Abstract:
A stand-alone calculator enables multi energy electron beam treatments with standard single beam electron beam radiotherapy equipment thereby providing improved dose profiles. By employing user defined depth-dose profiles, the calculator may work with a wide variety of existing standard electron beam radiotherapy systems.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
     BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to electron beam radiation therapy (radiotherapy) treatment of tumors and the like and in particular to a treatment planning tool providing improved dose profile for electron beam radiation therapy machines.  
         [0002]     Radiotherapy treats tissue with high-energy radiation. The amount of radiation and its placement must be accurately controlled to ensure both that the tumor receives sufficient radiation to be destroyed and that the damage to the surrounding and adjacent non-tumorous tissue is minimized.  
         [0003]     External source radiotherapy may use high-energy radiation such as photons or electrons. In electron beam radiotherapy, a source of electrons, for example from a linear accelerator, may be directed toward the patient at a given angle and collimated to a given beam size (cone size). The energy of the electrons may be set, typically to one or more discrete energy levels of 4, 6, 9, 12, 15, 16, 18, 20 or 22 MeV. Typical cone sizes include 4×4, 6×6, 10×10, 15×15, 20×20, and 25×25 cm. The cumulative flux and hence the dose may be controlled by controlling the monitor units (“MU”) of the radiation therapy machine through direct control of the linear accelerator current and/or control of the exposure time.  
         [0004]     Electrons have a particular advantage in the treatment of some superficial cancers such as skin, breast, head and neck tumors, and intraoperative surgical procedures in that they provide rapid falloff as a function of penetration depth. Control of the depth of falloff of the electron beam may be provided by use of a customized bolus, being typically a water equivalent or tissue mimicking material placed on the patient&#39;s skin to provide some initial interaction with the electrons before the electrons reach the region targeted for treatment.  
         [0005]     Standard electron beam radiation therapy machines are essentially “single-beam” devices providing a single, essentially constant and uniform electron beam. More sophisticated “intensity modulated” radiation treatment machines and treatment planning systems have been proposed in which the electron beam is divided into “beamlets”, each separately modulated by intensity and/or energy. A complex inverse treatment planning technique, for example, a Monte Carlo based algorithm, would be used to optimize the fluence and energy of the many beamlets over multiple treatments. Such equipment and the necessary planning tools are not available in most clinical settings.  
       BRIEF SUMMARY OF THE INVENTION  
       [0006]     The present invention provides a treatment planning tool that allows the benefits of treatments using the multiple electron beam energies or combined electron/photon energies available on a standard, single beam radiotherapy system. The tool, which may operate on a stand-alone desktop computer, accepts a simple characterization of a desired beam depth dose profile and produces a multiple energy treatment plan that can be entered into a radiation therapy treatment planning system and implemented using successive exposures from the radiation therapy machine.  
         [0007]     Specifically, the present invention provides a treatment planning tool for use with a standard electron beam radiation therapy machine of a type which produces a spatially unmodulated electron beam at one of a predetermined set of electron energies. The treatment tool comprises a program executing on a computer to accept a dose description indicating a desired dose as a function of depth along a central axis of the beam, and to determine from that desired dose description, a set of sequential electron energy exposures using a different electron energy and/or bolus that satisfies the dose description. Photon beams may optionally be combined with electron beams. The tool then outputs the expected depth dose distribution exposure data and the method by which the depth dose distribution may be achieved for input into a radiation therapy treatment planning system for the ultimate setting of the electron beam radiation therapy machine.  
         [0008]     Thus it is one object of at least one embodiment of the invention to improve the depth-dose profiles obtainable on standard radiation therapy machines by assisting a radiation therapy treatment planning system in combining multiple electron beam energies.  
         [0009]     The program may output a graph showing a plot of a dose of the combined sequential electron energy exposures as a function of depth.  
         [0010]     It is thus another object of at least one embodiment of the invention to provide a simple and intuitive display of the results of the electron treatment plan that may be used to verify the suitability of the treatment plan or to allow one treatment plan of multiple different treatment plans to be selected by the user.  
         [0011]     The graph may also show a plot of dose as a function of depth for each electron energy alone.  
         [0012]     It is thus another object of at least one embodiment of the invention to provide a simple method of visually confirming the results of the electron treatment plan.  
         [0013]     The tool may determine multiple combinations of sequential electron energy exposures with different energies and the program may output graphs having plots showing a dose as a function of depth for each combination. The user may select from these combinations or the tool may select along the combinations at least one best satisfying the dose description.  
         [0014]     Thus it is an object of at least one embodiment of the invention to provide a comprehensive view of a variety of electron combination treatment options obtainable on a particular electron beam radiation therapy machine.  
         [0015]     The dose description may comprise a first skin dose value and a second tissue dose value at a predetermined location beneath the skin.  
