- Email: [email protected]
The portable virtual simulator
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21. pp 475-482 Copyght
0360.3016191 $3.00 + .W C I99 I Pergamon Press plc
0 Technical Innovations and Notes THE PORTABLE VIRTUAL SIMULATOR GEORGE
W. SHEROUSE,
M.S. AND EDWARD L. CHANEY, PH.D.
Department of Radiation Oncology, University of North Carolina, Chapel Hill, NC 27599-75 I2 The Virtual Simulator is a software tool for support and management of the geometric component of 3-dimensional radiotherapy treatment design. The Virtual Simulator is a software implementation of a physical simulator with additional functionality not currently available on physical simulators. Treatment of a virtual patient, derived from CT or other source, is simulated using the Virtual Simulator in the same way a physical simulator would be used. The intent of this approach is to provide the user with a familiar working environment for radiotherapy treatment design. Key features include an effective and efficient user interface, and the use of computing techniques and software standards which enhance portability to a variety of computer workstations. The Virtual Simulator is implemented in the C programming language using the X Window System, and has been written with the generic UNIX workstation in mind. It has been demonstrated that it can be installed and run without modification on workstations from a number of vendors. 3-dimensional
treatment planning, 3-dimensional
treatment design, Simulation, Virtual simulation.
direct analogy to a physical procedure which the user already understands. This approach makes the system more intuitive by taking advantage of the experience of the user, and promotes acceptance and integration of RTD into the simulation process.
1. INTRODUCTION
A number of software tools and systems for 3-dimensional radiotherapy treatment design (RTD) have been described in recent years. Much of the early development was motivated by the demands for precise localization and dose planning ( 1-3, 8) associated with heavy charged particle beams. More recently, this technology has been used in planning conventional photon and electron therapy (1, 3, 5, 6, 7, 10, 12, 13, 17, 18, 28, 29). In general, most approaches treat geometric design (targeting or beam placement) as a separate step from dose calculation and dose evaluation. The software tool described here, the Virtual Simulator* (Fig. I), represents one approach to design of beam arrangements (15, 24, 25). A key feature of the Virtual Simulator is the user interface (14, 24). To the extent possible, the Virtual Simulator is a software implementation of a physical simulator augmented by additional functionality. In practice, the patient is represented by a 3-dimensional geometrical model, usually derived from CT scans, and the Virtual Simulator is used to simulate treatment the same way a physical simulator would be used. The intent of this approach is to provide the user with a familiar working environment for RTD. For example, radiotherapy professionals can readily anticipate the results of turning the gantry rotation knob of the Virtual Simulator. Thus the operation of the software can be quickly understood by
2.1. Simulator controls The Virtual Simulator currently presents nine graphic display panels to the user. Of those, four provide the essential elements of a physical simulator (Fig. 2). The beam’s-eye view (BEV) panel, similar to the fluoroscopic image in practice, displays the patient anatomy as projected from the radiation source, including overlays of the beam crosshairs (with 1 cm tic marks) and delineator wires showing the current jaw settings. The BEV panel is complemented by two views perpendicular to the central ray which allow the user to see and adjust the depth of isocenter. On a conventional simulator, isocenter depth is usually adjusted using the lateral lasers. The fourth element is the control panel, which contains virtual knobs and digital readouts for table, gantry, and
Reprint requests to: George W. Sherouse, Box 3295 DUMC, Durham, NC 277 10. Accepted for publication 25 January 199 I.
* The term “virtual simulation” is borrowed from the computer science concept of “virtual worlds,” computer simulations of real-world systems and environments.
2. FUNCTIONALITY The following sections describe some aspects of the appearance and functionality of the Virtual Simulator. Underlying design features that are not apparent to the user are discussed in Section 3.
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Fig. I. Example of the Virtual Simulator screen. The large bitmapped color display and the mouse pointing device are supported by the UNIX workstations for which this software is designed. The arrangement of the Virtual Simulator windows on the display is in its default configuration and can be rearranged to suit the preferences of the user.
collimator rotation. The knobs are used by moving the display cursor to the screen knob, pressing the mouse button to “grab” the knob, and using mouse motion to turn the knob. The table position is adjusted using three sliders which control the height and lateral and longitudinal translation. A separate button is provided to lock the table position, thus deactivating any functions requiring table adjustment. The delineator wires are controlled by sliders that operate according to properties of the treatment unit being used. If the unit has asymmetric jaws, the sliders can be operated independently. If not, the sliders are locked together so that, for instance, a change in the left wire automatically moves the right wire to the complementary position.
Fig. 2. Four panels that provide the functionality of physical simulation. The BEV display is on the right and two orthogonal views are on the left. The control panel (bottom) simulates the controls of a physical simulator. The user can interact with the displays in these four panels to manipulate the positions of the patient and treatment unit. Also the buttons along the top of the BEV panel can be used to set beam attributes such as choice of treatment machine, filters, blocking tray, etc.
