a modular software facility for performing patient localization and / or location tracking during radiation therapy (“ the facility ”) is described . in some embodiments , the facility employs a set of distinct software components that interact in order to collectively perform the tracking function . in some embodiments , these components are substitutable software elements , each performing a different role and providing one or more functions . components typically isolate dependencies of various types to a minimum number of components , abstracting their detailed operation away from their higher level function . components typically communicate in well - defined ways , including via a publish and subscribe mechanism . in some embodiments , the components communicate using standardized mechanism , such as a publish - and - subscribe mechanism . in these embodiments , the dependencies and interactions between components are defined based upon what information each component provides (“ publishes ”), and what information each component consumes (“ subscribes to ”). by limiting the dependencies that components may have on one another , this design minimizes the risk that a re - implemented component will fail to interact properly with other existing components . for example , this design may be used to isolate dependencies on the particular technology used to locate the tumor to a small subset of the components , so that the remaining components are free of dependencies on the technology used . if the technology used is subsequently changed , the software facility could be adapted to the new technology by re - implementing only the subset of components . the use of a publish - and - subscribe mechanism enables the set of components consuming particular information from a particular publisher component to expand or contract without any special processing by the publisher component . in some embodiments , the facility utilizes a publish - and - subscribe mechanism that supports location transparency , such as publish - and - subscribe using net remoting . in these embodiments , many of the components can be executed on any of a set of connected computer systems , providing a measure of flexibility in the deployment of the facility . in some embodiments , an update to the facility is performed by replacing a proper subset of the facility &# 39 ; s components with re - implemented versions of these components , without re - implementing the remaining components . in some embodiments , the facility may provide a number of different medical applications , such as patient tracking during radiation therapy for prostate cancer , lung cancer , breast cancer , cancer of the head and neck , liver cancer , pancreatic cancer , cervical cancer , orthopedic cancer , etc . in some such embodiments , each medical application is constituted of a particular set of components . for example , a particular medical application could add a new component not present in other medical applications , or replace a component present in other medical applications with a new implementation of the component . a particular medical application may also include additional hardware to be connected to the computing system . in these embodiments , the facility provides particular medical application by invoking the particular components used by the medical application . in such embodiments , the different applications may be distributed to customers separately . as one example , a new application may be distributed to a customer by providing a customer with a copy of each module . in some embodiments , the facility may provide a number of different activities relating to a localization and tracking system , including patient set up , patient treatment , patient biofeedback monitoring , patient treatment rehearsal , retrospective treatment data review , treatment data reporting , etc . in various embodiments , these activities are performed as described in u . s . patent application ser . no . 11 / 189 , 542 filed concurrently herewith and entitled user interface for guided radiation therapy , which is hereby incorporated by reference in its entirety . in some embodiments , each activity is constituted of a particular set of components . in such embodiments , the different activities may be distributed to customers separately . in some embodiments , the facility may enable the tracking system to interact with a number of add - on functionalities , such as an accessory for positioning a couch or other structure supporting the patient ; an accessory for controlling the intensity of the radiation beam ; an accessory for controlling the shape of the radiation beam ; an accessory for controlling devices securing the treatment room ; an accessory for collecting biofeedback input , such as pulse or respiration ; an accessory for performing pre - treatment or intra - treatment imaging ; an accessory for supporting the correlation of time - index data for multiple sources ; a quality assurance package for one or more accessories , etc . in some such embodiments , each such add - on functionality is constituted of a particular set of components . in some embodiments , the different add - on functionalities may be distributed to customers separately . in some embodiments , the facility is directed towards tracking a target , i . e ., measuring the position and / or the rotation of a target in substantially real time , in a patient in medical applications . in some such embodiments , the facility collects position data of a marker that is substantially fixed relative to the target , determines the location of the marker in an external reference frame ( i . e ., a reference frame outside the patient ), and provides an objective output in the external reference frame that is responsive to the location of the marker . the objective output is repeatedly provided at a frequency / periodicity that adequately tracks the location of the target in real time within a clinically acceptable tracking error range . as such , such tracking enables accurate tracking of the target during diagnostic , planning , treatment or other types of medical procedures . in many specific applications , the objective output is provided within a suitably short latency after collecting the position data and at a