Patent Publication Number: US-11648423-B2

Title: Radiation based treatment beam position calibration and verification

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 15/607,217, filed on May 26, 2017, the contents of which are incorporated in its entirety into this application. 
    
    
     TECHNICAL FIELD 
     Implementations of the present disclosure relate to radiation based treatment beam positions and, in particular, to calibration and verification of radiation based treatment beam positions. 
     BACKGROUND 
     A radiation source (e.g., linear accelerator (LINAC)) is used in radiation treatment to apply a beam of highly energized particles (e.g., a radiation beam) to a target within a patient. A mechanical positioning system positions the radiation source (e.g., LINAC) so that the radiation beam is emitted at specific angles and distances (e.g., nodes) relative to the target. Geometric beam delivery accuracy can be improved by performing calibration and verification of the mechanical positioning system. 
     Calibration techniques may use both a point detector and a raster scan. In a first calibration technique, a surrogate is used for the radiation treatment beam. A point detector (e.g., photodiode) or radiation sensor (e.g., stereotactic diode detector or point scintillation detector) is placed at an isocenter of the mechanical positioning system, a surrogate (e.g., a laser beam) for the radiation beam is emitted, a raster scan of a laser beam (e.g., from a central axis laser) is performed across the point detector or radiation sensor (e.g., an initial coarse scan at 0.8 millimeter (mm) resolution over a larger region and a subsequent finer 0.4 mm resolution scan over a smaller region), and the center of the radiation beam is defined from a resulting maximum optical signal intensity of the surrogate. Axis offsets (used to position the center of the radiation beam in the correct location) are determined and stored as pointing offsets to be applied during radiation treatment. 
     For a point detector, such a calibration and verification method using a laser as a surrogate may take 100-200 minutes for a node-set containing 100-200 nodes and 17-33 hours for a node-set for a dynamic path involving 1000 nodes. For a radiation sensor, such a calibration and verification method takes even longer. In the above described calibration and verification method, the laser beam acts as a surrogate for the center of the radiation beam, which introduces the uncertainty of coincidence of the laser beam and treatment beam (e.g., laser-to-radiation beam coincidence) in the calibration and verification results. Further uncertainty is added by any variation in instantaneous laser intensity when the maximum optical signal intensity (e.g., peak signal) is used (e.g., laser intensity stability). Uncertainties may also be introduced into the calibration and verification due to sensitivity varying with beam angle of incidence caused by anisotropic construction of the radiation sensor (e.g., detector sensitivity variation with beam orientation). 
     In a second calibration technique, the radiation treatment beam is used directly. A point detector or radiation sensor is placed at an isocenter of the mechanical positioning system, a radiation beam is emitted using the LINAC, a raster scan is performed across the point detector or radiation sensor, and the center of the radiation beam is defined from a resulting maximum optical signal intensity. Axis offsets are determined and stored as pointing offsets to be applied during radiation treatment. For the second calibration technique, there are not uncertainties from a laser-to-radiation beam coincidence, but the uncertainties caused by dose-rate stability and detector sensitivity variation with beam orientation may cause the time required for calibration and verification under the second calibration technique to be greater than the time required under the first calibration technique. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings. 
         FIG.  1    illustrates a calibration system including one or more cameras and a phantom to calibrate a position of a LINAC, in accordance with implementations of the present disclosure. 
         FIG.  2 A  illustrates a phantom including a spherical phantom body that includes an X-ray luminescent material, in accordance with implementations of the present disclosure. 
         FIG.  2 B  illustrates a phantom including a cylindrical phantom body that includes an X-ray luminescent material, in accordance with implementations of the present disclosure. 
         FIG.  2 C  illustrates a phantom including a spherical phantom body that includes an X-ray luminescent material overlaid with a pattern, in accordance with implementations of the present disclosure. 
         FIG.  3 A  illustrates a calibration system, in accordance with implementations of the present disclosure. 
         FIG.  3 B  illustrates incidence of the radiation beam on the phantom compared to view of a camera, in accordance with implementations of the present disclosure. 
         FIG.  3 C  illustrates the view of a camera of incidence of the radiation beam on the phantom, in accordance with implementations of the present disclosure. 
         FIG.  3 D  illustrates radiation luminescence generated at an entrance surface and an exit surface of the phantom, in accordance with implementations of the present disclosure. 
         FIG.  3 E  illustrates a calibration system, in accordance with implementations of the present disclosure. 
         FIG.  4    illustrates a flow diagram of a method for calibration of a position of a LINAC, in accordance with implementations of the present disclosure. 
         FIG.  5 A  illustrates a flow diagram of a method for calibration of a position of a LINAC using one or more cameras coupled to the LINAC to acquire images of an entrance surface of the phantom, in accordance with implementations of the present disclosure. 
         FIG.  5 B  illustrates a flow diagram of a method for calibration of a position of a LINAC using one or more cameras coupled to the LINAC to acquire images of an entrance surface and an exit surface of the phantom, in accordance with implementations of the present disclosure. 
         FIG.  5 C  illustrates a flow diagram of a method for calibration of a position of a LINAC using cameras positioned at static locations to acquire images of an entrance surface of the phantom, in accordance with implementations of the present disclosure. 
         FIG.  5 D  illustrates a flow diagram of a method for calibration of a position of a LINAC using cameras located at static locations to acquire images of an entrance surface and an exit surface of the phantom, in accordance with implementations of the present disclosure. 
         FIG.  6    illustrates a flow diagram of a method for verification of a position of a LINAC in accordance with implementations of the present disclosure. 
         FIG.  7    illustrates systems that may be used in performing calibration of a position of a LINAC, in accordance with implementations of the present disclosure. 
         FIG.  8    illustrates configurations of a calibration system, in accordance with implementations of the present disclosure. 
         FIG.  9    illustrates a gantry based intensity modulated radiotherapy system, in accordance with implementations of the present disclosure. 
         FIG.  10    illustrates a helical radiation delivery system, in accordance with implementations of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     A radiation source (e.g., LINAC) is used in radiation treatment to apply a radiation beam to a target within a patient. Implementations of the disclosure often reference LINAC for simplicity and brevity, however, the teaching of the present disclosure are applied to radiation sources generally and can be applied to various types of radiation sources, including for example, LINAC, radioactive isotopes (e.g., cobalt-60), cyclotron, etc. A radiation treatment plan is established by determining pointing vectors for each trajectory of the radiation beam (e.g., via in-room imaging of the target) and then determining positions of the LINAC (e.g., nodes, angle and distance relative to the target) to bring a radiation beam into coincidence with each pointing vector. The mechanical positioning systems have mechanical settings corresponding to positions of the LINAC. Geometric beam delivery accuracy is an aspect of any external beam radiation treatment, especially for techniques using high dose gradients and hypofractionation, and can be improved by performing calibration and verification of each mechanical setting of the one or more mechanical positioning systems that position the LINAC. 
     Calibration of each mechanical setting is defined by the physical design of the delivery system (e.g., LINAC and mechanical positioning systems) and the control systems. To improve accuracy of calibration, device specific measurements may be performed and applied as corrections to an original calibration of the mechanical positioning system or used to replace the original calibration entirely. 
     Described herein are methods, systems, and phantoms used for radiation-based treatment beam position calibration and verification. A phantom is a device for simulating the in vivo effect of radiation on tissues by absorbing and scattering x-rays in approximately the same way as the tissues of the body. The phantom includes an X-ray luminescent material. In one implementation, a phantom is coated with an X-ray luminescent material. In another implementation, a phantom includes an X-ray luminescent material (e.g., the X-ray luminescent material is integral to the phantom), where at least a portion of the X-ray luminescent material is on the surface of the phantom. One or more optical images of the radiation beam incident on the phantom are used to measure beam pointing offset. The beam pointing offset is calculated from each image, removing the need for a beam scanning procedure. After the beam pointing offset is applied to corresponding mechanical settings, a second image is acquired to measure the effectiveness of the correction (i.e., a verification procedure). The verification procedure can be iterated. The present disclosure is suitable for coplanar and non-coplanar treatment geometries. Implementations of the present disclosure may reduce path calibration time, for example, from about 100 minutes to between 10 and 40 minutes for 100 treatment positions. Extrapolating to a larger set of 1000 treatment positions, the implementations of the present disclosure may require about 2 hours instead of about 17 hours utilizing other methods. Alternatively, other path calibration times may be achieved. In addition to time saving, implementations of the present disclosure may remove uncertainties related to laser-to-radiation-beam coincidence and instantaneous laser intensity variation present in other methods. 