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Another control is the source-to-surface distance (SSD) slider, which, in addition to displaying the SSD, can also be used to move the patient along the central axis. If the gantry and/or table have previously been rotated, motion along the central axis may require table translation along more than one axis. The Virtual Simulator automatically computes the necessary motions of the table to achieve the new SSD. The details of this calculation have been presented elsewhere (22). Two other panels of the Virtual Simulator (Fig. 3) provide some additional visual feedback as to the position of the treatment machine. Displayed in these panels are iconic representations of the treatment unit as viewed facing the gantry and from above. When machine settings are modified, the icons change in accordance with the new settings. This can be quite useful for avoiding some undeliverable beam orientations. 2.2. Additional controls While the controls described above are necessary for operation of a treatment simulator, they do not constitute an efficient set for many kinds of planning. Consider the often-performed task of placing the isocenter at the desired location in the target volume. Using the BEV panel, a perpendicular view, and the three table sliders, it is relatively straightforward, albeit tedious, to make the successive table motions required to position the center of a target volume at isocenter. The Virtual Simulator allows the much more efficient mechanism of grabbing the desired location of isocenter in the BEV display and “dragging” it to the machine isocenter defined by the crosshairs (Fig. 4). This is analogous to “floating” the tabletop to position the patient with respect to isocenter on a physical simulator. A similar drag in one of the perpendicular views results in 3-dimensional isocenter placement with only two quick mouse motions. If a drag operation in one view corresponds to a compound table motion, the Virtual Simulator manages these calculations automatically.
Fig. 3. The treatment unit icons. These two panels give an en face view of the treatment machine (left) and a view from above (right). As the position of the machine is altered these views update automatically.
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Fig. 4. Using a brain tumor case as an example, the process of positioning isocenter in the center of the target volume is as follows: (a) grab the center of the target volume in the BEV panel (note the cursor position at the target center); (b) drag it to the isocenter marker, setting the table lateral and longitudinal positions (note that the cursor is now aligned with the central ray in the BEV panel): (c) grab the center of the target volume in either of the orthogonal views (lower right panel); (d) drag it to isocenter, setting the table height. Note that the table position and SSD sliders have been updated to reflect the new beam position.
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(4 Fig. 8. DRR for an anterior prostate field. Contours of anatomic structures taken from CT scans and used in the virtual simulation can be superimposed on the DRR as shown. Together with
printed setup instructions and computer-plotted templates for custom blocks, DRR’s make up a set of data similar to that which normally results from conventional simulation.
(b)
The Virtual Simulator also allows for rotation around the viewing axis of any of the three patient views (Fig. 5). In the BEV this is simply a collimator rotation. In the other views it may represent a compound rotation of table, gantry, and collimator. This functionality has two advantages in practice. The first is the ability to rotate the patient in the perpendicular views to find the “best shot” at the target while avoiding critical structures, a task often difficult to perform based solely on a BEV presentation. The second advantage is that it facilitates positioning of multiple coplanar fields in a nontransverse plane. An example might be a set of brain fields in a plane parallel to the base of skull. The user simply positions one of the beams and then rotates the patient in a perpendicular view by the desired angular increment. The complex motion of the table, gantry, and collimator are handled automatically. 2.3. Simultaneous 2- and 3-dimensional display Two other panels of the Virtual Simulator (Fig. 6) are devoted to display of the individual image cross-sections, usually CT scans, from which the 3-dimensional views are formed. A large panel is provided which displays all
Fig. 5. Positioning coplanar beams in an arbitrary plane. In (A) a beam enters the patient’s head 30” superior to AP. By rotating the patient in the appropriate orthogonal view, upper left in (B), beams can be rotated about isocenter in the plane which is tilted 30” superior. The machine motion required to achieve rotation in that plane is complex (note the machine settings on the control panel) but is managed automatically. Opposing beams can be created using a pop-up menu to complete a six-field plan (C).
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Fig. 6. The image viewing panels. The enlarged slice displayed in the panel at left is selected by clicking on the corresponding slice in the panel at right containing the full array of minified images. All images are overlayed with contours displayed in the BEV panel and with beam intersections. The bar on the right of the enlarged slice allows interactive greyscale windowing.
