Abstract:
A system for estimating a dose from a proton therapy plan includes a memory that stores machine instructions and a processor coupled to the memory that executes the machine instructions to subdivide a representation of a volume of interest in a patient anatomy traversed by a planned proton field into a plurality of voxels. The processor further executes the machine instructions to determine the distance from the source of the planned proton beam to one of the voxels. The processor also executes the machine instructions to compute the discrete contribution at the voxel to an estimated dose received by the volume of interest from the planned proton beam based on the distance between the source and the volume of interest.

Description:
TECHNICAL FIELD 
       [0001]    This description relates generally to radiation therapy, and more particularly to proton beam therapy treatment plan development. 
       BACKGROUND 
       [0002]    Radiation therapy treatment plan development generally employs medical imaging, such as X-ray, computed tomography (CT), magnetic resonance imaging (MRI), or the like. Typically, a series of two-dimensional patient images, each representing a two-dimensional cross-sectional “slice” of the patient anatomy, are used to reconstruct a three-dimensional representation of a volume of interest (VOI), or structure of interest, from the patient anatomy. 
         [0003]    The VOI typically includes one or more organs of interest, often including a planning target volume (PTV), such as a malignant growth or an organ including malignant tissue targeted for radiation therapy; a relatively healthy organ at risk (OAR) in the vicinity of a malignant growth at risk of radiation therapy exposure; or a larger portion of the patient anatomy that includes a combination of one or more PTVs along with one or more OARs. The objective of the radiation therapy treatment plan development typically aims to irradiate as much of the PTV as near the prescription dose as possible, while attempting to minimize irradiation of nearby OARs. 
         [0004]    The resulting radiation therapy treatment plans are used during medical procedures to selectively expose precise areas of the body, such as malignant tumors, to specific doses of radiation in order to destroy the undesirable tissues. During the development of a patient-specific radiation therapy treatment plan, information generally is extracted from the three-dimensional model to determine parameters such as the shape, volume, location, and orientation of one or more PTVs along with one or more OARs. 
         [0005]    Proton beam therapy permits relatively fine control regarding the three-dimensional concentration of energy release from the proton beam within the patient. In particular, the depth dose curve of proton beams has unique characteristics with regard to other types of radiation, such as electron, X-ray or photon beams. As a result, existing treatment plan development methodologies, such as those based on typical photon depth dose curves, may not accurately predict the results that may be achieved with proton beam therapy. 
       SUMMARY 
       [0006]    According to one embodiment of the present invention, a system for estimating a dose from a proton therapy plan includes a memory that stores machine instructions and a processor coupled to the memory that executes the machine instructions to subdivide a representation of a volume of interest into a first plurality of voxels. The processor further executes the machine instructions to determine a first distance associated with a first planned proton field and a first voxel of the first plurality of voxels. The processor also executes the machine instructions to compute a metric based on the first distance and a second distance associated with a target volume traversed by the first planned proton field. 
         [0007]    According to another embodiment of the present invention, a computer-implemented method of estimating a dose from a proton therapy plan includes subdividing a representation of a volume of interest into a first plurality of voxels, and determining a first distance associated with a first planned proton field and a first voxel of the first plurality of voxels. The method further includes computing a metric based on the first distance and a second distance associated with a target volume traversed by the first planned proton field. 
         [0008]    According to yet another embodiment of the present invention, a computer program product for estimating a dose from a proton therapy plan includes a non-transitory, computer-readable storage medium encoded with instructions adapted to be executed by a processor to implement subdividing a representation of a volume of interest into a first plurality of voxels. The instructions are further adapted to implement determining a first distance associated with a first planned proton field and a first voxel of the first plurality of voxels. The instructions are also adapted to implement computing a metric based on the first distance and a second distance associated with a target volume traversed by the first planned proton field. The metric represents a discrete contribution at the first voxel to an estimated dose received by the volume of interest. 
         [0009]    The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a block diagram illustrating an exemplary proton beam dose distribution planning tool that employs a proton geometry-expected-dose to develop and evaluate patient-specific proton therapy treatment plans in accordance with an embodiment of the present invention. 
           [0011]      FIG. 2  is an illustration of an exemplary radiation therapy patient anatomy that can be evaluated using the proton beam dose distribution planning tool of  FIG. 1 . 
           [0012]      FIG. 3  is graph depicting exemplary proton beam depth dose curve that can be implemented using the proton beam dose distribution planning tool of  FIG. 1 . 
           [0013]      FIG. 4  is a schematic view depicting an exemplary general computing system that can implement the proton beam dose distribution planning tool of  FIG. 1 . 