       FIG.  1    illustrates a calibration system  100  including one or more cameras  110  and a phantom  120  used to calibrate a position of a LINAC  150 , in accordance with implementations of the present disclosure. 
     The LINAC  150  emits a radiation beam  160  at a target (e.g., a phantom  120 , a patient, etc.). The LINAC  150  is coupled to a mechanical positioning system  170 . The mechanical positioning system  170  positions the LINAC  150  at one or more nodes (e.g., relative to the phantom  120 , relative to an isocenter  174  of the mechanical positioning system  170 , etc.). The node may include a distance from the phantom  120  and an angle relative to the phantom  120 . In one implementation, the mechanical positioning system  170  includes a robotic arm  172  (e.g., with degrees of rotation and translation, robotic manipulator joint rotations system, a frameless robotic radiation therapy system (e.g., CyberKnife® robotic radiosurgery system)). In another implementation, mechanical positioning system  170  includes a gantry-based system  900  (e.g., a C-arm gantry rotation system, LINAC  150  is coupled to a gantry  903  of gantry based system  900  of  FIG.  9   , etc.). In another implementation, the mechanical positioning system  170  is a helical radiation delivery system  1000  (see  FIG.  10   ). In another implementation, the mechanical positioning system  170  is a couch translation and rotation system. In another implementation, the mechanical positioning system  170  is a gimbal mount measurement system. Alternatively, other types of mechanical positioning systems may be used. 
     The calibration system  100  includes a camera system  110  having one or more cameras (e.g., camera  110 A, camera  110 B, etc.), a phantom  120 , and a processing device  130 . 
     In one implementation, one or more cameras  110 A are coupled to the LINAC  150  (e.g., on a distal end of the LINAC  150  proximate a collimator  152 ). In another implementation, cameras  110 B are located in a static location (e.g., mounted in a location in a treatment room, do not move in response to movement of the LINAC  150 ). In another implementation, one or more cameras  110  are located on a treatment couch. 
     In one implementation, the camera system  110  is a visual light camera. In another implementation, the camera system  110  is an infrared camera. In another implementation, the camera system  110  is a charge-coupled device (CCD) camera. In another implementation, the camera system  110  is an intensified CCD (ICCD) camera. In another implementation, the camera system  110  is an electron multiplied ICCD (emICCD) camera (e.g., Princeton Instruments PI-MAX4512 EM). In one implementation, the camera system  110  may be operated in a pulsed mode gated by the radiation beam  160 . In another implementation, the camera system  110  is an imaging scintillation or Cerenkov emission detector. 
     The camera system  110  may be designed to be positioned and shielded to maximize the lifetime of each camera system  110 . In one implementation, camera system  110  is positioned at the exit surface of the LINAC  150 , to the sides of the treatment beam where camera system  110  will be shielded by the collimator  152 . In another implementation, a lens (e.g., Canon EF 135 mm f/2 L USM) of each camera of camera system  110  is positioned adjacent to the collimator  152  and is shielded from the radiation beam  160  by the collimator  152 . Each lens may be coupled (e.g., using fiber-optics) to remotely positioned camera electronics (e.g., optics, an image sensor, an intensifier, and so forth), allowing the camera electronics to be positioned at greater distance from the treatment beam. The camera electronics may be positioned in a location where space and weight are less restricted to allow greater radiation shielding to be used that at the exit surface of the LINAC  150 . In one implementation, the camera system  110  is integrated (e.g., permanently integrated, non-removably integrated) into the housing  302  (e.g., treatment head) of the LINAC  150 . The camera system  110  may additionally be used for one or more of collision avoidance, external patient tracking, entrance patient dosimetry, etc. In another implementation, the camera system  110  is mounted on a removable accessory that attaches to the housing  302  (e.g., head) of the LINAC  150  for calibration. The camera system  110  may be removed during treatment to minimize radiation dose to which the camera system  110  is exposed and may extend the lifetime of the camera system  110 . 
     In one implementation, the phantom  120  is mechanically positioned around a reference point (e.g., positioned around a point in space using a high precision mechanical fixture). The reference point is used in calibration of the mechanical positioning system  170 . In one implementation, the reference point is an isocenter  174  (e.g., geometric isocenter) of the mechanical positioning system  170  (e.g., isocenter  174  of LINAC  150 ). In another implementation, the reference point is a known offset from the isocenter  174 . The position of the isocenter  174  of the mechanical positioning system  170  relative to the surface of the phantom  120  will be known from design of the phantom  120  and the method of mechanically positioning the phantom  120 . In one implementation, the phantom  120  is mounted on a support  176 . 
     The phantom  120  includes a phantom body  122 . In one implementation, the phantom body  122  is hollow and the thickness and material of a hollow phantom body  122  allows transmission of backscattered exit surface image (see  FIGS.  3 D and  5 B ). The phantom  120  may have a transparency that allows acquiring, using one camera system  110  at one location, of an image of an entrance feature  360  of a radiation beam  160  entering the phantom  120  and an exit feature  370  of the radiation beam  160  exiting the phantom  120  (see  FIG.  3 D ). 
     In another implementation, the phantom body  122  includes a substrate that is opaque (see  FIG.  5 D ). The opaqueness of the phantom may not allow acquiring, using one camera system  110  at one location, of an image of an entrance feature  360  of a radiation beam  160  entering the phantom  120  and an exit feature  370  of the radiation beam exiting the phantom  120  (see  FIG.  3 D ). 
     The phantom body  120  includes an X-ray luminescent material  124 . In one implementation, the phantom body  122  is coated with an X-ray luminescent material  124 . In another implementation, the X-ray luminescent material  124  is at least partially on the surface of the phantom body  122 . In another implementation, the X-ray luminescent material  124  is at least partially embedded in the outer layer of the phantom body  122 . In another implementation, the X-ray luminescent material  124  is integral to the material of the phantom body  122 . For example, the phantom body  122  may include a Terbium activated gadolinium oxysulphide (Gd 2 O 2 S 2 ) scintillator material  124 . In one implementation, the X-ray luminescent material  124  is an X-ray scintillation material with superficial build-up material. In another implementation, the X-ray luminescent material  124  is an X-ray scintillation material without superficial build-up material. In another implementation, the X-ray luminescent material  124  is a dielectric material (e.g., water, plastic, etc.) to generate a Cerenkov optical signal in response to a radiation beam  160  incident on the phantom  120 . In one implementation, the dielectric material is doped with a fluorescent compound (e.g., a wavelength shifter) to enhance light emission at a plurality of angles (e.g., most angles of the radiation beam  160  incident to the phantom  120 ) and to improve detection sensitivity. In another implementation, the dielectric material is not doped with fluorescent compound. 
     The surface of the phantom  120  is uniform. A relationship of optical signal (e.g., a measurement of a radiation beam incident to the surface) to absorbed dose (e.g., a measurement of absorption of the radiation beam in the phantom) of the radiation beam  160  incident on the phantom  120  is constant over the surface of the phantom  120 . The phantom  120  may include a pattern (e.g., checkerboard pattern, see  FIG.  2 C ) of visually identifiable features (e.g., squares of checkerboard pattern) at relative positions overlaid on the X-ray luminescent material  124 . 
     The phantom body  122  may be spherical (see  FIG.  2 A ), cylindrical (see  FIG.  2 B ), cubical, conical, or another shape. 
       FIG.  2 A  illustrates a phantom  120 A including a spherical phantom body  122 A that includes an X-ray luminescent material  124 , in accordance with implementations of the present disclosure. In one implementation, for calibration of a mechanical positioning system  170  coupled to a LINAC  150  that emits a radiation beam  160  that is non-coplanar, the phantom body  122 A may be spherical and the phantom  120 A is centered on the isocenter  174  of the mechanical positioning system  170  (e.g., isocenter  174  of the LINAC  150 ). 
       FIG.  2 B  illustrates a phantom  120 B including a cylindrical phantom body  122 B that includes an X-ray luminescent material  124 , in accordance with implementations of the present disclosure. In one implementation, for calibration of a mechanical positioning system  170  coupled to a LINAC  150  that emits a radiation beam  160  that is coplanar, the phantom body  122 B may be cylindrical (i.e., cylindrical phantom body  122 B) and includes a first circular end  210  and a second circular end  220  (not shown). A phantom axis  230  is aligned with a first center  212  of the first circular end  210  a second center  222  (not shown) of the second circular end  220 . The phantom axis  230  is coincident with an axis of rotation of the LINAC  150 . 