of the original image slices in miniature. This panel doubles as a menu for selecting, by the click of a mouse button, a slice for enlarged display in another window. A greyscale windowing scale is provided along the right side of the large cross-section panel and allows windowing of all the image cross-sections simultaneously. While a treatment beam is being designed, the intersections of the beam with each of the cross-sections are displayed dynamically in the image cross-section windows. The user thus has the ability to appreciate simultaneously both the 2- and 3-dimensional implications of a given beam geometry during the design process. Intersections of beams that have already been designed can also be displayed. The final panel of the Virtual Simulator (Fig. 7) controls aspects of the display. The most important of these is the attributes for each anatomic structure (anastruct) and beam. Those attributes currently include color, visibility (on or off), and display density for anastruct contours. The density control allows the user to specify the fraction of the contours defining an object to be displayed, thus providing a crude form of object transparency. The display panel also provides sliders which control near and far clipping planes in the BEV panel. Near clipping in particular can be very useful for removing clutter in front of a target during portal design. The position of the clipping planes is shown in the orthogonal patient-view panels. 2.4. Portal design The ultimate purpose of a tool such as the Virtual Simulator is to produce a set of beam descriptors which constitute an actual treatment setup. The beam descriptor files automatically generated during virtual simulation contain information such as the treatment machine to be used, the beam geometry and corresponding treatment unit setup parameters, and descriptions of wedges, com-
Fig. 7. The display panel (A) provides control of the way in which various objects are displayed. The “beams” and “anastructs” buttons pop up attribute panels (B) which control the color, visibility, and density ofdisplayed objects. The two sliders at the top in (A) control near and far clipping in the BEV panel. The positions of the near and far clipping planes are shown in the orthogonal patient-view windows as red lines.
pensators, blocks, trays, and other beam modifiers. A row of buttons across the top of the BEV panel allows the user to select or otherwise define these beam attributes, mostly from pop up menus. A number of options are provided for defining the shape of a beam. These include use of the collimator only, use of the mouse to draw a beam outline in the BEV panel, and use of a digitizer to input the outline of a field from some outside source such as a simulation film. This latter option makes it possible to reproduce treatments designed by physical simulation, a function sometimes useful for comparison studies or for incorporating a previous treatment setup into the planning of a new treatment. Other options for copying and/or opposing existing beams are also provided. Once the beam descriptor files are created, they may be used for computation of digitally reconstructed radiographs (DRR’s), the analog of the physical simulator film (26) (Fig. 8). The descriptor data are also required input for dose calculations, and can be used to produce other
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conventional outputs of the simulation process such as patient setup instructions and block templates. 3. IMPLEMENTATION
The following section is primarily concerned with the foundation on which the portable Virtual Simulator is built. The details presented here are transparent to the user but important in meeting the conflicting goals of good performance, wide portability, and adherence to the simulator paradigm. 3.1. Portability
The Virtual Simulator is written in the C programming language. The target computer environment is a UNIX workstation with at least eight planes of color display. Typical machines of this type have graphic displays of approximately 1000 pixels square and are equipped with mouse input devices. The assumed software environment includes a Berkeley-derived UNIX, an implementation of Version 11 of the X Window System (2 1), and, ideally, support for the Network File System (NFS). This environment is widely available today from a great number of equipment vendors and provides for maximum portability of computer-aided design (CAD) software such as the Virtual Simulator. The Virtual Simulator is known to run without modification on equipment from more than 10 vendors including Digital Equipment Corporation, Sun Microsystems, and Stellar Computer. As of this writing more than 50 tapes of the software have been distributed to sites worldwide. 3.2. Use of the X window system The Virtual Simulator uses the X Window System. This design choice provides at least four benefits. The first is portability. Version 11 of the X Window System is a de facto standard, which in practical terms means that manufacturers of modern graphic workstations provide operating systems that incorporate or support X 11. The second advantage is that X provides the basic mechanism required for constructing virtual devices, such as knobs and sliders, as part of a user interface. Once a virtual device is constructed it can be used in any suitable application without having to write new code. A set of such virtual devices called XMT (for X Mini-Toolkit) is the basis of the Virtual Simulator and several other associated software tools. One of the authors (GWS) developed XMT to encapsulate the X Window System functions that, at the computer program level, define virtual knobs, sliders, greyscale windowing controls, and other input devices. A third advantage is that the display layout is directly under the control of the user and can be easily modified as needed. For example, if the user is not actively using the image slice panels, a significant increase in software performance can be realized simply by closing those panels and reopening them when needed. Similarly, the user can
July 1991, Volume 21, Number 2
configure custom screen layouts by closing, rearranging, and resizing windows to suit individual preferences. A fourth advantage of using the X Window System is communication network transparency. It is straightforward to use the processor of one computer in a network to run the Virtual Simulator and have the results displayed on another computer. This allows for greater flexibility in the deployment of computing resources, and provides an extra layer of hardware redundancy. The benefits of the X Window System must be weighed against some distinct disadvantages. Currently the graphics supported by X is inherently 2-dimensional in nature and does not take full advantage the special-purpose hardware available on modern graphic workstations. This shortcoming will be resolved in the next several years with the incorporation of a recognized international standard for 3-dimensional graphics into X, most likely the Programmer’s Hierarchical Interactive Graphics System (PHIGS). 4. DISCUSSION More than 100 patients have undergone virtual simulation. We have developed a high level of confidence in the functionality provided by the Virtual Simulator and in our ability to setup accurately and reproducibly the treatment geometry designed with the Virtual Simulator (19, 23). One factor that strongly influences the usefulness of the Virtual Simulator is the speed of interaction. The Virtual Simulator can be used on slower workstations, but the sluggish interaction inhibits wide exploration of the various possible beam geometries. Use of a more powerful machine improves interactivity and encourages the user to explore alternative treatment geometries, thus taking full advantage of 3-dimensional treatment design. Other factors affecting usefulness are the quality of the wire-loop displays of patient anatomy and the ability of the user to control display properties such as color and density. However, there is need for significant improvement. In particular, the development of a 3-dimensional graphics standard compatible with X will allow use of shaded surface displays (14, 16, 20, 27) and/or volume displays (4, 9, 1 1). Another shortcoming of the current implementation, relative to physical simulation, is the lack of a radiographlike image as a backdrop to the BEV graphical display during portal design. In physical simulation it is standard practice to draw ports on simulator films. In our current implementation, DRR’s are computed only after ports are drawn on the BEV graphical display, primarily because the computation of DRR’s is compute intensive and would compromise the interactivity of the Virtual Simulator. The additional context provided by a greyscale image at the time of portal definition is important, however, and we are investigating methods for computing DRR’s at interactive or near-interactive speeds.