           [0014]      FIG. 5  is a flowchart representing an exemplary method of estimating the dose distribution of a proton beam in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    An embodiment of the present invention is shown in  FIG. 1 , which illustrates an exemplary proton beam dose distribution planning tool  10  that employs a proton beam geometry-expected-dose (GED) to develop and evaluate patient-specific proton therapy treatment plans. The proton beam dose distribution planning tool  10  predicts the dose received by a volume of interest, such as a target volume, during a proton beam therapy session. The proton beam dose distribution planning tool  10  predicts the three-dimensional dose distribution for a new patient based on the patient anatomy and the proton field geometry. 
         [0016]    The proton beam dose distribution planning tool  10  includes a patient geometry parametrization module  12 , a dose prediction module  14 , a knowledge assimilation module  16  and a knowledge base  18 . The patient geometry parametrization module  12  generates a set of parameters, or metrics, based on the individual patient anatomic geometry with respect to various factors that affect proton beam dose distribution. Various patient geometric features can affect proton beam dose distribution. Metrics that take into account these geometric features offer relatively improved correlation between predicted proton beam dose distribution and structure positions in patient geometry. 
         [0017]    Referring to  FIG. 2 , an illustration is shown of an exemplary radiation therapy patient anatomy  20  that can be evaluated using the proton beam dose distribution planning tool  10  of  FIG. 1 . A proton source  22  emits a proton beam or field  24  toward a target volume  26  in the patient anatomy  20 . The proton field  24  enters the proximal boundary  28  of the target volume  26  at distance T s  from the proton source  22  and reaches the distal boundary  30  of the target volume  26  at distance T e  from the proton source  22 . The proton field  24  travels distance Δ through the target volume  26 . 
         [0018]    The patient geometry parametrization module  12  of  FIG. 1  determines a set of parameters, or metrics, to represent the patient anatomy  20  of  FIG. 2 . In an embodiment, the patient anatomy  20  may be subdivided into a number of individual volume partitions, or voxels, such as exemplary voxel  32 , that are individually evaluated with regard to dose distribution. Metrics may be assigned to each voxel based, for example, on the distance from each field origin to the voxel, the prescription dose of the target volume  26 , and field parameters based on the planned proton field. The metrics may take into account the field position and orientation. 
         [0019]    Examples of expected dose metrics in addition to organ at risk (OAR)-target proximity include but are not limited to the volume of the target structure, the volume of the OAR, any portion of the OAR that is not located within the field of the proton beam, the number and orientation of applied proton beam fields, field geometry, target and OAR tissue densities, the prescription dose, and the like. For example, various metrics can take into account the number of fields that converge at each point in the patient geometry, or any organ passed through by a field before reaching the target volume. Additional metrics can account for tissue characteristics, for example, the Hounsfield unit (HU) scale can represent energy deposition and dispersion characteristics. 
         [0020]    For example, the dose attenuation, or fall-off, profile is not symmetric, but rather, drops off much more rapidly as location extends beyond the vicinity of the target volume toward an out-of-field region. In addition, application of multiple fields with differing target dose levels at varying distances from a point in an OAR further complicate the determination of an equivalent distance using combined metrics. 
         [0021]    An embodiment of the present invention generates metrics with respect to the target volume and OARs based on a geometrically-expectable dose (GED) distribution. GED metrics incorporate general assumptions regarding how a clinical dose delivery is organized with respect to the target shape. GED metrics also take into account the geometry of the field setup. The dose at any point in an OAR is equal to the sum of the individual contributory doses at that location, or voxel, due to each of the applied target-level fields. 
         [0022]    In an embodiment, OAR geometry is included in GED metrics, for example, the number of voxels in OARs that are crossed by a field before reaching the target volume. In another embodiment, conformal dose metrics are employed, including descriptive setup and patient geometry factors. The beam orientation module  14  evaluates the patient geometry metrics and determines preferred beam geometry in the form of one or more beam orientations that meet the constraints for the target volume and OARs. 
         [0023]    In an embodiment, the field intensity is modulated to tailor the dose distribution to the specific target form. Target anatomical features, including, for example, the shape, elongation, and position of the target with respect to multiple fields, preferred directions, or beam orientations, are defined. Thus, for example, fields that are perpendicular to the target main direction are allowed to deliver less energy than fields that are parallel to the target main direction. 
         [0024]    Further, in an embodiment, the energy level delivered across a field is modulated with respect to GED metrics assigned to each beamlet within a field. For example, metrics can be defined regarding the number of voxels the beamlet crosses, or the distance the beamlet travels through the patient before reaching the target volume, and the intensity of the beamlet can be adjusted according to the metrics. 