       FIG.  2 C  illustrates a phantom  120 A including a spherical phantom body  122 A that includes an X-ray luminescent material  124  and overlaid with a pattern  200 , in accordance with implementations of the present disclosure. The pattern  200  is an optical calibration object containing visually identifiable features at known relative positions (e.g., a checkerboard pattern) that is overlaid on the X-ray luminescent material  124  on the outer surface of the phantom body  122 . In one implementation, the pattern  200  is used for calculating the pose of the camera with respect to the beam axis  306  of the radiation beam (see method  500  of  FIG.  5 A  and method  540  of  FIG.  5 C ). 
     Although  FIG.  2 C  illustrates pattern  200  overlaid on a spherical phantom body  122 A, pattern  200  can be overlaid on any other shape of phantom body  122 . Although a checkerboard pattern is illustrated in  FIG.  2 C , alternative types of patterns may be used. 
       FIGS.  3 A and  3 E  illustrates a calibration system  100  coupled to a LINAC  150 , in accordance with implementations of the present disclosure. The calibration system  100  includes a camera system  110  and a phantom  120 . The phantom  120  is radiated by a radiation beam  160  emitted by LINAC  150 . The LINAC  150  has a housing  302  coupled to a collimator  152 . One or more radiation beams  160  may be emitted from a distal end  310  of the LINAC  150  along one or more beam axes  306  to a target location  320 . In one implementation, the target location  320  is located in or on a phantom  120 . In another implementation, the target location  320  is located in or on a surface of a patient. 
     In one implementation, one or more of the beam axes  306  may be substantially normal to the target location  320  (e.g., perpendicular to the phantom surface  308  overlaying the target location, forming a ninety degree beam incident angle  150  with the phantom surface  308 ). The one or more radiation beams  160  may be emitted through collimation (e.g., an aperture between banks of leaves in the collimator  152 , rectangular variable collimation, circular variable collimation, fixed collimation (e.g., cones), etc.). 
     In one implementation, the distal end  310  of the housing  302  of LINAC  150  may be the radiation source  304 . In another implementation, a distal end  310  of the housing  302  of LINAC  150  may be the area proximate where the housing  302  is coupled to the collimator  152 . In another implementation, a distal end  310  of the housing  302  of LINAC  150  may be the area proximate where the radiation beam  160  is emitted from the housing  302 . In another implementation, the distal end  310  of housing  302  may be where a one or more cameras  110  are coupled to the housing  302 . 
     A source-to-axis distance (SAD)  330  is measured from the radiation source  304  to the target location  320 . One or more of the support  176  or LINAC  150  may be used to vary the SAD  330 . In one implementation, support  176  is a stage that moves the phantom  120  relative to the LINAC  150  to vary the SAD  330 . In another implementation, the support  176  is a couch and motion of the couch alters the SAD  330 . In another implementation, support  176  is floor or a wall of a treatment room and a robotic manipulator (e.g., robotic arm  172  of  FIG.  1   ) is used to vary the SAD  330 . 
     In some implementations, the one or more cameras  110  may be coupled to the housing  302  of LINAC  150  at locations that do not interfere with the removal and attachment of the collimator  152 . In one implementation, each camera of camera system  110  may be coupled to the housing  302  at a distal end  310  of the LINAC  150 . In another implementation, each camera of camera system  110  may include a lens  312  disposed at a distal end  310  of the LINAC  150  proximate exit of the radiation beam  160  from the collimator  152  (e.g., exit surface  311 ) at a location that does not interfere with removal and attachment of the collimator  152 . Each lens  312  may be shielded from the one or more radiation beams  160  by the collimator  152 . Each camera of camera system  110  may capture a set of images (e.g., live images) of the radiation beam  160  incident to the phantom  120  (e.g., optical Cerenkov emission generated at the phantom  120  by charged particles of the radiation beam  160  moving in a medium of the phantom  120  with a phase speed greater than the speed of light in the medium). 
     As shown in  FIG.  3 E , in some implementations, the one or more cameras  110  may be positioned at an angle (e.g., 90 degrees) relative to the beam axis  306 . The one or more cameras  110  may be disposed proximate the distal end  310  of housing  302 . In one implementation, the one or more cameras  110  are coupled to the housing  302 . In another implementation, the one or more cameras  110  are not coupled to the housing. In some implementations, the camera system  110  and the radiation beam  160  have in-line geometry along the beam axis  306 . The in-line geometry (e.g., shared axis) may be achieved using a mirror  392 . The mirror  392  may be in the beam path of the radiation beam  160 . The camera system  110  may be disposed at an angle (e.g., 90 degrees) relative to the beam axis  306  and a mirror  392  (e.g., a 45-degree mirror) may be used to align the optical axis and the beam axis  306 . The mirror  392  may be calibrated (e.g., a one-time calibration) to provide aligning of the optical axis and the beam axis  306 . 
     In one implementation, an image of the radiation beam  160  incident on the phantom  120  may be acquired with a phantom  120  including a scintillator material (e.g., a Terbium activated Gd 2 O 2 S 2  scintillator material) at least partially on the surface of the phantom  120 , using 5 monitor units (MUs) (a measure of machine output from a LINAC  150 ) with a 6× radiation beam  160  (e.g., photon beam produced by the acceleration of electrons to 6 megaelectron-volts (MeV)) at approximately 1000 millimeter (mm) SAD  330 , and with no build-up. This may result in approximately 0.5 seconds per image acquisition and allowing an additional 2.5 seconds for optical image acquisitioning and processing. This is approximately ten times faster than other calibration and verification methods (excluding robot motion time). In another implementation, perspective correction is used, resulting in 30 minutes of perspective calibration followed by 100 nodes at 3 seconds per node for calibration, followed by verification, so the total time with the methods disclosed herein would be 40 minutes (excluding robot traversal) instead of 100 minutes with the other calibration and verification methods. For calibration and verification of 1,000 nodes (e.g., for a dynamic treatment delivery method of delivering radiation beams from a continuous range of beam source locations around the patient), the same comparison becomes approximately 2 hours by the methods disclosed herein instead of 17 hours by other methods. 
       FIG.  3 B  illustrates incidence of the radiation beam  160  on the phantom  120  compared to view of a camera system  110 , in accordance with implementations of the present disclosure. 
     The LINAC  150  emits a radiation beam  160  from the radiation source  304  to the target location  320  in or on the phantom  120 . The camera system  110  acquires an image of the radiation beam  160  incident on the phantom  120  at the phantom surface (e.g., radiation pattern  352  on entrance surface in  FIG.  3 C ). Camera system  110  has a camera axis  314  (e.g., center of lens  312  of camera  110 , center of the image acquired by the camera, center of the projection plane  316 , etc.) and radiation beam  160  has a beam axis  306  (e.g., center of the radiation beam  160 ). In one implementation, the camera axis  314  and beam axis  306  may both intersect the phantom  120  at the target location  320  (e.g., at the center of the phantom  120 ). 
     The camera  110  is not coincident with the beam axis  306  of radiation beam  160 . The camera system  110  has a camera pose including translation (e.g., distance  318  between camera axis  314  and beam axis  306 ) and rotation (e.g., angle of camera axis  314  in relation to beam axis  306 ). The distance  318  between a camera axis  314  of the camera system  110  and the beam axis  306  results in a shift between the phantom centroid  354  of the phantom  120  and the pattern centroid  356  of a radiation pattern  352  (e.g., radiation scintillation pattern) in images acquired by camera system  110  (see image  350  of  FIG.  3 C ). A projection plane  316  is a view of the camera system  110  of the phantom  120  and a radiation pattern  352  the phantom surface  308 . The projection plane  316  corresponds with an image  350  acquired by camera system  110  (see  FIG.  3 C ). 
       FIG.  3 C  illustrates the view of a camera system  110  of a radiation pattern  352  from the incidence of the radiation beam  160  on the phantom  120 , in accordance with implementations of the present disclosure. Image  350  is an image of the phantom  120  and radiation pattern  352  as acquired by camera system  110 . The phantom centroid  354  of the phantom  120  (e.g., center of a spherical phantom) and the pattern centroid  356  of the radiation pattern  352  in image  350  do not coincide because of the distance  318  and angle between the beam axis  306  and the camera axis  314 , and the finite size of the phantom. The offset between the phantom centroid  354  and the pattern centroid  356  can be modeled based on the camera pose (e.g., translation and rotation of camera system  110 ) relative to the phantom  120 . 
       FIG.  3 D  illustrates radiation luminescence generated at an entrance surface and an exit surface of the phantom  120 , in accordance with implementations of the present disclosure. 