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5. CONCLUSIONS We have implemented a software tool for 3-dimensional radiotherapy beam placement which, to as great an extent as possible, imitates a physical treatment simulator. The Virtual Simulator has been constructed with two primary goals in mind: (a) to provide an intuitive and efficient user interface by using physical simulation as a paradigm for the actions of the software, and (b) to emphasize portability of the software to other computers and other clinics. Our software design incorporates effective mechanisms for achieving both of these design goals. Use of the physical
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simulator model in the Virtual Simulator has proven to be valuable in eliminating user discomfort with the new technology of computer-based 3-dimensional treatment design. As for portability, the current software is portable to a number of commercial UNIX workstations. This portability is enhanced by the decision to eliminate physical input devices such as knob boxes, and to implement those devices virtually, using graphic icons and mouse actions. The portability of the Virtual Simulator and of its virtual devices derives directly from the choice of the industry standard X Window System as part of the software platform.
REFERENCES I. Chen, G. T. Y.; Castro, J. R. Charged particle treatment planning. In: Syllabus: a categorical course in radiation therapy treatment planning. Oak Brook, IL: RSNA; 1986: 101-109. 2. Chen, G. T. Y.; Goitein, M. Treatment planning for heavy charged particle beams. In: Advances in radiation therapy treatment planning, AAPM monograph no. 9. New York: American Institute of Physics; 19835 14-54 1. 3. Chen, G. T. Y.; Verhy, L. J. Charged particle dosimetry. In: Radiation oncology physics- 1986, AAPM monograph no. 15. New York: American Institute of Physics; 1987: 393-426. 4. Drebin, R.: Carpenter, L.: Hanrahan, P. Volume rendering. Comp. Graph. 22(4):65-74: 1988. 5. Fraas, B. A.; M&hart, D. L. 3-D treatment planning: I. Overview of a clinical planning system. In: Proceedings of 9th International Conference of the Use of Computers in Radiation Therapy (ICCR). Scheveningen, The Netherlands: North Holland Publishing Co.; 1987:273-276. 6. Fraas, B. A.; McShan, D. L.; Weeks, K. J. 3-D treatment planning: III. Complete beam’s-eye view planning capabilities. In: Proceedings of 9th International Conference of the Use of Computers in Radiation Therapy (ICCR). Scheveningen, The Netherlands: North Holland Publishing Co.: 1987:193-196. 7. Goitein, M.; Abrams, M. Multi-dimensional treatment planning: I. Delineation of anatomy. Int. J. Radiat. Oncol. Biol. Phys. 9:777-787; 1983. 8. Goitein. M.: Abrams, M.: Rowe& D.; Pollari, H.; Wiles, J. Multi-dimensional treatment planning: II. Beam’s eye-view, back projection, and projection through CT sections. Int. J. Radiat. Oncol. Biol. Phys. 9(6):789-797; 1983. 9. Hoehne, K.-H.; Bomans, M.; Tiede, U.; Riemer, M. Display of multiple 3D objects using the generalized voxel-model. Med. Imaging II, Proc. SPIE 914:850-854; 1988. 10. Kalet, I. J.; Jacky, J. P. A research-oriented treatment planning program system. Comput. Prog. Biomed. 14:85-98; 1982. 11. Levoy, M. Display of surfaces from volume data. IEEE Comp. Graph. Appl. 8(3):29-37; 1988. 12. McShan, D. L.; Silverman, A.; Lanza, D. M.; Reinstein, L. E.; Glicksman, A. S. A computerized three-dimensional treatment planning system utilizing interactive color graphics. Br. J. Radiol. 52(618):478-481; 1979. 13. Mohan, R.; Barest, G.; Brewster, L.; Chui, C.; Kutcher, G.; Laughlin, J.; Fuks, Z. A comprehensive three-dimensional radiation treatment planning system. Int. J. Radiat. Oncol. Biol. Phys. 15(2):48 l-495: 1988.