         [0016]     It is thus another object of at least one embodiment of the invention to provide a simple description of the desired dose such as may be manually entered into a stand-alone computer running the present program.  
         [0017]     The dose description may include a homogeneity limit defining a maximum variation in specified dose over a depth range and/or a range defining a maximum dose at a range location.  
         [0018]     Thus it is another object of at least one embodiment of the invention to provide a simple method of characterizing a depth-dose curve as may be manually entered into a stand alone computer running the present program.  
         [0019]     The program may accept multiple files providing percent depth dose profiles for different electron or photon energies for a specific electron beam radiation therapy machine. These files may be used in determining the dose produced by the combined sequential electron or electron/photon energy exposures.  
         [0020]     Thus it is an object of at least one embodiment of the invention to provide a tool that may be used with a wide variety of standard electron beam radiation therapy machines as characterized by the files without the need for complex mathematical modeling or the like.  
         [0021]     A different file may be provided for each different cone size for the standard electron beam radiation therapy machine.  
         [0022]     Thus it is another object of at least one embodiment of the invention to capture changes in depth-dose profiles caused by changes in cone sizes.  
         [0023]     The exposures of the combined exposure may each employ a different bolus thickness.  
         [0024]     Thus it is another object of at least one embodiment of the invention to provide the ability to tailor both energy and bolus thickness to optimize the depth dose profiles.  
         [0025]     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  
       [0026]      FIG. 1  is a simplified cross-sectional representation of an electron beam radiation therapy machine treating a patient having a skin placed bolus;  
         [0027]      FIG. 2  is a percent depth dose profile of two energies of the electron beams and the resultant combined percent depth dose profile produced by the present invention;  
         [0028]      FIG. 3  is a block diagram of a standard desktop computer system suitable for implementing the present invention as a program;  
         [0029]      FIG. 4  is a flow chart showing the steps of the execution of the program of  FIG. 4  and implementing the present invention; and  
         [0030]      FIG. 5  is a simplified display produced on the computer system of  FIG. 3  as facilitates the entering and outputting of treatment data. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0031]     Referring now to  FIG. 1 , a standard electron beam radiation therapy machine  10  may include a linear accelerator  12  (not shown) producing an electron beam  14  centered along central beam axis  16  as collimated by cone  17  and directed toward a patient  18 . The electron beam  14  may pass through a bolus  20  placed on the patient&#39;s skin and then pass into a treatment region  22  of the patient. A target (not shown) can be placed in the electron beam  14  to allow for the production of x-ray photons.  
         [0032]     Referring now to  FIG. 2 , the electron beam  14  deposits a dose in the treatment region  22  described by a depth-dose profile  24  providing dose as a function of depth defined with respect to the patient&#39;s skin surface. Typically, the depth-dose profile  24  rises slightly to a peak and then falls off abruptly. The exact shape of the depth-dose profile  24  will vary (e.g. depth-dose profile  24 ′) based on the energy of the electron beam  14 , the size of the cone  17  and the model of linear accelerator  12 .  
         [0033]     The height of the depth-dose profile  24  and  24 ′ (thus the absolute dose) may be changed by controlling the monitor units (MU) of the exposure. The energy of the electron beam  14  and the MU of the exposure are controlled by settings on the radiation therapy machine  10  as is well understood in the art.  
         [0034]     The standard electron beam radiation therapy machine  10  is characterized by the fact that the electron beam  14  is relatively constant in profile, unmodulated over its area by multi-leaf collimators or the like.  
         [0035]     In the present invention, two or more electron beams  14  having different energies are used sequentially to produce two or more depth-dose profiles  24  and  24 ′ which combine to produce a composite depth-dose profile  30  with superior characteristics, for example, better homogeneity inside the treatment region  22  and/or improved fall off outside of the treatment region. Generally the composite depth-dose profile  30  will be a point-by-point summing of the values of two sequential depth-dose profiles  24  and  24 ′.  
         [0036]     Referring now to  FIG. 3 , the present invention provides a tool to generate settings for a standard electron beam radiation therapy machine  10  that will produce a composite depth-dose profile  30 . In the preferred embodiment, the tool uses a standard desktop computer system  32  having a graphic display terminal  34  including a display screen  36  and a data entry device  38  such as a keyboard and trackball or mouse. The terminal  34  may communicate with a processor unit  40 , the latter having an interface circuit  42  of a type well known in the art. The interface circuit  42  in turn, communicates with a processor  44  and memory  46 , the latter holding data files  56  and a program  50  reading the data files  56  and executing the method of the present invention. As will be understood in the art, the computer system  32  need not and, in the preferred embodiment, does not have direct electrical connection to the radiation therapy machine  10  of  FIG. 1 , but rather receives data solely through the data entry device  38  from a human user and displays data on the display screen  36  or through a printer (not shown) to a human user for manual setting of the standard electron beam radiation therapy machine  10 .  