sufficiently high frequency to use the data for such medical procedures . in some or all of the above - described embodiments , the facility provides significant flexibility and adaptability to a system for tracking patient position for radiation therapy . fig1 is a network diagram showing computer systems and other hardware devices used by the facility . a number of the computer systems and devices are connected by one or more data networks 110 . these include a magnetic localizer front end 101 for controlling a magnetic localizer device 102 . a tracking station 111 combines information from the magnetic localizer front end , as well as one or more camera , such as cameras 104 - 106 . the tracking station also provides a visual user interface . a console pc 103 provides an additional visual user interface . computer systems and devices 101 - 106 are typically located inside a radiation treatment vault 100 , while the tracking station 111 is located outside . those skilled in the art will appreciate that the facility may use sets of devices that differ from those shown here , including sets of devices that omit devices shown here , those that include devices not shown here , and / or those that combine or divide the functionality of devices shown here . one or more suitable , exemplary patient localization systems are described in the following , each of which is hereby incorporated by reference in its entirety : u . s . patent application ser . no . 10 / 334 , 700 , entitled panel - type sensor / source array assembly , filed dec . 30 , 2002 ; u . s . patent application ser . no . 09 / 877 , 498 , entitled guided radiation therapy system , filed jun . 8 , 2001 ; u . s . patent application ser . no . 10 / 679 , 801 , entitled method and system for marker localization , filed oct . 6 , 2003 ; u . s . patent application ser . no . 10 / 746 , 888 , entitled implantable marker with wireless signal transmittal , filed dec . 24 , 2003 ; and u . s . patent application ser . no . 10 / 749 , 478 , entitled receiver used in marker localization sensing system , filed dec . 31 , 2003 . fig2 is a block diagram showing some of the components typically incorporated in at least some of the computer systems and other devices on which the facility executes . these computer systems and devices 200 may include one or more central processing units (“ cpus ”) 201 for executing computer programs ; a computer memory 202 for storing programs and data — including data structures — while they are being used ; a persistent storage device 203 , such as a hard drive , for persistently storing programs and data ; a computer - readable media drive 204 , such as a cd - rom drive , for reading programs and data stored on a computer - readable medium ; and a network connection 205 for connecting the computer system to other computer systems , such as via the internet , to exchange programs and / or data — including data structures . while computer systems configured as described above are typically used to support the operation of the facility , one of ordinary skill in the art will appreciate that the facility may be implemented using devices of various types and configurations , and having various components . fig3 is a component diagram showing a typical set of components used by the facility to provide a medical application for tracking patient location during radiation treatment delivered to a prostate tumor . those skilled in the art will appreciate that the facility could provide this application using a different set of components , such as a set of components that includes components not shown , a set of components that omits shown components , and / or a set of components in which the functionality of components is consolidated or divided . fig3 shows components 301 - 308 , as well as dependencies between these components . a guidance system controller component 301 is the top - level component for the facility . the guidance system controller component acts as a registry for the various services , and represents the entire facility . a target tracker component 302 tracks the target isocenter of the patient by integrating the location of a sensor array provided by an array tracker component 303 with magnetic transponder locations provided by a front - end controller component 304 . target isocenter tracking information may be used in a variety of ways , such as those described in u . s . patent application no . 60 / 590 , 693 filed jul . 23 , 2004 , u . s . patent application ser . no . 11 / 190 , 205 filed concurrently herewith and entitled data processing for real - time tracking of a target in radiation therapy , u . s . patent application no . 60 / 590 , 503 filed jul . 23 , 2004 , and u . s . patent application ser . no . 11 / 189 , 431 , filed concurrently herewith and entitled dynamic / adaptive treatment planning for radiation therapy , each of which is hereby incorporated by reference in its entirety . the target tracker component outputs the location of the treatment isocenter relative to the machine isocenter , and publishes transponder locations and sensor array locations in the same coordinate system . the array tracker component 303 controls and provides access to cameras used to track the location of the sensor array , as well as a hub , and a vision tracking library used to analyze the input from the cameras . the front - end controller component 304 controls and provides information from a front - end microcontroller program component 305 . the front - end controller component may include and / or use a front - end interface and a magnetic localization algorithm library . the front - end controller component provides services , including logging , to the front - end microcontroller program component . the front - end controller component provides transponder locations relative to the sensor array to various subscribers . the front - end microcontroller program component drives a sensor array to acquire raw magnetic transponder data . a system database component 306 stores and provides access to system data such as patient records . a tracking station ui component 307 provides a graphical user interface providing capabilities such as patient data access , treatment room monitoring , and reporting . console ui component 308 provides a graphical user interface for users in the radiation treatment vault , including patient session management , array positioning , tracking , quality assurance testing , and calibration . sample behaviors provide by such ui components are described in u . s . patent application no . 