14. Mosher. C.; Sherouse, G. W.; Chaney, E.; Rosenman, J. 3D Displays and user interface design for a radiation therapy treatment planning CAD tool. Proc. 1987 SPIE 902:64-7 I ; 1988. 15. Mosher. C.; Sherouse, G. W.: Mills, P.; Novins, K.: Pizer, S.; Rosenman, J.; Chaney, E. The Virtual Simulator. In: Proc. 1986 Workshop on Interactive 3D Graphics. Association for Computing Machinery; 1987:37-42. 16. Pizer, S.; Fuchs, H.; Mosher, C.; Lifshitz, L.: Abram, G.; Ramanathan, S.; Whitney, B.: Rosenman, J.; Staab, E.; Chaney, E.; Sherouse, G. W. 3D shaded graphics in radiotherapy and diagnostic imaging. In: National Computer Graphics Association Technical Sessions Proceedings, Anaheim, CA, Vol. III; 1986: 107-I 13. 17. Purdy, J. A.; Wong, J. W.; Harms, W. B.; Drzymala, R. E.; Emami, B.; Matthews, J. W.; Krippner, K.; Ramchander, P. K. Three dimensional radiation treatment planning system. In: Proceedings of 9th International Conference of the Use of Computers in Radiation Therapy (ICCR), Scheveningen, The Netherlands: North Holland Publishing Co.; 1987:277-279. 18. Reinstein, L. E.; McShan, D.; Webber, B. M.; Glicksman, A. S. A computer-assisted three-dimensional treatment planning system. Radiology 127( 1):259-264; 1978. 19. Rosenman. J.; Sailer, S. L.; Sherouse, G. W.; Chaney. E. L.: Tepper, J. E. Virtual simulation: Initial clinical results. Int. J. Radiat. Oncol. Biol. Phys. 20(4):843-85 1: 199 1. 20. Rosenman. J.; Sherouse, G. W.; Fuchs. H.; Pizer. S.; Skinner, A.: Mosher, C.; Novins, K.: Tepper, J. Three-dimensional display techniques in radiation therapy treatment planning. Int. J. Radiat. Oncol. Biol. Phys. 16:263-269: 1989. 2 I. Scheifler, R. W.; Gettys, J. The X Window system. ACM Trans. Graph. 5(2):79- 109; 1986. 22. Sherouse. G. W. Coordinate transformation as a primary representation of radiotherapy beam geometry. Med. Phys. (In press). 23. Sherouse, G. W.; Bourland, D. J.; Reynolds, K.: McMurry, H. L.; Mitchell, T.; Chaney, E. L. Virtual simulation in the clinical setting: some practical considerations. Int. J. Radiat. Oncol. Biol. Phys. 19: 1059- 1065: 1990. 24. Sherouse, G. W.; Mosher, C. User interface issues in radiotherapy CAD software. In: Proceedings of 9th International Conference of the Use of Computers in Radiation Therapy (ICCR). Scheveningen, The Netherlands: North Holland Publishing Co.; 1987:429-432. 25. Sherouse, G. W.; Mosher, C.; Novins, K.; Rosenman, J.; Chaney, E. Virtual simulation: concept and implementation. In: Proceedings of 9th International Conference of the Use
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of Computers in Radiation Therapy (ICCR). Scheveningen, The Netherlands: North Holland Publishing Co.; 1987:433436. 26. Sherouse, G. W.; Novins, K.; Chaney, E. L. Computation of digitally reconstructed radiographs for use in radiotherapy treatment design. Int. J. Radiat. Oncol. Biol. Phys. 18(3): 651-658; 1990. 27. Sontag, M. R.; Reynolds, R. A.; Talton, G. K.; Waxler, G. K.; Wallace, R. E.; Cheng, E. C. 3D graphics in radiation therapy treatment planning. Med. Phys. 13(4):574; 1986.
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28. Sontag, M. R.; Altshuler, M. D.; Bloch, P.; Reynolds, R. A.; Wallace, R. E. Design and clinical implementation of a second generation three-dimensional treatment planning system. In: Proceedings of 9th International Conference of the Use of Computers in Radiation Therapy (ICCR). Scheveningen, The Netherlands: North Holland Publishing Co.; 1987:285-288. 29. Viitanen, J. Development and evaluation of a dose planning system for radiation therapy. Technical Research Center of Finland, Publication no. 52; Espoo, Finland; 1989.