     In one implementation, the phantom  120  may be constructed so that both entrance feature  360  and exit feature  370  are visible in the same projection (e.g., projection plane  316 , each of the first set of images is of an entrance surface and an exit surface of the phantom  120 , etc.). For example, a camera system  110  may acquire an image of the phantom  120 , where the image displays both the entrance feature  360  and the exit feature  370 . The thickness and material of the phantom  120  allow transmission of backscattered exit surface image. In one implementation, the entrance feature  360  and the exit feature  370  are separated in an image by the dimensions of the phantom  120  being greater than a first threshold size and the radiation beam  160  being less than a second threshold size. The material and thickness of the phantom  120  may separate the entrance feature  360  and exit feature  370  by optical intensity. 
     In another implementation, two or more images are acquired by one or more cameras of camera system  110  in different locations (e.g., a camera  110  in a first location and a second location, or a first camera  110  in a first location and a second camera in a second location). In one implementation, the position of the entrance feature  360  or exit feature  370  of the phantom  120  relative to the array of cameras  110  is triangulated using a first orientation of a first image from a first camera  110  and a second orientation of a second image from a second camera. The position of the entrance feature  360  or exit feature  370  relative to the radiation beam  160  may be triangulated using a first orientation of a first image from a first camera  110  and a second orientation of a second image from a second camera. The phantom  120  may be an opaque substrate. 
     One or more images (or views via camera  110 ) of the phantom  120  may be of the radiation beam  160  incident on the entrance surface and exit surface of the phantom  120 . The radiation beam  160  incident on the entrance surface of the phantom  120  generates an entrance feature  360  (e.g., a teardrop shape) and the radiation beam  160  incident on the exit surface of the phantom  120  (e.g., exiting the phantom  120 ) generates an exit feature  370  (e.g., a teardrop shape). The entrance feature  360  has a first centroid  362  and the exit feature  370  has a second centroid  372 . The first centroid  362  and the second centroid  372  create a line  380  through the phantom and the line  380  has a half-way point  382 . A distance between the half-way point  382  and the center  384  of the phantom  120  is the beam pointing offset  390 . 
     In one implementation, the center  384  of the phantom  120  may be a projected isocenter of the LINAC  150  based on first images of the phantom  120  while the phantom  120  is not being irradiated, geometry of the phantom  120 , and position of the phantom  120 . The half-way point  382  between the first centroid  362  and the second centroid  372  is based on second images of the phantom  120  while the radiation beam  160  is incident on the phantom  120 . The lighting may be turned off or dimmed during the acquisition of images of the phantom  120  with the phantom is being irradiated and the lighting may be turned on during the acquisition of images of the phantom  120  while the phantom  120  is not being irradiated. 
     In one implementation, lighting is constant (e.g., one room lighting state for the whole procedure) and a camera system  110  captures one image that includes both the outline of the phantom  120  and the scintillation or Cerenkov signal. A beam pointing offset is determined from the image and a position of the radiation source (e.g., LINAC  150 ) is calibrated based on the beam pointing offset. 
     In one implementation, using one or more cameras of camera system  110  coupled to the LINAC  150 , images are acquired of an entrance feature  360  and no exit feature  370  (see  FIG.  5 A ). In another implementation, using one or more cameras of camera system  110  coupled to the LINAC  150 , images are acquired of an entrance feature  360  and an exit feature  370  (see  FIG.  5 B ). In another implementation, using camera system  110  located in static positions, images are acquired of an entrance feature  360  and no exit feature  370  (see  FIG.  5 C ). In another implementation, using camera system  110  located in static positions, images are acquired of an entrance feature  360  and an exit feature  370  (see  FIG.  5 D ). 
       FIGS.  4 - 5 D  illustrate flow diagrams of methods  400 ,  500 ,  520 ,  540 , and  560  for calibration of a position of a LINAC  150 , in accordance with implementations of the present disclosure.  FIG.  6    illustrates a flow diagram of method  600  for verification of a position of a LINAC  150 , in accordance with implementations of the present disclosure. Methods  400 ,  500 ,  520 ,  540 ,  560 , and  600  are described in relation to the calibration or verification of a position of a LINAC  150 . However, it should be understood that methods  400 ,  500 ,  520 ,  540 ,  560 , and  600  may also be used to calibrate or verify a position of other systems that emit radiation, in particular, a radiation beam  160 . The methods  400 ,  500 ,  520 ,  540 ,  560 , and  600  may be performed by processing logic that comprises hardware (e.g., circuitry, dedicated logic, programmable logic, microcode, etc.), software (e.g., instructions run on a processing device to perform hardware simulation), or a combination thereof. 
     In one implementation, prior to any of the methods in  FIGS.  4 - 5 D , intrinsic properties (e.g., intrinsic camera properties, sensor distortions, lens distortions, etc.) of the one or more cameras of camera system  110  are determined. This is a one-off procedure that may be performed prior to installation of the one or more cameras of camera system  110  (e.g., prior to coupling one or more cameras  110  to the LINAC  150 , prior to locating the cameras of camera system  110  in stationary locations relative to the phantom  120 , etc.). Corrections for the distortions in the intrinsic properties of the camera system  110  are applied to all images described in  FIGS.  4 - 5 D . For example, the processing device applies corrections for the sensor and lens distortions to the first set of images and the second set of images. 
       FIG.  4    illustrates a flow diagram of a method  400  for calibration of a position of a LINAC  150 , in accordance with implementations of the present disclosure. 
     At block  410 , processing logic acquires, using a camera system  110 , an image of a radiation beam  160  incident on a phantom  120 . In one implementation, block  410  includes acquiring, using one or more cameras of camera system  110 , a first set of images of a phantom  120  while the phantom  120  is not being irradiated and acquiring, using one or more cameras of camera system  110 , a second set of images of a radiation beam  160  incident on the phantom. The phantom  120  includes an X-ray luminescent material  124  at least partially on the surface of the phantom body  122 . The radiation beam  160  is emitted by a radiation source (e.g., a LINAC  150 ). 
     At block  420 , processing logic determines a beam pointing offset based on the image. In one implementation, block  410  includes determining the beam pointing offset based on the first set of images and the second set of images. 
     At block  440 , processing logic calibrates a position of the LINAC  150  based on the beam pointing offset. In some implementations, the beam pointing error from block  630  of  FIG.  6    may be applied as a beam pointing offset to mechanical beam positioning devices (e.g., devices of mechanical positioning system  170 ) to adjust the position of the LINAC  150 . The calibration method of any one of methods  400 ,  500 ,  520 ,  540 , or  560  and the verification method of method  600  may be iterated. A list of beam pointing offsets that can be applied by the mechanical positioning systems during treatment (e.g., used to amend or replace the existing pointing calibration) may be output (e.g., as a report). 
     Blocks  410 - 440  may be repeated (e.g., after block  440 , the method  400  may restart at  410 ) if a set of a radiation beams is to be calibrated. 
       FIGS.  5 A-D  illustrate flow diagrams for calibration of a position of a radiation source using one or more cameras of camera system  110  coupled to the radiation source. In one implementation, the methods of  FIGS.  5 A-D  include one camera of system  110  acquiring one image of the radiation beam incident on the phantom  120 , a beam pointing offset being determined based on the image, and the position of the radiation source being calibrated based on the beam pointing offset. In another implementation, the methods of  FIGS.  5 A-D  include one or more cameras acquiring a plurality of images of the radiation beam incident on the phantom  120 , a beam pointing offset being determined based on the plurality of images, and the position of the radiation source being calibrated based on the beam pointing offset. 
       FIG.  5 A  illustrates a flow diagram of a method  500  for calibration of a position of a LINAC  150  using one or more cameras of camera system  110  coupled to the LINAC  150  to acquire images of an entrance surface of the phantom  120 , in accordance with implementations of the present disclosure. 
     At block  508 , processing logic acquires, using one or more cameras of camera system  110 , a first set of images of a phantom while the phantom  120  is not being irradiated. The phantom  120  may be mounted at the isocenter  174  of the mechanical positioning system  170  (e.g., isocenter  174  of the LINAC  150 ). 
     At block  510 , processing logic acquires, using the one or more cameras of camera system  110 , a second set of images of the radiation beam  160  incident on the phantom. The second set of images may be of the radiation beam  160  incident on a portion of the phantom surface  308  of phantom  120  that is most proximate to the radiation source  304  (e.g., the entrance feature  360 ). The radiation beam  160  is collimated to be symmetric about the beam axis  306  (e.g., using a fixed circular collimator). 