         [0037]     Referring now to  FIG. 4 , the method of the present invention starts with the collection of data for the data files  56 . These data files  56  hold percent depth dose data (PDD) taken along the central axis  16  of the standard electron beam radiation therapy machine  10 . The PDD data is essentially a depth-dose profile  24  normalized to a standard MU value and is collected for each energy of electron beam  14 , each cone size, and each linear accelerator  12 . In an alternative embodiment, the PDD data for different cone sizes may be measured for each cone  17  and applied to collected PDD data for each energy. The PDD data can be obtained by measurements of the particular linear accelerator  12  using standard phantoms and calibration techniques well known in the art, and once obtained, may be imported by the program in memory  46  as shown in process block  54 .  
         [0038]     The PDD data provides dose values every millimeter and may be formatted and accepted by the program  50  in standard spread sheet formats for operator convenience.  
         [0039]     During the importation process of process block  54 , each file of PDD data is identified by cone size, beam energy, and a description of the particular linear accelerator by model number, serial number, and location so that this information can be read by the program  50  as well as the PPD data. This identification process is indicated by process block  60 . The entry identifying information about the PDD data may be facilitated by a graphical menu appearing on the display screen  36  and generated by the program  50  and prompting the user as necessary.  
         [0040]     After the collection of the PDD data, an electron treatment plan may be initiated. As indicated by process block  62  at the beginning of the treatment process, the user is prompted to enter descriptions of a desired depth dose profile. Referring again also to  FIG. 2 , this description does not require a full set of data points per the PDD but in the preferred embodiment is captured by a first dose value  61  at the skin and a second dose value  63  at a designated depth below the skin.  
         [0041]     At this time, the user also enters a range value  65  (in the form of a percent of first dose value  63 ) at a predetermined depth outside the treatment region  22 . The range value will be used to test for a desired fall off in the dose in the composite depth-dose profile  30  to be produced. Finally the user will enter a homogeneity value  67  being a percent deviation in dose between the skin and the location of second dose value  63  for the composite depth-dose profile  30 .  
         [0042]     Once this data is entered, and as indicated by process block  64 , the user may set two desired beam energies that will be considered in the treatment plan typically from a set of fixed energies provided by the standard electron beam radiation therapy machine  10 . The user may also enable the use of the boluses in developing a treatment plan by checking an appropriate menu check box on the display screen  36 .  
         [0043]     Once this data is collected, a treatment plan within these constraints is determined as indicated by process block  66 .  
         [0044]     Generally, the dose D(x) at any depth x in standard tissue of a patient at an electron beam energy E n  will be equal to:  
         [0045]     D n (x)=MU n PDD n (x) 
        where MU n  is monitor units of the electron beam  14 , PDD n  is percent depth dose along the central axis  16  of a specific electron energy, measured as described above. All calculations are done on the central axis  16  substantially simplifying the problem and the evaluation of the solutions.        
 
         [0047]     Accordingly the dose at a series of depth location points x 1 , x 2 , . . . , x m  defining a composite depth-dose profile  30  for the two or more selected beam energies will be defined by a series of equations as follows:  
                           D   ⁡     (     x   0     )       =         MU   1     ⁢       PDD   1     ⁡     (     x   0     )         +       MU   2     ⁢       PDD   2     ⁡     (     x   0     )       ⁢           ⁢   …   ⁢           ⁢     MU   n     ⁢       PDD   n     ⁡     (     x   0     )                         D   ⁡     (     x   1     )       =         MU   1     ⁢       PDD   1     ⁡     (     x   1     )         +       MU   2     ⁢       PDD   2     ⁡     (     x   1     )       ⁢           ⁢   …   ⁢           ⁢     MU   n     ⁢       PDD   n     ⁡     (     x   1     )                             ⋮   ⁢                               D   ⁡     (     x   m     )       =         MU   1     ⁢       PDD   1     ⁡     (     x   m     )         +       MU   2     ⁢       PDD   2     ⁡     (     x   m     )       ⁢           ⁢   …   ⁢           ⁢     MU   n     ⁢       PDD   n     ⁡     (     x   m     )                     
 
         [0048]     As will be understood to one of ordinary skill in the art, these equations may be generalized for treatment at more than one energy in an alternative embodiment of the invention.  
         [0049]     Dose values  61  and  63  are substituted into the appropriate D(x) values and these equations are solved by standard matrix algebra techniques to yield a set of solutions providing MU values for the two or more electron beams  14 . This set of solutions is tested against the range and homogeneity values previously input.  