21. pp 475-482 Copyght
0360.3016191 $3.00 + .W C I99 I Pergamon Press plc
0 Technical Innovations and Notes THE PORTABLE VIRTUAL SIMULATOR GEORGE
W. SHEROUSE,
M.S. AND EDWARD L. CHANEY, PH.D.
Department of Radiation Oncology, University of North Carolina, Chapel Hill, NC 27599-75 I2 The Virtual Simulator is a software tool for support and management of the geometric component of 3-dimensional radiotherapy treatment design. The Virtual Simulator is a software implementation of a physical simulator with additional functionality not currently available on physical simulators. Treatment of a virtual patient, derived from CT or other source, is simulated using the Virtual Simulator in the same way a physical simulator would be used. The intent of this approach is to provide the user with a familiar working environment for radiotherapy treatment design. Key features include an effective and efficient user interface, and the use of computing techniques and software standards which enhance portability to a variety of computer workstations. The Virtual Simulator is implemented in the C programming language using the X Window System, and has been written with the generic UNIX workstation in mind. It has been demonstrated that it can be installed and run without modification on workstations from a number of vendors. 3-dimensional
treatment planning, 3-dimensional
treatment design, Simulation, Virtual simulation.
direct analogy to a physical procedure which the user already understands. This approach makes the system more intuitive by taking advantage of the experience of the user, and promotes acceptance and integration of RTD into the simulation process.
1. INTRODUCTION
A number of software tools and systems for 3-dimensional radiotherapy treatment design (RTD) have been described in recent years. Much of the early development was motivated by the demands for precise localization and dose planning ( 1-3, 8) associated with heavy charged particle beams. More recently, this technology has been used in planning conventional photon and electron therapy (1, 3, 5, 6, 7, 10, 12, 13, 17, 18, 28, 29). In general, most approaches treat geometric design (targeting or beam placement) as a separate step from dose calculation and dose evaluation. The software tool described here, the Virtual Simulator* (Fig. I), represents one approach to design of beam arrangements (15, 24, 25). A key feature of the Virtual Simulator is the user interface (14, 24). To the extent possible, the Virtual Simulator is a software implementation of a physical simulator augmented by additional functionality. In practice, the patient is represented by a 3-dimensional geometrical model, usually derived from CT scans, and the Virtual Simulator is used to simulate treatment the same way a physical simulator would be used. The intent of this approach is to provide the user with a familiar working environment for RTD. For example, radiotherapy professionals can readily anticipate the results of turning the gantry rotation knob of the Virtual Simulator. Thus the operation of the software can be quickly understood by
2.1. Simulator controls The Virtual Simulator currently presents nine graphic display panels to the user. Of those, four provide the essential elements of a physical simulator (Fig. 2). The beam’s-eye view (BEV) panel, similar to the fluoroscopic image in practice, displays the patient anatomy as projected from the radiation source, including overlays of the beam crosshairs (with 1 cm tic marks) and delineator wires showing the current jaw settings. The BEV panel is complemented by two views perpendicular to the central ray which allow the user to see and adjust the depth of isocenter. On a conventional simulator, isocenter depth is usually adjusted using the lateral lasers. The fourth element is the control panel, which contains virtual knobs and digital readouts for table, gantry, and
Reprint requests to: George W. Sherouse, Box 3295 DUMC, Durham, NC 277 10. Accepted for publication 25 January 199 I.
* The term “virtual simulation” is borrowed from the computer science concept of “virtual worlds,” computer simulations of real-world systems and environments.
2. FUNCTIONALITY The following sections describe some aspects of the appearance and functionality of the Virtual Simulator. Underlying design features that are not apparent to the user are discussed in Section 3.
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Fig. I. Example of the Virtual Simulator screen. The large bitmapped color display and the mouse pointing device are supported by the UNIX workstations for which this software is designed. The arrangement of the Virtual Simulator windows on the display is in its default configuration and can be rearranged to suit the preferences of the user.
collimator rotation. The knobs are used by moving the display cursor to the screen knob, pressing the mouse button to “grab” the knob, and using mouse motion to turn the knob. The table position is adjusted using three sliders which control the height and lateral and longitudinal translation. A separate button is provided to lock the table position, thus deactivating any functions requiring table adjustment. The delineator wires are controlled by sliders that operate according to properties of the treatment unit being used. If the unit has asymmetric jaws, the sliders can be operated independently. If not, the sliders are locked together so that, for instance, a change in the left wire automatically moves the right wire to the complementary position.
Fig. 2. Four panels that provide the functionality of physical simulation. The BEV display is on the right and two orthogonal views are on the left. The control panel (bottom) simulates the controls of a physical simulator. The user can interact with the displays in these four panels to manipulate the positions of the patient and treatment unit. Also the buttons along the top of the BEV panel can be used to set beam attributes such as choice of treatment machine, filters, blocking tray, etc.