     At block  512 , processing logic determines a projected isocenter of the radiation source (e.g., LINAC  150 ) based on the first set of images, geometry of the phantom  120 , and position of the phantom  120 . For example, for a phantom  120 A with a spherical phantom body  122 A that is surrounding the isocenter  174  of the mechanical positioning system  170 , the projected isocenter (e.g., phantom centroid  354 ) is the center  384  of the circular outline of the phantom  120 A. 
     At block  514 , processing logic determines a third centroid of the radiation beam  160  incident on the phantom  120  (e.g., pattern centroid  356 ) based on the second set of images. 
     At block  516 , processing logic determines a beam pointing offset based on comparing the projected isocenter and the third centroid. The processing logic determines a direction of the beam pointing offset. In one implementation, the direction is found by an iterative search. The robot pointing is adjusted by the offset magnitude (e.g., the targeting location  320  is shifted by the magnitude of the beam pointing offset and the location of the radiation source  304  is not adjusted) and the beam pointing offset is applied in a random direction or is guided by the shape of the projected beam aperture onto the phantom surface  308  (e.g., for a circular radiation beam  160  projected onto a spherical phantom body  122 A, the beam pointing offset should be applied along the major axis of the projected shape, in the direction of the fat end of the tear drop shape (entrance feature  360 )). In another implementation, the direction is found by optical fiducial marks placed on the surface of the phantom  120  from which the orientation of the phantom  120  in room space can be calculated in each optical image which allows the direction of the beam pointing offset to be calculated as well as the magnitude. In another implementation, the direction is found by camera extrinsic parameters (e.g., pose of the camera  110 ) that describe the orientation of the image relative to the radiation beam  160  and if the nominal orientation of the radiation beam  160  with respect to the room is also known, then the camera extrinsic parameters and the nominal orientation of the radiation beam  160  can be combined to give the offset direction. 
     At block  518 , processing logic calibrates a position of the LINAC based on the beam pointing offset and the relationship. In one implementation, the calibrating the position of the LINAC  150  may be by storing the beam pointing offset for later use. In another implementation, the calibrating the position of the LINAC  150  may be by adjusting the position of the LINAC  150  via the mechanical positioning system  170 . 
     In some implementations, the beam pointing error from block  630  of  FIG.  6    may be applied as a beam pointing offset to mechanical beam positioning devices (e.g., devices of mechanical positioning system  170 ) to adjust the position of the LINAC  150 . The calibration method of any one of methods  400 ,  500 ,  520 ,  540 , or  560  and the verification method of method  600  may be iterated. A list of beam pointing offsets that can be applied by the mechanical positioning systems during treatment (e.g., used to amend or replace the existing pointing calibration) may be output (e.g., as a report). 
     Blocks  508 - 518  may be repeated (e.g., after block  518 , the method  500  may restart at  508 ) if a set of a radiation beams is to be calibrated. 
     In one implementation, the one or more cameras of camera system  110  are in-line with the beam axis of the radiation beam  160 . In another implementation, the one or more cameras are not in-line with a beam axis of the radiation beam and the method further includes calculating pose of the one or more cameras of camera system  110  with respect to the radiation beam axis (e.g., blocks  502 - 506 , not shown in  FIG.  5 A ). At block  502 , processing logic acquires, using the one or more cameras of camera system  110 , a third set of images of a pattern  200  (see  FIG.  2 C ) overlaid on the X-ray luminescent material  124  while the phantom  120  is not being irradiated. The third set of images is acquired at one or more SAD  330 . The one or more SAD  330  may be provided by phantom  120  being mounted on support  176  and the support  176  or LINAC  150  moving relative to each other. Processing logic determines pose (e.g., translation and rotation of camera axis  314  relative to beam axis  306 ) and focal length of the one or more cameras of camera system  110  from the third set of images of pattern  200 . 
     At block  504 , processing logic acquires, using the one or more cameras, a fourth set of images of the radiation beam incident on the pattern at one or more SAD  330  (see  FIG.  3 A ). Processing logic determines a location of the beam axis  306  with respect to the pattern  200  from the fourth set of images. One or more SAD  330  in block  504  may correspond with the one or more SAD in block  502 . Each of the third set of images corresponds to one or more images of the fourth set of images. 
     At block  506 , processing logic determines, based on the pose, the focal length, and the location of the beam axis, a relationship between a first centroid of the phantom  120  (e.g., phantom centroid  354  of  FIG.  3 C ) and a second centroid of the radiation beam  160  (e.g., pattern centroid  356  of  FIG.  3 C ) incident on the pattern  200  (e.g., scintillation pattern as seen by the camera). Since the camera axis  314  is not coincident with the beam axis  306 , an image  350  of phantom  120  and the radiation pattern  352  will not be concentric even when the radiation beam  160  is pointing exactly at the center of the phantom  120  (e.g., phantom centroid  354 ). In one implementation, the relationship determined by block  506  may be an expected offset (e.g., distance between phantom centroid  354  and pattern centroid  356  of  FIG.  3 C ) and may be used as a goal (e.g., target value) during determining of the beam pointing offset. In another implementation, the relationship determined by block  506  may be an expected offset (e.g., distance between phantom centroid  354  and pattern centroid  356 ) may be converted into a beam pointing offset such that the phantom  120  and radiation patterns  352  become concentric in the image  350  acquired by camera system  110  by application of the beam pointing offset. 
     In one implementation, blocks  502 - 506  may be a camera calibration setup that is a one-off procedure (if pose is repeatable). In another implementation, blocks  502 - 506  may be required before each calibration procedure (e.g., blocks  508 - 518 ) or verification procedure (e.g., method  600 ) is performed. In one implementation, blocks  502 - 506  may be generalized to multiple camera configurations and the accuracy of the calibration may improve with multiple cameras for camera system  110 . 
       FIG.  5 B  illustrates a flow diagram of a method  520  for calibration of a position of a LINAC  150  using one or more cameras of camera system  110  coupled to the LINAC  150  to acquire images of an entrance surface and an exit surface of the phantom  120 , in accordance with implementations of the present disclosure. 
     Method  520  does not require pose information of the camera system  110  (e.g., does not require blocks  502 - 506  of method  500 ). 
     At block  522 , processing logic acquires, using one or more cameras of system  110 , a first set of images of a phantom  120  while the phantom  120  is not being irradiated. 
     At block  524 , processing logic acquires, using the one or more cameras of system  110 , a second set of images of a radiation beam  160  incident on the phantom  120 . Each of the second set of images is of the radiation beam incident on an entrance surface and an exit surface of the phantom (e.g., display both entrance feature  360  and exit feature  370  (see  FIG.  3 D )). In one implementation, the phantom  120  used in method  500  may be constructed so that both entrance feature  360  and exit feature  370  are visible in the same projection (e.g., projection plane  316 , each of the first set of images is of an entrance surface and an exit surface of the phantom  120 , etc.). In another implementation, two or more of the first set of images are a super-position of an entrance surface and an exit surface of the phantom  120 . The thickness and material of the phantom  120  allow transmission of backscattered exit surface image. In one implementation, the entrance feature  360  and the exit feature  370  are separated in an image by the phantom  120  having a first size greater than a first threshold size and the radiation beam  160  having a second size less than a second threshold size. The material and thickness of the phantom  120  may separate the entrance feature  360  and exit feature  370  by optical intensity. 
     At block  526 , processing logic determines a projected isocenter of the LINAC  150  onto an image plane based on the first set of images, geometry of the phantom, and position of the phantom. The projected isocenter of the LINAC  150  may be coincident with the center of the phantom. The projected isocenter of the LINAC  150  may be a property of the treatment device as a whole and not of each individual radiation beam  160 . For example, the projected isocenter may be the center  384  of phantom  120  that has a spherical phantom body  122 A (see  FIG.  3 D ). 
     At block  528 , processing logic determines a half-way point  382  between a first centroid  362  of the radiation beam  160  incident on the entrance surface (e.g., entrance feature  360 ) and a second centroid  372  of the radiation beam  160  incident on the exit surface (e.g., exit feature  370 ) based on the second set of images. 
     At block  530 , processing logic determines a beam pointing offset  390  based on a distance between the projected isocenter (e.g., center  384  of phantom  120 ) and the half-way point  382 . 
     In one implementation, the direction of the beam pointing offset  390  may be determined by an iterative search as described above. In another implementation, the direction of the beam pointing offset  390  may be determined by optical fiducial marks placed on the surface of the phantom  120  as described above. 
