Abstract:
A volume phantom for radiation therapy verification employs film held in a spiral configuration within a equalizing ring of attenuating material. The ring provides improved uniformity in radiation measurement and may be extended, for example, to a hemisphere to provide improved modeling and simulation of treatments in the region of the head.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    [0001] 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002] This invention was made with United States government support awarded by the following agencies: NIH CA14520. The United States has certain rights in this invention. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0003]    This invention relates to the evaluation of radiation therapy phantoms in particular to a phantom using film and providing radiation measurement throughout a volume.  
           [0004]    External beam radiation therapy treats cancerous tissue by exposing the tissue to a high-energy radiation from an external source. Normally, a number of different external beams are employed, each approaching the tissue at a different angle, simultaneously or in sequence. The use of multiple beams and angles minimizes the radiation exposure of any given area of the skin and of nearby, possibly radiation-sensitive organs. The selection of the angles and the exposure times for each beam comprises a radiation treatment plan.  
           [0005]    Whereas some treatment plans may have a relatively low number of beams and exposure times, the latest generation of radiation therapy equipment allows for extremely complex radiation treatment plans employing many independently controllable beams throughout a range of angles. Multiple beams of varying average intensity may be formed by a multileaf collimator or similar mechanism.  
           [0006]    Such complex radiation treatment plans provide precise placement of dose upon tumor tissue, but place severe demands on phantoms used to verify the dose produced by the treatment plan. A conventional radiation therapy phantom incorporates an attenuating material, such as plastic or water, interacting with radiation in a manner equivalent to that of human tissue. One or more radiation detectors, for example, ionization detectors or flat sheets of radiation sensitive film are located within the attenuating material to measure the radiation at different locations.  
           [0007]    Conventional phantoms are cumbersome or expensive when accurate characterization of a dose throughout a volume is required, requiring repeated measurements and repositioning of the phantom or its detectors. Accordingly, the present inventors have developed a “spiral” phantom using a single sheet of radiation sensitive film rolled in a spiral to provide dose measurements in a volume rather than a single plane. Knowledge of the mathematical description of the spiral and the properties of the material in which the spiral is cut, allows the radiation measured by the film at different locations upon its two dimensional surface to be related to the doses at different volumes within the three dimensions of the phantom. The spiral phantom is particularly useful for complex intensely modulated radiation therapy protocols and is described in the article: “Spiral Phantom for IMRT and Tomotherapy Treatment Delivery Verification” by Bhudatt Paliwal and Wolfgang Tomé, Susan Richardson and T. Rockwell, Med. Phys. 27(11), November 2000, pp. 2503-2507. These papers are hereby incorporated by reference.  
           [0008]    As noted in this paper, although the prototype spiral phantom provided good qualitative assessment of the treatment plan, deviation in the prediction of dose and in the measured dose of the spiral phantom, particularly at the outer arm of the spiral, limited its use in precise quantitative applications.  
         BRIEF SUMMARY OF THE INVENTION  
         [0009]    The inventors have determined that the quantitative accuracy of the spiral phantom may be significantly improved by the addition of a ring of phantom material outside the furthest radial extent of the film. This extra material promotes photon scatter before the radiation strikes the outermost film, providing more uniform sensitivity of the film to radiation over the entire length of the spiral.  
           [0010]    The outer ring of phantom material also allows improved clamping and registration of the film, allowing the spiral to be rotated as desired within a fixed outer shell attached to a patient table or the like. The ring may be extended to a hemispherical shell to provide simplified modeling of the expected dose on the phantom and improved simulation for radiation treatment of the head region.  
           [0011]    Specifically then, the present invention provides a radiation phantom having a film holder providing a spiral support for radiation sensitive film within an attenuating material. The radiation sensitive film, when placed in the film holder, extends along the spiral support to an outer film limit at which point a housing surrounds the film holder and provides a build up region equalizing radiation sensitivity of the radiation sensitive film near the outer film limit and the radiation sensitive film removed from the outer film limit, i.e., at the center of the spiral.  
           [0012]    Thus, it is one object of the invention to provide for greater uniformity in the radiation measurements over the length of the spiral and to improve the quantitative accuracy of the spiral phantom.  
           [0013]    The housing may be constructed of a material having radiation attenuation properties similar to those of the material of the spiral support. The materials may mimic the radiation attenuation provided by human tissue.  