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Another control is the source-to-surface distance (SSD) slider, which, in addition to displaying the SSD, can also be used to move the patient along the central axis. If the gantry and/or table have previously been rotated, motion along the central axis may require table translation along more than one axis. The Virtual Simulator automatically computes the necessary motions of the table to achieve the new SSD. The details of this calculation have been presented elsewhere (22). Two other panels of the Virtual Simulator (Fig. 3) provide some additional visual feedback as to the position of the treatment machine. Displayed in these panels are iconic representations of the treatment unit as viewed facing the gantry and from above. When machine settings are modified, the icons change in accordance with the new settings. This can be quite useful for avoiding some undeliverable beam orientations. 2.2. Additional controls While the controls described above are necessary for operation of a treatment simulator, they do not constitute an efficient set for many kinds of planning. Consider the often-performed task of placing the isocenter at the desired location in the target volume. Using the BEV panel, a perpendicular view, and the three table sliders, it is relatively straightforward, albeit tedious, to make the successive table motions required to position the center of a target volume at isocenter. The Virtual Simulator allows the much more efficient mechanism of grabbing the desired location of isocenter in the BEV display and “dragging” it to the machine isocenter defined by the crosshairs (Fig. 4). This is analogous to “floating” the tabletop to position the patient with respect to isocenter on a physical simulator. A similar drag in one of the perpendicular views results in 3-dimensional isocenter placement with only two quick mouse motions. If a drag operation in one view corresponds to a compound table motion, the Virtual Simulator manages these calculations automatically.
Fig. 3. The treatment unit icons. These two panels give an en face view of the treatment machine (left) and a view from above (right). As the position of the machine is altered these views update automatically.
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Fig. 4. Using a brain tumor case as an example, the process of positioning isocenter in the center of the target volume is as follows: (a) grab the center of the target volume in the BEV panel (note the cursor position at the target center); (b) drag it to the isocenter marker, setting the table lateral and longitudinal positions (note that the cursor is now aligned with the central ray in the BEV panel): (c) grab the center of the target volume in either of the orthogonal views (lower right panel); (d) drag it to isocenter, setting the table height. Note that the table position and SSD sliders have been updated to reflect the new beam position.
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(4 Fig. 8. DRR for an anterior prostate field. Contours of anatomic structures taken from CT scans and used in the virtual simulation can be superimposed on the DRR as shown. Together with
printed setup instructions and computer-plotted templates for custom blocks, DRR’s make up a set of data similar to that which normally results from conventional simulation.
(b)
The Virtual Simulator also allows for rotation around the viewing axis of any of the three patient views (Fig. 5). In the BEV this is simply a collimator rotation. In the other views it may represent a compound rotation of table, gantry, and collimator. This functionality has two advantages in practice. The first is the ability to rotate the patient in the perpendicular views to find the “best shot” at the target while avoiding critical structures, a task often difficult to perform based solely on a BEV presentation. The second advantage is that it facilitates positioning of multiple coplanar fields in a nontransverse plane. An example might be a set of brain fields in a plane parallel to the base of skull. The user simply positions one of the beams and then rotates the patient in a perpendicular view by the desired angular increment. The complex motion of the table, gantry, and collimator are handled automatically. 2.3. Simultaneous 2- and 3-dimensional display Two other panels of the Virtual Simulator (Fig. 6) are devoted to display of the individual image cross-sections, usually CT scans, from which the 3-dimensional views are formed. A large panel is provided which displays all
Fig. 5. Positioning coplanar beams in an arbitrary plane. In (A) a beam enters the patient’s head 30” superior to AP. By rotating the patient in the appropriate orthogonal view, upper left in (B), beams can be rotated about isocenter in the plane which is tilted 30” superior. The machine motion required to achieve rotation in that plane is complex (note the machine settings on the control panel) but is managed automatically. Opposing beams can be created using a pop-up menu to complete a six-field plan (C).
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Fig. 6. The image viewing panels. The enlarged slice displayed in the panel at left is selected by clicking on the corresponding slice in the panel at right containing the full array of minified images. All images are overlayed with contours displayed in the BEV panel and with beam intersections. The bar on the right of the enlarged slice allows interactive greyscale windowing.