         [0025]    The dose prediction module  14  evaluates the proton beam dose distribution with respect to a specific set of metrics and a specific grouping of proton beam orientations. The dose prediction module  14  employs a knowledge-based proton dose prediction algorithm that predicts the viable proton beam dose distribution on a structure of interest based on a set of anatomical features. The proton dose prediction algorithm estimates the quality of treatment plan achieved based on detailed planning for specific target geometries and field geometries. 
         [0026]    In particular, the dose prediction module  14  creates and applies proton-specific dose-volume histogram (DVH) estimation models. The dose prediction module  14  takes into account the characteristics of the proton beam in order to evaluate the field geometry. Referring to  FIG. 3 , a graph  40  depicts an exemplary proton beam depth dose curve  42  that can be used by the dose prediction module  14  to create a DVH estimation model. In an embodiment, the depth dose curve  42  represents the approximate dose distribution under the assumption of uniform target dose coverage from each beam. 
         [0027]    According to the depth dose curve  42 , the amount of energy released by the proton beam along its path increases exponentially from the proton beam source  44  to the point where the proton beam enters the target volume, T s ,  46 . In addition, according to the dose curve  42 , the dose delivered by the proton beam traveling distance, Δ,  48  through the target volume is uniform from the point at which the proton beam enters the target volume, T s ,  46  to the point where the proton beam reaches the far extreme of the target volume, T e ,  50 . The asymptotic value can be assumed to depend on the distance, Δ,  48  the ray travels inside the target volume. Further, according to the dose curve  42 , no energy is released by the proton beam beyond the far extreme of the target volume, T e ,  50 . 
         [0028]    For example, in an embodiment, the dose prediction module  14  applies the following definition of a proton beam geometry-expected-dose (GED) metric representing the dose contribution from a proton beam with respect to a location, or voxel: 
         [0000]    
       
         
           
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         [0029]    where G x is the single proton field contribution to the geometry-expected dose, x is the distance from the proton beam source, the target volume starts at distance T s  and ends at distance T e , Δ is the distance the ray travels in the target volume, d is the prescription dose level from the specified field to the target, and parameters a and b are phenomenological coefficients. In an embodiment, distances are measured in units that correspond to the energy release of a proton beam far before the Brachy peak. 
         [0030]    The coefficients, a and b, are exponential parameters that depend on the selected dimensions of the proton therapy setup. In an embodiment, the coefficients, a and b, may be determined using statistical analysis of patient geometry and dose information from historical proton beam treatment plans. For example, depending on the selected dimensions, the value of the coefficient a may be approximately in the range of 0.4 to 0.8, and the value of the coefficient b may be approximately in the range of 2.75 to 12. In one embodiment the empirically determined coefficients, a=0.6, and b=5, have produced useful predictions. 
         [0031]    In an embodiment, the dose prediction module  14  permits interactive definition and fine-tuning of the target volume to be treated while providing an immediate estimate of the achievable plan quality. The quality can be described, for example, by predicting the dose-volume histograms (DVHs) that would be achieved for a specific target volume. The dose prediction module  14  can facilitate decisions, for example, regarding the clinical tradeoff between the size of spatial region to be irradiated and sparing of critical organs. 
         [0032]    In another embodiment, the dose prediction module  14  permits interactive definition and modification of the proton beam field geometry while providing an immediate estimate of the achievable plan quality. Thus, independent dose optimization would not be required for each candidate target volume and field geometry. Further, in an embodiment, the proton beam dose distribution is evaluated with respect to multiple target volumes, for example, adding weighted contributions, or fractionations, corresponding to the various modified distances from an OAR to multiple target volumes with different dose levels. 
         [0033]    In various embodiments, the GED metrics are used in dose-volume histogram (DVH) estimation, fluence estimation, or three-dimensional dose estimation. In an embodiment, estimated dose distributions based on GED metrics are compared with corresponding proton beam dose distributions actually achieved in clinical treatments to tune an actual knowledge model. 
         [0034]    The knowledge assimilation module  16  extracts major dosimetric features from existing proton therapy datasets representing the actual historical patient population. In knowledge-based dose prediction, information gleaned from actual historical plans is used to estimate the achievable proton dose distribution regarding a new patient. For example, patient geometry and dose information of multiple historical proton beam treatment plans is distilled into a prediction model that can be used for proton beam therapy dose prediction without storing all of the information from the original set of plans. 
         [0035]    The knowledge base  18  stores the existing proton therapy datasets representing a historical population of actual patient anatomical and achieved dose information. Thus, the proton beam dose distribution planning tool  10  can create proton-specific DVH estimation models based on a set of existing proton plans. The systems described herein can offer advantages such as evaluating plans with different proton field geometries, evaluating plans with multiple target volumes with differing dose levels, and analyzing the effect of target volume shape on dose distribution. 