           [0014]    Thus, it is another object of the invention to provide a uniform phantom that is easily modeled for simulations and that provides a dose distribution similar to that which would be found in a human patient.  
           [0015]    The spiral support may be a slot following an Archimedean spiral. The film holder may optionally include a second slot interleaved with the first slot.  
           [0016]    Thus, it is another object of the invention to provide for simple structure for supporting the film that similarly provides uniform sampling over a volume. Multiple slots allow arbitrary sampling density to be obtained.  
           [0017]    The film holder may be a cylinder and the housing may be a tube fitting around the film holder.  
           [0018]    Thus, it is another object of the invention to provide for simple structure allowing preloading of film within the phantom in a protected light-tight configuration.  
           [0019]    A clamping means may fit between the cylinder and the housing, pressing the slot about the radiation sensitive film.  
           [0020]    It is thus another object of the invention to provide a clamping mechanism for the film. The clamping means may be a wedge inserted between the housing and the film holder and constructed of a material similar to both.  
           [0021]    The film holder may include a keying element locking rotation of the film holder with respect to the housing.  
           [0022]    Thus, it is another object of the invention to provide positive registration of the film with respect to the housing so that rotation of the housing may be used to accurately position the sampling points of the film within the volume to be measured.  
           [0023]    The housing may be a hemispherical outer shell having radiation attenuation properties mimicking human tissue.  
           [0024]    It is yet a further object of the invention to provide for a simple phantom shape amenable to simulations and particularly suitable for use in simulations of radiation treatment of the human head.  
           [0025]    The foregoing and other objects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings which form a part hereof and in which there is shown by way of illustration a preferred embodiment of the invention. Such embodiment does not necessary represent the full scope of the invention, however, and reference must be made to the claims herein for interpreting the scope of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]    [0026]FIG. 1 is a perspective view of the phantom of the present invention mounted on a patient table showing the attachment of an optical target for alignment of the phantom with an external reference;  
         [0027]    [0027]FIG. 2 is an exploded, partial cross-sectional view of the components of the phantom of FIG. 1 showing a slotted film holder fitting within an inner tubular housing held by an outer hemispherical outer housing;  
         [0028]    [0028]FIG. 3 is a cross-sectional view of the film holder and housing of FIG. 2 taken along lines 3--3 of FIG. 2 showing the locking of the film holder and inner housing by means a of cylindrical key and the intermitting of a wedge between the film holder and inner housing to compress the slot about the film;  
         [0029]    [0029]FIG. 4 is a view similar to FIG. 3 showing the use of two spiral slots to obtain a greater sampling density and showing locations of optional ionization detectors for normalizing the data of the film to quantitative measurements;  
         [0030]    [0030]FIG. 5 is a flowchart of the steps of using the phantom of the present invention in verifying complex radiation therapy treatment plans;  
         [0031]    [0031]FIG. 6 is an outline of a patient&#39;s head showing the positioning of a bite bar holding an optical target similar to that of the phantom of FIG. 1 (also shown in outline) for alignment of the phantom and patient with the radiation therapy isocenter; and  
         [0032]    [0032]FIG. 7 is a simplified representation of the film exposed in the phantom after processing, such as represents, when flattened, a spiralogram, and showing a mapping of locations on the spiralogram to the volume of the phantom. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0033]    Referring now to FIG. 1, in a preferred but non-limiting embodiment, the spiral phantom  10  of the present invention includes a hemispherical outer housing  12  having a vertically oriented flat face  14 . During use, the flat face  14  may be arranged perpendicularly to a longitudinal axis  16  extending along the length of a patient table  20 .  
         [0034]    Referring also to FIG. 2, the flat face  14  may include two mounting holes  22  along its lower edge, the mounting holds  22  being threaded to receive turn screws  24 . The turn screws  24  may be used attach the flat face  14  to a bracket  26  extending upward from one end of a patient table  20 . The bracket  26  is sized so that the hemispherical outer housing  12  is roughly in the same height above the surface of patient table  20  as a patient&#39;s head when supported on the surface of patient table  20 . The hemispherical outer housing  12  is moved, however, longitudinally beyond the end of the patient table  20  so as not to interfere with a patient location.  