     At block  532 , processing logic calibrates a position of the LINAC  150  based on the beam pointing offset. 
     Blocks  522 - 532  may be repeated (e.g., after block  532 , the method  520  may restart at  522 ) if a set of a radiation beams is to be calibrated. 
       FIG.  5 C  illustrates a flow diagram of a method  540  for calibration of a position of a LINAC  150  using cameras of camera system  110  positioned at static locations to acquire images of an entrance surface of the phantom  120 , in accordance with implementations of the present disclosure. 
     At block  550 , processing logic acquires, using one or more cameras of system  110 , a first set of images of a phantom  120  while the phantom  120  is not being irradiated. In one implementation, the phantom  120  is located about the isocenter  174  of mechanical positioning system  170 . 
     At block  552 , processing logic acquires, using the one or more cameras of camera system  110 , a second set of images of a radiation beam  160  incident on the phantom  120 . The second set of images is of radiation beam  160  incident on the entrance surface of the phantom  120 . 
     At block  554 , processing logic determines a center, triangulated in 3D, of the radiation beam  160  incident on the phantom  120  (e.g., pattern centroid  356 ) based on the second set of images. 
     At block  556 , processing logic determines a beam pointing offset based on the center triangulated in 3D and a location of a source of the radiation beam  160  (e.g., radiation source  304 , node of LINAC, spatial location in a room). 
     At block  558 , processing logic calibrates a position of the LINAC  150  based on the beam pointing offset and the relationship (from block  548 ). In one implementation, a beam vector is determined based on center triangulated in 3D and the location of the radiation source  304 . The beam pointing offset magnitude and direction are determined from the beam vector. 
     Blocks  550 - 558  may be repeated (e.g., after block  558 , the method  540  may restart at  550 ) if a set of a radiation beams is to be calibrated. 
     In one implementation, the one or more cameras of camera system  110  are in-line with the beam axis of the radiation beam  160 . In another implementation, the one or more cameras are not in-line with a beam axis of the radiation beam and the method further includes calculating pose of the one or more cameras of camera system  110  with respect to the radiation beam axis (e.g., blocks  540 - 548 , not shown in  FIG.  5 C ). At block  542 , processing logic acquires, using the plurality of cameras, a third set of images of a pattern  200  overlaid on the X-ray luminescent material while the phantom  120  is not being irradiated to determine pose and focal length of the plurality of cameras of camera system  110 . The plurality of cameras of camera system  110  is an array of cameras fixed within a room (e.g., a treatment room). A threshold number of cameras of camera system  110  are needed such that the phantom  120  and radiation pattern  352  at the entrance surface of the phantom  120  are visible on multiple cameras of camera system  110  with each direction of the radiation beam  160 . The third set of images may be at a plurality of SAD between the pattern  200  and the LINAC  150 . 
     At block  544 , processing logic maps the third set of images to stationary spatial camera positions of the plurality of cameras of camera system  110  to perform a three-dimensional (3D) calibration. The 3D calibration is based on multiple optical checkerboard positions and orientations (e.g., images from a plurality of cameras of camera system  110  at a plurality of SAD). Each camera of camera system  110  of the plurality of cameras of camera system  110  is mapped to a corresponding spatial position. 
     At block  546 , processing logic acquires, using the plurality of cameras of camera system  110 , a fourth set of images of the radiation beam  160  incident on the pattern  200  at a plurality of SAD to determine a location of a beam axis  306  with respect to the pattern  200 . 
     At block  548 , processing logic determines, based on the pose, the focal length, and the location of the beam axis  306 , a relationship between a first centroid  354  of the phantom  120  and a second centroid  356  of the radiation beam  160  incident on the pattern  200 . In one implementation, blocks  542 - 548  may be a one-off procedure if the cameras remain static between tests. 
     In one implementation, blocks  540 - 548  may be a camera calibration setup that is a one-off procedure (if pose is repeatable). In another implementation, blocks  540 - 548  may be required before each calibration procedure (e.g., blocks  550 - 558 ) or verification procedure (e.g., method  600 ) is performed. In one implementation, blocks  502 - 506  may be generalized to multiple camera configurations and the accuracy of the calibration may improve with multiple cameras  110 . 
       FIG.  5 D  illustrates a flow diagram of a method  560  for calibration of a position of a LINAC  150  using camera system  110  located at static locations to acquire images of an entrance surface and an exit surface of the phantom  120 , in accordance with implementations of the present disclosure. 
     At block  562 , processing logic acquires, using a plurality of cameras of camera system  110 , a first set of images of a phantom  120  while the phantom  120  is not being irradiated. In one implementation, the phantom  120  of method  560  may be substrate that is opaque. In one implementation, one or more of the plurality of cameras of camera system  110  acquires an image of the unirradiated entrance surface. The image of the unirradiated entrance surface may further display the light signal at the exit surface. In another implementation, the plurality of cameras of camera system  110  acquires an image of the unirradiated entrance surface and an image of the unirradiated exit surface. 
     At block  564 , processing logic acquires, using the one or more cameras of camera system  110 , a second set of images of a radiation beam  160  incident on the phantom  120 . Two or more of the second set of images may be used to generate a super-position of the radiation beam  160  incident on an entrance surface (e.g., entrance feature  360 ) and an exit surface (e.g., exit feature  370 ) of the phantom  120 . 
     At block  566 , processing logic determines a projected isocenter of the radiation source (e.g., LINAC  150 ) based on the first set of images, geometry of the phantom  120 , and position of the phantom  120 . 
     At block  568 , processing logic determines a first center, triangulated in 3D, of the radiation beam  160  incident on the entrance surface (e.g., entrance feature  360 ) and a second center, triangulated in 3D, of the radiation beam incident on the exit surface (e.g., exit feature  370 ) based on the second set of images. 
     At block  570 , processing logic determines a beam pointing offset based on the first center triangulated in 3D, the second center triangulated in 3D, and the projected isocenter. In one implementation, a beam vector is determined from the first center and the first center without the location of the radiation source  304 . The beam vector and projected isocenter are used to determine the beam pointing offset. 
     At block  572 , processing logic calibrates a position of the LINAC  150  based on the beam pointing offset. In some implementations, the beam pointing error from block  630  of  FIG.  6    may be applied as a beam pointing offset to mechanical beam positioning devices (e.g., devices of mechanical positioning system  170 ) to adjust the position of the LINAC  150 . The calibration method of any one of methods  400 ,  500 ,  520 ,  540 , or  560  and the verification method of method  600  may be iterated. A list of beam pointing offsets that can be applied by the mechanical positioning systems during treatment (e.g., used to amend or replace the existing pointing calibration) may be output (e.g., as a report). 
     Blocks  562 - 572  may be repeated (e.g., after block  572 , the method  560  may restart at  562 ) if a set of a radiation beams is to be calibrated. 
     It should be noted that the above described operations are just one method of calibrating a position of a LINAC  150  and that, in alternative implementations, certain ones of the operations of  FIG.  4 - 5 D  may be optional or take a simpler form. 
       FIG.  6    illustrates a flow diagram of a method  600  for verification of a position of a LINAC  150 , in accordance with implementations of the present disclosure. In some implementations, method  600  may occur after any one of methods  400 ,  500 ,  520 ,  540 , or  560 . In some implementations, method  600  may occur without any of methods  400 ,  500 ,  520 ,  540 , or  560 . Method  600  may occur after a different method for calibration (e.g., a slower method of calibration, a calibration technique using a point detector and a raster scan). Method  600  may be iterated. 
     At block  610 , processing logic acquires, using a camera of camera system  110  of the one or more cameras of camera system  110 , a third image of the radiation beam  160  incident on the phantom  120  subsequent to the calibrating of the position of the LINAC  150 . The calibrating of the position of the LINAC  150  may include updating the offsets used by the mechanical positioning system  170  in positioning the LINAC  150  for emitting of a radiation beam  160 . 
     At block  620 , processing logic calculates a beam pointing error based on the third image and a corresponding image of the second set of images. The second set of images may be of the radiation beam  160  incident on the phantom  120  acquired in any one of methods  400 ,  500 ,  520 ,  540 , or  560 . 
     At block  630 , processing logic outputs the beam pointing error. The beam pointing error may be output as a list of verification results describing the resulting beam pointing error at each position after the final calibration (e.g., the final calibration after any iterations of calibration) is applied. If only a verification procedure is performed (e.g., as part of system Quality Assurance), then only a report of the verification results may be generated. 