         [0050]     If the use of boluses has been enabled as described above, this calculation is repeated for each of a set of different bolus thicknesses by modifying the above equations as follows:  
                           D   ⁡     (     x   0     )       =         MU   1     ⁢       PDD   1     ⁡     (       x   0     +   δ     )         +       MU   2     ⁢       PDD   2     ⁡     (       x   0     +   δ     )       ⁢           ⁢   …   ⁢           ⁢     MU   n     ⁢       PDD   n     ⁡     (       x   0     +   δ     )                         D   ⁡     (     x   1     )       =         MU   1     ⁢       PDD   1     ⁡     (       x   1     +   δ     )         +       MU   2     ⁢       PDD   2     ⁡     (       x   1     +   δ     )       ⁢           ⁢   …   ⁢           ⁢     MU   n     ⁢       PDD   n     ⁡     (       x   1     +   δ     )                             ⋮   ⁢                               D   ⁡     (     x   m     )       =         MU   1     ⁢       PDD   1     ⁡     (       x   m     +   δ     )         +       MU   2     ⁢       PDD   2     ⁡     (       x   m     +   δ     )       ⁢           ⁢   …   ⁢           ⁢     MU   n     ⁢       PDD   n     ⁡     (       x   m     +   δ     )                     
        where δ is an effective offset in depth created by the bolus thickness. For simplicity, only bolus thicknesses differing by at least one millimeter, are considered and a predetermined range of bolus thickness ranges are determined (by a look up table) integrated into the program  50  tailored to each energy level as will be understood to those of ordinary skill in the art. Generally lower electron beam energies will have smaller maximum bolus sizes. Thus, for example, at 4 MeV a maximum bolus of 16 millimeters may be provided in this table, whereas at 22 MeV a maximum bolus size of 105 millimeters will be considered.        
 
         [0052]     The solutions obtained using boluses are added to the solution set to be tested against the requirements of range and homogeneity. Generally, when a bolus is used, a solution may include two different beam energies that pass through different thicknesses of boluses, or a solution may include two beam energies that are the same but that pass though different thicknesses of boluses, or a bolus may be used with only one beam and the second beam may have no bolus, or two different energies may be used with boluses of the same thickness.  
         [0053]     In the event that there is no solution, the user is informed of such and prompted to consider relaxing the homogeneity and/or range requirements.  
         [0054]     More typically referring to  FIG. 5 , when multiple solutions are obtained, the program may select one solution according to predetermined criteria or, in the preferred embodiment, a predetermined number of these solutions may be presented on the display screen  36  by plots of the composite depth-dose profiles  30  corresponding to the solutions. Each composite depth-dose profile  30  is of a different color and associated with a tab  70  of corresponding color. When a given tab  70  is pressed, the graphic display presents detailed data associated with that solution.  
         [0055]     Referring to  FIGS. 2 and 5 , the data available upon pressing a tab  70  will include a depiction of normalized PDD depth-dose profile  24  and  24 ′ associated with the user selected beam energies and a depiction of the composite depth-dose profile  30  associated with that tab  70 . The user may quickly verify the plausibility of the solution and may see the extent to which it improves upon either of the two beam energies used individually. Note that generally the composite depth-dose profile  30  will be in absolute dose values whereas the depth-dose profile  24  and  24 ′ will be in percent dose values.  
         [0056]     Pressing a tab  70  also provides detailed numeric information about the selected choice including relative monitor units needed for each exposure with the different beams in text box  72 . These monitor units may be as an input to a radiation therapy treatment planning system prior to being used to manually set the standard electron beam radiation therapy machine  10  to implement the composite depth-dose profile  30 .  
         [0057]     The display screen  36  also provides numeric readings at text box  76  giving the total dose contribution provided at each electron beam  14  at each energy level, tabular numeric values of the composite depth-dose profile  30  indicated by text box  80  and a summary  82  of the values that will then be put into a radiation therapy treatment planning system, providing a description of the cone size, the particular energies selected and the other information entered at process block  60  and  62 .  
         [0058]     Referring now to  FIG. 4 , once the desired composite depth-dose profile  30  is selected as indicated by process block  84 , the screen and output data may be printed to provide a permanent record of the electron solutions treatment plan.  
         [0059]     Typically, the output data will be input to treatment planning software that may model a dose in three dimensions using the beam energies selected. Such software may be part of a treatment planning system, for example, that is normally used for inverse treatment planning. After the modeled dose is checked and possibly refined, the output data is used to control the radiation therapy machine.  
         [0060]     As will be understood from the above description, the invention may be readily extended to combinations of electron beams and one or more photon beams simply by preparing the necessary PDD files for the photon beams and allowing the program to consider combinations of electron and photon beams with different depth dose profiles. Photon beams will not typically use boluses.  
         [0061]     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.