         [0035]    The hemispherical outer housing  12  has a cylindrical bore  28  perpendicular to and centered in the flat face  14  to provide a cavity receiving an inner housing  30 . The inner housing  30  is cylindrical about an axis of symmetry parallel to longitudinal axis  16  to fit tightly within the cylindrical bore  28 .  
         [0036]    Referring specifically to FIG. 2, the inner housing  30  is composed of a tubular body  32  having first and second cylindrical end caps  34  and  36  fitting against either end of the tubular body  32  so as to provide an enclosed cylindrical volume therein. The ends of tubular body  32  may include a longitudinally extending circumferential ridge  38  that is received by a corresponding groove  40  in each of the inwardly facing surfaces of end caps  34  and  36 . The ridge  38  and groove  40  serve to center the end caps  34  and  36  on the tubular body  32  and to provide a light trap preventing light leakage into the inner volume of the tubular body  32 . The end caps  34  and  36  are held to the tubular body by a press fit.  
         [0037]    The material of the hemispherical outer housing  12 , inner housing  30  and film holder  42  is preferentially selected to mimic human tissue and thus to have a density and anomic number similar to that of water. These materials may be, for example, Lucite or Solid Water, the latter commercially available from Gammex of Middleton, Wis. Preferably, the material of the inner housing  30  is opaque to visible light preventing light exposure problems during the handling of the device.  
         [0038]    A cylindrical film holder  42  fits within the volume defined by the tubular body  32  and end caps  34  and  36  and is thus protected from light. The film holder  42  provides a spiral slot  44  extending a full length of the film holder between the cylinder bases. The spiral slot  44  preferably conforms to an Archimedean spiral meaning that its radius from a longitudinal center axis of the cylindrical film holder  42  increases linearly with angle without the center axis of the cylindrical film holder  42 . This results in the spiral arms having constant radial separation producing more uniformity of sampling when a detector film is placed within the spiral slot  44 . Ideally, this spiral extends at least two revolutions or about 6.6 radians about the axis.  
         [0039]    Referring now also to FIG. 3, the spiral slot  44  begins at a center point  46  at the center of the film holder  42  and proceeds outward to an outer film limit  50  being the edge of the film holder  42 . At the outer film limit  50 , the film  85  is captured by the interfitting of a cylindrical key  54  and a hemicylindrical groove  52  extending axially along the periphery of film holder  42 . Deformation of the film  85  between these surfaces, when the cylindrical key  54  is pressed inward by the inner surface of the tubular body  32 , holds the film securely.  
         [0040]    The key  54  also located the film holder  42  at a predetermined rotational orientation with respect to the tubular body  32  which has a corresponding hemicylindrical groove  52  cut in its inner surface. Further, the inner surface of end cap  36  may include a blind bore  62  receiving an end of the key  54  so as to lock the rotation of the cap  36  to match the orientation of the film  85  held by the key  54 . Thus, key  54  locates the beginning of the film  85  with respect to the tubular body  32  and end cap  36 .  
         [0041]    A wedge  56  may also be fit between the inner surface of the tubular body  32  and the outer surface of the film holder  42  to provide a radial compression to the film holder  42  holding the film  85  securely without movement and with minimal air gaps within the spiral slot  44 .  
         [0042]    Referring still to FIG. 3, the thickness of the tubular body  32 , indicated by dimension A, is sized so as to provide necessary scattering so that radiation striking film  85  within the spiral slot  44  near the outer film limit  50  experiences an exposure per given amount of radiation, similar to the exposure of film  85  near the center point  46  for the same amount of radiation.  
         [0043]    It will be understood that the film holder  42  may be preloaded and stored within the inner housing  30  and easily inserted into the hemispherical outer housing  12  as needed so that multiple studies may be readily conducted and time required to load film holder  42  may be avoided. The film  85  may be EDR film from Kodak having a high dynamic range.  
         [0044]    Referring to FIG. 4, it will be understood that an arbitrary spatial sampling of a given volume may be achieved by constructing the spiral slot  44  to be of greater or lesser length and thus of a greater or lesser number of turns. Additional sampling can also be obtained, while fixing the slot length and thus retaining the ability to use conventional film sizes, by producing a second spiral slot  44 ′ interleaved with the first. Holes my be bored in the film holder  42  to receive ionization detectors  58  that can provide for quantitative measurements of dose at particular locations within the spiral phantom  10  that may be used to normalize measurements obtained from the film as will be described. The ionization gauges may be inserted into the film holder  42  before placement in the inner housing  30  and appropriate light-tight conduits for the signal wires provided.  
