view of a three-dimensional array of detector elements; 
         FIG. 8  is a plot of isodose lines for a standard Bragg peak being part of a set of isodose surfaces that may be fit to the isodose surfaces measured by the detector of  FIG. 7  to locate the Bragg peak in the phantom; and 
         FIG. 9  is a flow chart for use of the phantom of the present invention for calibrating the gantry system and/or for treatment planning. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to  FIG. 1 , a ion therapy system  10  may include a cyclotron or synchrotron  12  or other proton source providing a pencil beam of protons  14  to a gantry unit  16 . 
     Referring also to  FIG. 2 , the proton beam  14  may be received along an axis  22  into an axial portion of a rotating arm  20  rotating about the axis  22 . The rotating arm  20  includes guiding magnet assemblies of a type known in the art to bend the proton beam  14  radially away from the axis  22  then parallel but spaced from the axis  22  to a treatment head  26 . The treatment head  26  orbits about the axis  22   c  with rotation of the rotating arm  20  bending the proton beam  14  back toward the axis  22 . 
     The treatment head  26  may include a modulation assembly  30  for forming the proton beam  14  into a wider treatment beam (for example a fan or cone beam) and for modulating rays of the beam in energy and intensity to produce a modulated treatment beam  24  as will be described. 
     Referring still to  FIG. 2 , a patient  32  may be positioned on a support table  34  extending along the axis  22  so that a modulated treatment beam  24  may irradiate the patient  32  at a variety of angles  36  about the axis  22 . A cylindrical neutron shield  40  having a bore for receiving the table  34  and the rotating arm  20  may surround the gantry unit  16  to block generated neutrons. 
     In one embodiment, a second rotating arm (not shown) may rotate with or independently of the rotating arm  20  to support an x-ray source  42  and x-ray detector  44  opposed across the axis  22  to illuminate the patient  32  at a range of angles to provide a CT scan of the patient  32  according techniques well-known in the art. 
     Referring now to  FIG. 3 , the modulation assembly  30  may include a fan beam former  46 , followed by an energy or range modulator  47 , followed in turn by an intensity modulator  48 . The fan beam former  46  may receive the monoenergetic pencil beam of protons  14  having a generally small circular cross-section to widen this beam into a treatment beam  50  being for example a larger circle (a cone beam) or a thin rectangle extending perpendicular to axis  22  (a fan beam). The range modulator  47 , receives the treatment beam  50  and changes the energy of the protons in different rays  52  of the treatment beam  50 , for example by selective insertion of different thicknesses of materials as taught in U.S. Pat. No. 5,668,371 entitled: “Method and Apparatus for Proton Therapy” assigned to the same assignee as the present invention, and hereby incorporated by reference. 
     The treatment beam  50  then is received by the intensity modulator  48  which may, for example, be a set of shutters, one for each ray  52 , controlling the amount of time the protons may pass along that ray  52  thus defining an average intensity of protons in each ray  52  as it also taught by the above referenced patent. 
     The modulated treatment beam  24  having rays  52  that are both intensity and energy modulated may be directed toward a phantom  60  of the present invention fixed at a known location with respect to the table  34 . This precise location of the phantom may be assisted by means of laser line projectors  62  directing lasers toward an isocenter along axis  22 , the isocenter being  64  defining the center of rotation of the treatment head  26  and matching with fiducial markings on the outside of the phantom  60  as will be described. 
     The range modulator  47  and intensity modulator  48  may be controlled by a control system  66  including a calibration memory  68  providing a conversion between the desired intensity and actual physical shutter settings of the range modulator  47  and intensity modulator  48 . The control system  66  may receive a treatment plan  70  from a general purpose computer  72  executing a stored program as will the described to generate the treatment plan from one or more CT images  74  having a user-defined dose pattern  76  superimposed thereon. Computer  72  may also provide for a CT reconstruction by controlling and monitoring of x-ray source  42  and detector  44  described above and may provide other control functions generally understood in the arc. 