         [0036]    An embodiment of the present invention correlates achieved dose distributions in actual historical patient proton therapy treatment plans to specific underlying patient anatomy and stores the resultant correlation information in the knowledge base  18 . The correlation information in the knowledge base  18  is accessed in order to predict three-dimensional dose distribution for new patients based on patient anatomy. Thus, the proton beam dose distribution planning tool  10  correlates proton beam dose distributions achieved in actual patients in radiotherapy treatments with the underlying patient anatomy, or geometry. 
         [0037]    As illustrated in  FIG. 4 , an exemplary general computing device  52  that can be employed in the proton beam dose distribution planning tool  10  of  FIG. 1  includes a processor  54 , a memory  56 , an input/output device (I/O)  58  storage  60  and a network interface  62 . The various components of the computing device  52  are coupled by a local data link  64 , which in various embodiments incorporates, for example, an address bus, a data bus, a serial bus, a parallel bus, or any combination of these. 
         [0038]    The computing device  52  communicates information to and requests input from the user or other devices by way of the I/O  58 , which in various embodiments incorporates, for example, an interactive, menu-driven, visual display-based user interface, or graphical user interface (GUI), a pointing device, such as a, with which the user may interactively input information using direct manipulation of the GUI. 
         [0039]    The computing device  52  can be coupled to a communication network by way of the network interface  62 , which in various embodiments incorporates, for example, any combination of devices—as well as any associated software or firmware—configured to couple processor-based systems, including modems, access points, network interface cards, LAN or WAN interfaces, wireless or optical interfaces and the like, along with any associated transmission protocols, as may be desired or required by the design. 
         [0040]    The computing device  52  can be used, for example, to implement the functions of the components of the proton beam dose distribution planning tool  10  of  FIG. 1 . In various embodiments, the computing device  52  can include, for example, a server, a controller, a workstation, a mainframe computer, personal computer (PC), a note pad, a computing tablet, a personal digital assistant (PDA), a smart phone, a wearable device, or the like. Programming code, such as source code, object code or executable code, stored on a computer-readable medium, such as the storage  60  or a peripheral storage component coupled to the computing device  52 , can be loaded into the memory  56  and executed by the processor  54  in order to perform the functions of the proton beam dose distribution planning tool  10 . 
         [0041]    Referring now to  FIG. 5 , an exemplary process flow is illustrated that may be performed, for example, by the proton beam dose distribution planning tool  10  of  FIG. 1  to implement an embodiment of the method described in this disclosure for employing a proton beam geometry-expected-dose (GED) to develop and evaluate patient-specific proton therapy treatment plans. The process begins at block  70 , where a volume of interest, such as a target volume or an organ at risk (OAR), is subdivided into a group of voxels, for example, equal-sized, three-dimensional units, as described above. 
         [0042]    As further described above, in block  72 , the distance from the planned proton beam field origin to each voxel is determined. The prescription dose for the target is ascertained, in block  74 , and the value of one or more phenomenological coefficients that depend on the selected dimensions of the proton therapy setup are ascertained, in block  76 . 
         [0043]    In block  78 , as explained above, geometry-based expected dose (GED) metrics are computed for all voxels with respect to each planned proton field. In block  80 , the GED metrics are summed for all voxels in the volume of interest with respect to each field. The summation of the GED metrics provides an estimation of the total dose received by the volume of interest in the planned proton beam therapy. 
         [0044]    Aspects of this disclosure are described herein with reference to flowchart illustrations or block diagrams, in which each block or any combination of blocks can be implemented by computer program instructions. The instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to effectuate a machine or article of manufacture, and when executed by the processor the instructions create means for implementing the functions, acts or events specified in each block or combination of blocks in the diagrams. 
         [0045]    In this regard, each block in the flowchart or block diagrams may correspond to a module, segment, or portion of code that includes one or more executable instructions for implementing the specified logical function(s). It should also be noted that, in some alternative implementations, the functionality associated with any block may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or blocks may sometimes be executed in reverse order. 
         [0046]    A person of ordinary skill in the art will appreciate that aspects of this disclosure may be embodied as a device, system, method or computer program product. Accordingly, aspects of this disclosure, generally referred to herein as circuits, modules, components or systems, or the like, may be embodied in hardware, in software (including firmware, resident software, micro-code, etc.), or in any combination of software and hardware, including computer program products embodied in a computer-readable medium having computer-readable program code embodied thereon. 
         [0047]    It will be understood that various modifications may be made. For example, useful results still could be achieved if steps of the disclosed techniques were performed in a different order, and/or if components in the disclosed systems were combined in a different manner and/or replaced or supplemented by other components. Accordingly, other implementations are within the scope of the following claims.