         [0045]    Referring again to FIG. 2, the hemispherical outer housing  12  may be sized to contain end cap  36  and tubular body  32  but to expose end cap  36  slightly from the flat face  14  to allow for rotation of the inner housing  30  with respect to the hemispherical outer housing  12 . This rotation can bring the spiral slot  44  into a configuration where greater mounts of film cut through a region of interest depending on the particular procedure. The exposed surface of the cap  36  may include angular graduations  60  to be used to control this rotation.  
         [0046]    A hole  64  may pass axially through the hemispherical outer housing  12  to the cylindrical bore  28  so as to facilitate the removal of the inner housing  30  using a pusher rod  66  inserted through the axial hole  64  to press against the outer surface of end cap  34 .  
         [0047]    Referring now to FIG. 5, in use, the phantom  10  may be used to verify a radiation treatment plan developed for a particular patient. As indicated by process block  70 , CT data from that patient is to calculate the necessary beams and intensities for a radiation treatment plan according to well-known techniques.  
         [0048]    The same radiation treatment plan may then be applied to the phantom  10  of FIG. 1 in a simulation as indicated by process block  72  based on the known materials and geometry of the phantom  10 . The hemispherical shape of the phantom  10  makes this simulation process relatively simple and differences between the phantom and the patient are minimized by adopting a simple head-like outer structure and materials that mimic human tissue. Referring to FIG. 7 a mathematical mapping process can relate individual latitude bands  84  crossing the film  85  to similar axial paths through the phantom of the film holder  42 . In this way, at process block  72 , a simulated film may be created showing exposures of the film per the simulation.  
         [0049]    At process block  74 , the phantom  10  is located at a treatment isocenter in the radiation therapy machine where the radiation treatment plan is to be effected.  
         [0050]    Referring momentarily to FIG. 1, this location of the phantom  10  may be facilitated by the attachment of an optical target  76  to the top of the phantom  10 . Such optical targets  76  are well known in the art and make use of triangulation of a series of reflective spheres  78  positioned on the optical target  76  by infrared sensitive scanner camera assemblies (not shown) positioned in a fixed location on the radiation therapy machine. The center  15  of the sphere defining the hemispherical outer housing  12  may thus be located at the isocenter of the radiation treatment plan.  
         [0051]    Referring again to FIG. 5, at process block  80 , the radiation treatment plan is conducted on the phantom having been preloaded with film. The film is then removed and developed to show on its surface a series of exposure zones  82  having exposure corresponding with radiation received at those zones  82 . The actual dose values may be normalized to readings obtained from the ionization detectors  58  with those quantitative measurements interpolated or extrapolated to particular locations on the spiral slot  44 .  
         [0052]    The measured dose is compared against the expected dose at process block  83 . In a first method, the actual film dose may be compared with the simulated film produced at process block  72  and differences highlighted through a subtraction process indicating differences between the actual and expected doses. Such differences may indicate, for example, improper functioning of a mechanical multileaf collimator of a radiation therapy machine or computational errors in the simulation for radiation treatment planning process. In addition, this comparison process establishes that the proper treatment plan was loaded.  
         [0053]    Alternatively or in addition, the data collected from the phantom  10  may be used to construct a three dimensional dose by interpolation to regular Cartesian coordinate points, to be compared against the desired dose map forming the basis for the radiation treatment plan. While generally the dose over the volume of the phantom  10  as used to produce the radiation treatment plan will be slightly different from that computed from the phantom data, the similarities between these doses will be sufficient to allow for a simple quantitative assessment.  
         [0054]    When the radiation treatment plan is verified, then at process block  85 , the patient may be place in the radiation therapy machine and treated. Referring to FIG. 6, the location of the patient is facilitated by a bite block  88  that may be held within the patient&#39;s mouth having attached to it an optical target  76 ′ similar to the optical target  76  used on the phantom  10  thus providing a closed correlation between the phantom data and the patient treatment.  
         [0055]    The description has been that of a preferred embodiment of the present invention. It will occur to those that practice the art that many modifications may be made without departing from the spirit and scope of the invention. In order to apprise the public of the various embodiments that may fall within the scope of the invention, the following claims are made.