     Referring now to  FIG. 4  the phantom  60  may for example be generally cylindrical with the axis of the cylinder aligned, when it is placed on the table  34 , with axis  22 . And outer surface of the cylinder may include fiducial markings  78  that may align with the laser scans of laser line projectors  62  to allow precise and known placement of the phantom on the table  34  and thus its location with respect to the treatment head  26 . The phantom in one embodiment may include a set of axial bores  80  of cylindrical dimension that may receive corresponding cylindrical inserts  82 . The main body of the phantom  60  may be, for example, a water mimicking material such as Plexiglas or polyethylene or other hydrocarbon material while the inserts may be of the same material or different materials for example those simulating bone or lungs, these latter materials compounding, for example, calcium and phosphorus compounds mixed in with plastic or micro spheres to provide for air inclusions decreasing the density of the plastic material. 
     One of the inserts  82  may be a detection insert  82 ′ providing a series of ion detectors or MOSFET or scintillation detectors embedded in the insert  82 ′ and communicating through cabling  84  with a data acquisition system associated with computer  72 , as shown in  FIG. 3 , or a freestanding data acquisition system. 
     Referring now to  FIG. 5 , a CT image of the phantom  60  will provide for image areas  86  associated with the main body of the phantom  60  and each of the inserts  82 . Each image area may provide a characteristic CT number or image grayscale value. The inserts  82  may include radio-opaque fiducial markings  88 , for example, allowing them to be automatically recognized in the CT image, for example, as may be fashioned from as lead beads or wire. Improved discernment of the type of material may be provided, for example, through dual energy CT techniques or the like. 
     Referring now to  FIG. 6 , a ray  52  extending along a ray axis  90  may pass through the plug  82 ′ to be measured by one or more detector elements  92 . Each of these detector elements  92  provides a signal  94  having a strength I and a spatial location x. The spatial location will generally describe up to three dimensions, but as shown is a single dimension along the known axis  90 . These signals  94  may be fit to a Bragg peak template  96  having a characteristic shape that may be scaled in the x and I dimensions to provide a best fit to the signals  94 . Once the Bragg peak template is fit to the signals  94 , Bragg peak maximum  98  and Bragg peak location  100  may then be determined. 
     Referring now to  FIG. 7  the detector elements  92  may be distributed in three dimensions within the plug  82 ′ to provide not only a Bragg peak location but a three-dimensional characterization of the proton beam distribution. 
     Referring now to  FIG. 8  a two-dimensional implementation of this process for example takes the signals  94  from each of the detectors in multiple dimensions to generate an isodose surface  102  which may be fit to a template standard isodose surface  104  with scaling, translation and rotation which may be used to determine proton beam characteristics including beam axis  106 , beam peak  108 , beam limit  110 , and beam width  112  byte reading from the template as so scaled, translated, and rotated. It will be understood that this process may be readily extended to a complete three-dimensional characterization of the proton beam even though the beam does not exit the phantom  60 . 
     Referring now to  FIG. 9 , the phantom  60  may be used in a procedure with the system of  FIG. 1  to provide improved characterization of the ion beam and of the patient for planning purposes. At a first process block  111 , a planning CT images taken of the patient in a manner similar to that done for standard x-ray Tomotherapy. This is followed or preceded by a calibration CT scan indicated by process block  113  of the phantom  60 . 
     As indicated by process block  114 , the radiation therapy machine  10  is then used to irradiate the phantom  60  to determine the Bragg peak location for the phantom  60  as calibrated at step  113 . This Bragg peak information is used for two purposes. In its initial purpose, as indicated by process block  116 , the Bragg peak measurements are extracted and used as to adjust and calibrate the modulation assembly  30  through, for example, adjustment of calibration factor table  68  described with respect to  FIG. 3 . In this way the radiation therapy machine can produces the energies and beam spread expected in its operation. 
     The Bragg peak information is also used as indicated by process block  118  to improve the radiation planning process. In this use, materials of the planning CT of the patient taken at process block  111  are matched to the known materials on the phantom  60  by matching grayscale values of the calibration CT image taken at process block  113  with the planning CT image taken at process block  111 . An improved model of the patient is thereby produced and used to evaluate different beam intensities and ranges according to a planning loop  120  which in its simplest form uses a Monte Carlo or simple similar technique to perturb intensities and ranges of the beams and to model those iteratively against the model produced. Alternatively as disclosed in the previously cited U.S. Pat. No. 5,668,371 the beam ranges may be placed using distal edge algorithm described in that patent and the intensities alone may be iteratively modeled using the attenuating properties all the materials alone. 
     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.