60 / 590 , 699 filed jul . 23 , 2004 entitled user interface for guided radiation therapy , and u . s . patent application ser . no . 11 / 189 , 542 , filed concurrently herewith and entitled user interface for guided radiation therapy , each of which is incorporated herein by reference in its entirety . component 305 typically executes on the magnetic localizer front end component 313 , typically located inside the radiation treatment vault . component 308 typically executes on the console pc 312 , typically located inside the radiation treatment vault . components 301 , 302 , 303 , 304 , 306 , and 307 typically execute on the tracking station 311 , typically located outside the radiation treatment vault . the direction ( s ) of the arrows between pairs of components indicate the dependency relationship ( s ) between the components . for example , the double - headed arrow between components 301 and 308 indicates both that component 301 has a dependency on component 308 and component 308 has a dependency on component 301 . the single - headed arrow between components 301 and 307 indicates that component 307 has a dependency on component 301 . the design for some embodiments omits various inter - component dependencies shown in fig3 , such as the dependency of component 301 on components 303 and 304 . in some embodiments , dependencies between components indicate a publish / subscribe relationship between the components . for instance , the dependency of component 307 on component 301 may indicate that component 307 subscribes to data published by component 301 . in some embodiments , these dependencies are also used as a basis for module invocation , as is discussed in greater detail below . fig3 further identifies a version number of each component , shown in a circle inside the component &# 39 ; s rectangle . for example , fig3 shows that the guidance system controller component 301 is of version 1 . 1 . fig4 is a flow diagram showing steps typically performed by the facility in order to launch a particular application . in step 401 , the facility displays a list of applications obtained from a manifest data structure , discussed below in conjunction with fig7 and 8 . in step 402 , the facility receives a user selection of one of the applications displayed in step 401 . in step 403 , if the user has purchased any license keys needed for the application , then the facility continues in step 404 , else the facility continues in step 401 . in some embodiments , the facility omits step 403 , and does not enforce a license key requirement in order to launch an application . in steps 404 - 406 , the facility loops through each component listed in the manifest for the selected application . in step 405 , the facility invokes the current component , such as is described below in connection with fig5 . in step 406 , if additional components remain to be processed , then the facility continues step 404 to process the next component , else these steps conclude . fig5 is a flow diagram showing steps typically performed by the facility in order to invoke an individual component . in step 501 , the facility identifies other components upon which the present component is dependent . in steps 502 - 504 , the facility loops through each identified component . in step 503 , the facility invokes the identified component . in step 504 , if additional identified components remain to be processed , then the facility continues to step 502 to process the next identified component , else these steps conclude . fig6 is a component diagram showing a set of components typically used by the facility to provide a medical application for tracking patient location during radiation treatment of a lung tumor . by comparing fig6 with fig3 , one can discern that there is intersection between components used to provide the prostate application and components used to provide the lung application . the differences are as follows : array tracker component 303 shown in fig3 is omitted from fig6 ; guidance system controller component 601 shown in fig1 has version number 1 . 2 as opposed to version number 1 . 1 of guidance system controller component 301 shown in fig3 ; the tracking station ui component 607 shown in fig6 is of version 2 . 0 , whereas the tracking station ui component 307 shown in fig3 is of version 1 . 1 ; the console ui component 608 shown in fig6 is of version 2 . 0 , whereas the console ui component 308 shown in fig3 is of version 1 . 1 ; and an additional respiration monitor component 609 , executing on a new respiration station computer station 614 , has been added . fig7 is a table diagram showing sample contents of a component manifest data structure used in a first embodiment of the facility . component manifest 700 is a table made up of rows 701 - 716 , each corresponding to a component employed in a particular application . in particular , component manifest 700 contains a row for every component used in each application . each row is divided into an application column 751 identifying the application , a component column 752 identifying the component , and a version number column 753 identifying the version . for example , row 701 indicates that the prostate application uses version 1 . 1 of the guidance system controller component . in using component manifest 700 , the facility would invoke the versions of the components shown in row 701 - 708 directly as part of invoking the prostate application . fig8 is a data structure diagram showing sample contents of a component manifest data structure used in a second embodiment of the facility . the component manifest data structure 800 is similar to component manifest 700 , except that fewer components are identified for each application . for instance , component manifest 800 contains a single row 801 identifying a component for the prostate application . when component manifest 800 is used , version 1 . 