     Blocks  610 - 630  may be repeated (e.g., after block  630 , the method  600  may restart at  610 ) if a set of positions (e.g., positions of the LINAC for emitting radiation beams) is to be verified. 
     It should be noted that the above described operations are just one method of verifying a position of a LINAC  150  and that, in alternative implementations, certain ones of the operations of  FIG.  6    may be optional or take a simpler form. 
     The methods described in  FIGS.  4 - 6    may be used in systems other than a radiation beam  160  incident on a phantom  120 . 
       FIG.  7    illustrates systems that may be used in performing radiation treatment, in accordance with implementations of the present disclosure. These systems may be used to perform, for example, the methods described above. As described below and illustrated in  FIG.  7   , a system  700  may include a calibration system  100  and a treatment delivery system  715 . 
     In one implementation, calibration system  100  includes an imaging detector  730  (e.g., one or more cameras of camera system  110 ) to acquire a first set of images of a phantom  120  without being irradiated and a second set of images of a radiation beam  160  incident on the phantom  120 . 
     In one implementation, imaging detector  730  may be coupled to processing device  740  to control the imaging operation and process image data. In one implementation, calibration system  100  may receive imaging commands from treatment delivery system  715 . 
     Calibration system  100  includes a processing device  740  to calibrate the position of the LINAC  150 . Processing device  740  may represent one or more general-purpose processors (e.g., a microprocessor), special purpose processor such as a digital signal processor (DSP) or other type of device such as a controller or field programmable gate array (FPGA). Processing device  740  may be configured to execute instructions for performing beam profile measurement generating operations discussed herein. Processing device  740  may also include other components (not shown) such as memory, storage devices, network adapters and the like. Processing device  740  may be configured to generate digital diagnostic images in a standard format, such as the Digital Imaging and Communications in Medicine (DICOM) format, for example. In other implementations, processing device  740  may generate other standard or non-standard digital image formats. Processing device  740  may transmit diagnostic image files (e.g., the aforementioned DICOM formatted files) to treatment delivery system  715  over a data link  790 , which may be, for example, a direct link, a local area network (LAN) link or a wide area network (WAN) link such as the Internet. In addition, the information transferred between systems may either be pulled or pushed across the communication medium connecting the systems, such as in a remote diagnosis or treatment planning configuration. In remote diagnosis or treatment planning, a user may utilize implementations of the present disclosure to diagnose or treat a patient despite the existence of a physical separation between the system user and the patient. 
     Calibration system  100  may also include system memory  735  that may include a random access memory (RAM), or other dynamic storage devices, coupled to processing device  740  by bus  786 , for storing information and instructions to be executed by processing device  740 . System memory  735  also may be used for storing temporary variables or other intermediate information during execution of instructions by processing device  740 . System memory  735  may also include at least one of a read only memory (ROM) or other static storage device coupled to bus  786  for storing static information and instructions for processing device  740 . 
     Calibration system  100  may also include storage device  745 , representing one or more storage devices (e.g., a magnetic disk drive or optical disk drive) coupled to bus  786  for storing information and instructions. Storage device  745  may be used for storing instructions for performing the beam profile measurement steps discussed herein. 
     Processing device  740  may also be coupled to a display device  750 , such as a cathode ray tube (CRT) or liquid crystal display (LCD), for displaying information (e.g., beam profile offset of  FIGS.  4 - 6   , beam profile error of  FIG.  6   , etc.) to the user. An input device  755 , such as a keyboard, may be coupled to processing device  740  for communicating at least one of information or command selections to processing device  740 . One or more other user input devices (e.g., a mouse, a trackball or cursor direction keys) may also be used to communicate directional information, to select commands for processing device  740  and to control cursor movements on display  750 . Processing device  740  may be coupled to system memory  735 , storage device  745 , display device  750 , and input device  755  by a bus  786  or other type of control and communication interface. 
     In one implementation, the input device  755  may receive input from a user to perform one or more of calibration or verification of a position of a LINAC  150  (e.g., one or more of calibration or verification of the mechanical positioning system  170  coupled to the LINAC  150 , etc.). The processing device  740  may transmit a first command to the one or more cameras of camera system  110  to acquire a first set of images of phantom  120  while the phantom is not being irradiated, transmit a second command to emit a radiation beam  160  using the LINAC  150 , transmit a third command to the one or more cameras of camera system  110  to acquire a second set of images of a radiation beam  160  incident on the phantom  120 , determine a beam pointing offset based on the first set of images and the second set of images, and transmit a fourth command to calibrate a position of the LINAC  150  based on the beam pointing offset. The processing device  740  may generate a list of beam pointing offsets and a list of beam pointing errors to be displayed via display device  750 . 
     Calibration system  100  may share its database (e.g., data stored in storage  745 ) with a treatment delivery system, such as treatment delivery system  715 , so that it may not be necessary to export from the treatment planning system prior to treatment delivery. Calibration system  100  may be linked to treatment delivery system  715  via a data link  790 , which in one implementation may be a direct link, a LAN link or a WAN link. 
     In one implementation, treatment delivery system  715  includes one or more of a therapeutic or surgical radiation source  304  (e.g., LINAC  150 ) to administer a prescribed radiation dose (e.g., radiation beam  160 ) to a target volume (e.g., patient, phantom  120 , etc.). Treatment delivery system  715  may also include imaging system  765  to perform computed tomography (CT) such as cone beam CT, and images generated by imaging system  765  may be two-dimensional (2D) or three-dimensional (3D). 
     Treatment delivery system  715  may also include a processing device  770  to control radiation source  304 , receive and process data from calibration system  100 , and control a support device such as a support  176 . Processing device  770  may include one or more general-purpose processors (e.g., a microprocessor), a special purpose processor such as a digital signal processor (DSP) or other type of device such as a controller or field programmable gate array (FPGA). The processing device  770  may be configured to execute instructions to position the LINAC  150  (e.g., via calibration of the mechanical positioning system  170 ). 
     Treatment delivery system  715  also includes system memory such as a random access memory (RAM), or other dynamic storage devices, coupled to a processing device, for storing information and instructions to be executed by the processing device. The system memory also may be used for storing temporary variables or other intermediate information during execution of instructions by the processing device  770  (e.g., instructions received from calibration system  100 ) or processing device  740 . The system memory may also include one or more of a read only memory (ROM) or other static storage device for storing static information and instructions for the processing device. 
     Treatment delivery system  715  also includes a storage device, representing one or more storage devices (e.g., a magnetic disk drive or optical disk drive) for storing information and instructions (e.g., instructions received from calibration system  100 ). Processing device  770  may be coupled to radiation source  304  and support  176  by a bus  792  or other type of control and communication interface. 
     Processing device  770  may implement methods to manage timing of diagnostic x-ray imaging in order to maintain alignment of a target with a radiation treatment beam delivered by the radiation source  304 . Processing device  770  may implement methods to manage timing of diagnostic x-ray imaging in order to maintain alignment of a target with a set of radiation treatment beams delivered by the radiation source  304 . 
     In one implementation, the treatment delivery system  715  includes an input device  778  and a display  777  connected with processing device  770  via bus  792 . The display  777  can show trend data that identifies a rate of target movement (e.g., a rate of movement of a target volume that is under treatment). The display  777  can also show a current radiation exposure of a patient and a projected radiation exposure for the patient. The input device  778  can enable a clinician to adjust parameters of a treatment delivery plan during treatment. 
     It should be noted that when data links  786  and  790  are implemented as LAN or WAN connections, at least one of calibration system  100  or treatment delivery system  715  may be in decentralized locations such that the systems may be physically remote from each other. Alternatively, at least one of calibration system  100  or treatment delivery system  715  may be integrated with each other in one or more systems. 
       FIG.  8    illustrates configurations of calibration system  800 , in accordance with implementations of the present disclosure. In one implementation, the calibration system  800  includes camera  110 A coupled to a LINAC  150 . In another implementation, the calibration system  800  includes cameras  110 B that are stationary. LINAC  150  acts as a radiation treatment source. LINAC  150  is coupled to a mechanical positioning system  170  including a robotic arm  172 . In one implementation, the LINAC  150  and camera  110 A are mounted on the end of a robotic arm  172  having multiple (e.g., 5 or more) degrees of freedom in order to position the LINAC  150  to irradiate a pathological anatomy (e.g., target location  320 ) with radiation beams  160  delivered from many angles, in many planes, in an operating volume around a phantom  120 , and to capture images by the camera  110  of the radiation beam  160  incident on the phantom  120 . Treatment may involve beam paths with a single isocenter, multiple isocenters, or with a non-isocentric approach. Alternatively, other types of image guided radiation treatment (IGRT) systems may be used. In one alternative implementation, the LINAC  150  and one or more cameras  110  may be mounted on a gantry based system (e.g., robotic gantry) to provide isocentric beam paths (see  FIG.  9   ). In one particular implementation, the IGRT system is the Vero SBRT System (referred to as TM200 in Japan), a joint product of Mitsubishi Heavy Industries Ltd., of Tokyo Japan and BrainLAB AG of Germany, that utilizes a rigid O-ring based gantry (see  FIG.  9   ). 
     In one implementation, the LINAC  150  and camera  110  may be positioned at multiple different nodes (predefined positions at which the robot stops and radiation may be delivered) during treatment by moving the robotic arm  172 . At the nodes, the LINAC  150  can deliver one or more radiation beams  160  to a target location  320 . The nodes may be arranged in an approximately spherical distribution about a phantom  120 . The particular number of nodes and the number of radiation beams  160  applied at each node may vary as a function of the location and type of pathological anatomy to be treated. For example, the number of nodes may vary from 50 to 300, or more preferably 15 to 100 nodes and the number of treatment beams  114  may vary from 700 to 3200, or more preferably 50 to 300. In one implementation, there are at least 1000 nodes. 
     Referring to  FIG.  8   , calibration system  700 , in accordance with one implementation of the present disclosure, includes fixed cameras  110 B coupled to a processing device  670 . Alternatively, the cameras  110 B may be mobile, in which case they may be repositioned to at least one of maintain alignment with the target location  320 , image the target location  320  from different orientations, or to acquire many images and reconstruct a three-dimensional (3D) cone-beam CT. In one implementation the cameras  110  are not point cameras, but rather camera arrays, as would be appreciated by the skilled artisan. In one implementation, LINAC  150  serves as an imaging source (whether gantry or robot mounted), where the LINAC power level is reduced to acceptable levels for imaging. 
     Calibration system  800  may perform computed tomography (CT) such as cone beam CT, and images generated by calibration system  800  may be two-dimensional (2D) or three-dimensional (3D). The cameras  110 B may be mounted in fixed positions on the ceiling of an operating room and may be aligned to acquire images from two different angular positions (e.g., separated by 90 degrees) to intersect at a machine isocenter (referred to herein as a treatment center, which provides a reference point for positioning the phantom  120  on a support  176  during emitting of radiation beams  160 ). In one implementation, calibration system  800  provides stereoscopic imaging of the target location  320  and the surrounding volume of interest (VOI). In other implementations, calibration system  800  may include more than cameras  110 B, and any of the cameras  110 B may be movable rather than fixed. Phantom  120  may be fabricated from or coated with a scintillating material that converts the radiation beam  160  to visible light (e.g., amorphous silicon), and the light may be converted to a digital image that can be compared with a reference image during an image registration process that transforms a coordinate system of the digital image to a coordinate system of the reference image, as is well known to the skilled artisan. The reference image may be, for example, a digitally reconstructed radiograph (DRR), which is a virtual x-ray image that is generated from a 3D CT image based on simulating the x-ray image formation process by casting rays through the CT image. 
       FIG.  9    illustrates a gantry based intensity modulated radiotherapy (IMRT) system  900 , in accordance with implementations of the present disclosure. In one implementation, the LINAC  150  is mounted on a gantry  903  (e.g., a mechanical positioning system  170 ). In a gantry based system  900 , a radiation source (e.g., a LINAC  150 ) having a head assembly  901  is mounted on a gantry  903  in such a way that they rotate in a plane corresponding to an axial slice of the phantom  120 . Radiation beams  160  are then delivered from several positions on the circular plane of rotation (e.g., around an axis of rotation). In one implementation, one or more cameras  110  may be coupled to the LINAC  150 . In another implementation, cameras are statically located. In IMRT, the camera  110  may acquire a first set of images of the phantom  120  without being irradiated and a second set of images of a radiation beam  160  incident on the phantom  120 . The images may be acquired at different positions of the LINAC  150 . The resulting system generates arbitrarily shaped radiation beams  160  that intersect each other at the isocenter to deliver a dose distribution to the target location. In one implementation, the gantry based system  900  may be a c-arm based system. 
       FIG.  10    illustrates a helical radiation delivery system  1000 , in accordance with implementations of the present disclosure. The helical radiation delivery radiotherapy system  1000  includes a LINAC  150  mounted to a ring gantry  1020 . The ring gantry  1020  has a toroidal shape and the target location  320  (e.g., phantom  120 , a patient, etc.) is moved through a bore of the toroidal shape of the ring gantry  1020 . A central axis passes through the center of the bore. In one implementation, a radiation beam  160  is generated by a LINAC  150  that is mounted to a ring gantry  1020  that rotates around the central axis to deliver the radiation beam  160  to a phantom  120  from various angles. While the radiation beams  160  are being delivered, the phantom  120  is on a treatment couch  1040  (e.g., an adjustable table, support  176 ) and the phantom  120  is simultaneously moved through the bore of the ring gantry  1020  allowing horizontal movement of the radiation beam  160  in relation to the phantom  120  without horizontally moving the LINAC  150  or the phantom  120 . The treatment couch  1040  may move the phantom in a vertical direction so that images may be acquired at different SAD  330 . 
     In some implementations, the LINAC  150  may be mounted to a C-arm gantry in a cantilever-like manner, which rotates the LINAC  150  about the axis passing through the isocenter of the ring gantry  1020 . In other implementations, the LINAC  150  may be mounted to a robotic arm having multiple (e.g., 5 or more) degrees of freedom in order to position the LINAC  150  around the ring gantry  1020  to irradiate the phantom  120  that is moved (e.g., horizontally, vertically) by the treatment couch  1040 . 
     It will be apparent from the foregoing description that aspects of the present disclosure may be embodied, at least in part, in software. That is, the techniques may be carried out in a computer system or other data processing system in response to a processing device  770 , for example, executing sequences of instructions contained in a memory. In various implementations, hardware circuitry may be used in combination with software instructions to implement the present disclosure. Thus, the techniques are not limited to any specific combination of hardware circuitry and software or to any particular source for the instructions executed by the data processing system. In addition, throughout this description, various functions and operations may be described as being performed by or caused by software code to simplify description. However, those skilled in the art will recognize what is meant by such expressions is that the functions result from execution of the code by processing device  770 . 
     A machine-readable medium can be used to store software and data which when executed by a general purpose or special purpose data processing system causes the system to perform various methods of the present disclosure. This executable software and data may be stored in various places including, for example, system memory and storage or any other device that is capable of storing software programs and/or data. Thus, a machine-readable medium includes any mechanism that provides (i.e., stores) information in a form accessible by a machine (e.g., a computer, network device, personal digital assistant, manufacturing tool, any device with a set of one or more processors, etc.). For example, a machine-readable medium includes recordable/non-recordable media such as read only memory (ROM), random access memory (RAM), magnetic disk storage media, optical storage media, flash memory devices, etc. The machine-readable medium may be a non-transitory computer readable storage medium. 
     Unless stated otherwise as apparent from the foregoing discussion, it will be appreciated that terms such as “acquiring,” “determining,” “calibrating,” “mapping,” “outputting,” “applying,” or the like may refer to the actions and processes of a computer system, or similar electronic computing device, that manipulates and transforms data represented as physical (e.g., electronic) quantities within the computer system&#39;s registers and memories into other data similarly represented as physical within the computer system memories or registers or other such information storage or display devices. Implementations of the methods described herein may be implemented using computer software. If written in a programming language conforming to a recognized standard, sequences of instructions designed to implement the methods can be compiled for execution on a variety of hardware platforms and for interface to a variety of operating systems. In addition, implementations of the present disclosure are not described with reference to any particular programming language. It will be appreciated that a variety of programming languages may be used to implement implementations of the present disclosure. 
     It should be noted that the methods and apparatus described herein are not limited to use only with medical diagnostic imaging and treatment. In alternative implementations, the methods and apparatus herein may be used in applications outside of the medical technology field, such as industrial imaging and non-destructive testing of materials. In such applications, for example, “treatment” may refer generally to the effectuation of an operation controlled by the treatment planning system, such as the application of a beam (e.g., radiation, acoustic, etc.) and “target” may refer to a non-anatomical object or area. 
     In the foregoing specification, the disclosure has been described with reference to specific exemplary implementations thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the disclosure as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative sense rather than a restrictive sense.