1 of the guidance system controller component is directly invoked as part of launching the prostate application . when the guidance system controller component is invoked , it in turn invokes the components upon which it has dependencies , until all of the components needed for the prostate application are invoked . fig9 is a data structure diagram showing a component dependency used in the recursive invocation of components discussed above in conjunction with fig8 . a first component 900 contains a reference 901 to a second component 910 . in connection with loading the first component , the facility uses the reference from the first component to the second component to invoke the second component . if the second component contains one or more of its own references 911 , the facility in turn uses the references 911 to invoke the depended - on components . the references may be stored either internally or externally to the components , or both . the references may identify the referred to components in a variety of ways , such as using a file name , a fully - qualified path , a global unique identifier , a name and version number , or any of a number of other types of references . the localization system and at least one marker enables real time tracking of a target relative to a machine isocenter or another external reference frame outside of the patient during treatment planning , set up , radiation sessions , and at other times of the radiation therapy process . in many embodiments , real time tracking means collecting position data of the markers , determining the locations of the markers in an external reference frame , and providing an objective output in the external reference frame that is responsive to the location of the markers . the objective output is provided at a frequency that adequately tracks the target in real time and / or a latency that is at least substantially contemporaneous with collecting the position data ( e . g ., within a generally concurrent period of time ). for example , several embodiments of real time tracking are defined as determining the locations of the markers and calculating the location of the target relative to the machine isocenter at ( a ) a sufficiently high frequency so that pauses in representations of the target location at a user interface do not interrupt the procedure or are readily discernable by a human , and ( b ) a sufficiently low latency to be at least substantially contemporaneous with the measurement of the location signals from the markers . alternatively , real time means that the location system 10 calculates the absolute position of each individual marker 40 and / or the location of the target at a periodicity of 1 ms to 5 seconds , or in many applications at a periodicity of approximately 10 - 100 ms , or in some specific applications at a periodicity of approximately 20 - 50 ms . in applications for user interfaces , for example , the periodicity can be 12 . 5 ms ( i . e ., a frequency of 80 hz ), 16 . 667 ms ( 60 hz ), 20 ms ( 50 hz ), and / or 50 ms ( 20 hz ). alternatively , real time tracking can further mean that the location system 10 provides the absolute locations of the markers and / or the target to a memory device , user interface , linear accelerator or other device within a latency of 10 ms to 5 seconds from the time the localization signals were transmitted from the markers . in more specific applications , the location system generally provides the locations of the markers and / or target within a latency of about 20 - 50 ms . the location system accordingly provides real time tracking to monitor the position of the markers and / or the target with respect to an external reference frame in a manner that is expected to enhance the efficacy of radiation therapy because higher radiation doses can be applied to the target and collateral effects to healthy tissue can be mitigated . alternatively , real - time tracking can further be defined by the tracking error . measurements of the position of a moving target are subject to motion - induced error , generally referred to as a tracking error . according to aspects of the present invention , the localization system and at least one marker enable real time tracking of the target relative to the machine isocenter or another external reference frame with a tracking error that is within clinically meaningful limits . tracking errors are due to two limitations exhibited by any practical measurement system , specifically ( a ) latency between the time the target position is sensed and the time the position measurement is made available , and ( b ) sampling delay due to the periodicity of measurements . for example , if a target is moving at 5 cm / s and a measurement system has a latency of 200 ms , then position measurements will be in error by 1 cm . the error in this example is due to latency alone , independent of any other measurement errors , and is simply due to the fact that the target has moved between the time its position is sensed and the time the position measurement is made available for use . if this exemplary measurement system further has a sampling periodicity of 200 ms ( i . e ., a sampling frequency of 5 hz ), then the peak tracking error increases to 2 cm , with an average tracking error of 1 . 5 cm . in some embodiments , the facility uses structure and / or techniques described in u . s . patent application ser . no . 11 / 166 , 801 , filed on jun . 24 , 2005 , which is hereby incorporated by reference in its entirety . it will be appreciated by those skilled in the art that the above - described facility may be straightforwardly adapted or extended in various ways . for example , the facility may be used in conjunction with a wide variety of components , executing on a wide variety of computer systems or other devices , and may provide a variety of different medical applications relating to radiation therapy . while the foregoing description makes reference to preferred embodiments , the scope of the invention is defined solely by the claims that follow and the elements recited therein .
a facility for performing patient localization for radiation therapy is described . the facility activates two or more discrete software modules , and performs interactions between the activated software modules . based upon the performed interactions , the facility performs patient localization for a radiation therapy session .