of the original image slices in miniature. This panel doubles as a menu for selecting, by the click of a mouse button, a slice for enlarged display in another window. A greyscale windowing scale is provided along the right side of the large cross-section panel and allows windowing of all the image cross-sections simultaneously. While a treatment beam is being designed, the intersections of the beam with each of the cross-sections are displayed dynamically in the image cross-section windows. The user thus has the ability to appreciate simultaneously both the 2- and 3-dimensional implications of a given beam geometry during the design process. Intersections of beams that have already been designed can also be displayed. The final panel of the Virtual Simulator (Fig. 7) controls aspects of the display. The most important of these is the attributes for each anatomic structure (anastruct) and beam. Those attributes currently include color, visibility (on or off), and display density for anastruct contours. The density control allows the user to specify the fraction of the contours defining an object to be displayed, thus providing a crude form of object transparency. The display panel also provides sliders which control near and far clipping planes in the BEV panel. Near clipping in particular can be very useful for removing clutter in front of a target during portal design. The position of the clipping planes is shown in the orthogonal patient-view panels. 2.4. Portal design The ultimate purpose of a tool such as the Virtual Simulator is to produce a set of beam descriptors which constitute an actual treatment setup. The beam descriptor files automatically generated during virtual simulation contain information such as the treatment machine to be used, the beam geometry and corresponding treatment unit setup parameters, and descriptions of wedges, com-
Fig. 7. The display panel (A) provides control of the way in which various objects are displayed. The “beams” and “anastructs” buttons pop up attribute panels (B) which control the color, visibility, and density ofdisplayed objects. The two sliders at the top in (A) control near and far clipping in the BEV panel. The positions of the near and far clipping planes are shown in the orthogonal patient-view windows as red lines.
pensators, blocks, trays, and other beam modifiers. A row of buttons across the top of the BEV panel allows the user to select or otherwise define these beam attributes, mostly from pop up menus. A number of options are provided for defining the shape of a beam. These include use of the collimator only, use of the mouse to draw a beam outline in the BEV panel, and use of a digitizer to input the outline of a field from some outside source such as a simulation film. This latter option makes it possible to reproduce treatments designed by physical simulation, a function sometimes useful for comparison studies or for incorporating a previous treatment setup into the planning of a new treatment. Other options for copying and/or opposing existing beams are also provided. Once the beam descriptor files are created, they may be used for computation of digitally reconstructed radiographs (DRR’s), the analog of the physical simulator film (26) (Fig. 8). The descriptor data are also required input for dose calculations, and can be used to produce other
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conventional outputs of the simulation process such as patient setup instructions and block templates. 3. IMPLEMENTATION
The following section is primarily concerned with the foundation on which the portable Virtual Simulator is built. The details presented here are transparent to the user but important in meeting the conflicting goals of good performance, wide portability, and adherence to the simulator paradigm. 3.1. Portability
The Virtual Simulator is written in the C programming language. The target computer environment is a UNIX workstation with at least eight planes of color display. Typical machines of this type have graphic displays of approximately 1000 pixels square and are equipped with mouse input devices. The assumed software environment includes a Berkeley-derived UNIX, an implementation of Version 11 of the X Window System (2 1), and, ideally, support for the Network File System (NFS). This environment is widely available today from a great number of equipment vendors and provides for maximum portability of computer-aided design (CAD) software such as the Virtual Simulator. The Virtual Simulator is known to run without modification on equipment from more than 10 vendors including Digital Equipment Corporation, Sun Microsystems, and Stellar Computer. As of this writing more than 50 tapes of the software have been distributed to sites worldwide. 3.2. Use of the X window system The Virtual Simulator uses the X Window System. This design choice provides at least four benefits. The first is portability. Version 11 of the X Window System is a de facto standard, which in practical terms means that manufacturers of modern graphic workstations provide operating systems that incorporate or support X 11. The second advantage is that X provides the basic mechanism required for constructing virtual devices, such as knobs and sliders, as part of a user interface. Once a virtual device is constructed it can be used in any suitable application without having to write new code. A set of such virtual devices called XMT (for X Mini-Toolkit) is the basis of the Virtual Simulator and several other associated software tools. One of the authors (GWS) developed XMT to encapsulate the X Window System functions that, at the computer program level, define virtual knobs, sliders, greyscale windowing controls, and other input devices. A third advantage is that the display layout is directly under the control of the user and can be easily modified as needed. For example, if the user is not actively using the image slice panels, a significant increase in software performance can be realized simply by closing those panels and reopening them when needed. Similarly, the user can
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configure custom screen layouts by closing, rearranging, and resizing windows to suit individual preferences. A fourth advantage of using the X Window System is communication network transparency. It is straightforward to use the processor of one computer in a network to run the Virtual Simulator and have the results displayed on another computer. This allows for greater flexibility in the deployment of computing resources, and provides an extra layer of hardware redundancy. The benefits of the X Window System must be weighed against some distinct disadvantages. Currently the graphics supported by X is inherently 2-dimensional in nature and does not take full advantage the special-purpose hardware available on modern graphic workstations. This shortcoming will be resolved in the next several years with the incorporation of a recognized international standard for 3-dimensional graphics into X, most likely the Programmer’s Hierarchical Interactive Graphics System (PHIGS). 4. DISCUSSION More than 100 patients have undergone virtual simulation. We have developed a high level of confidence in the functionality provided by the Virtual Simulator and in our ability to setup accurately and reproducibly the treatment geometry designed with the Virtual Simulator (19, 23). One factor that strongly influences the usefulness of the Virtual Simulator is the speed of interaction. The Virtual Simulator can be used on slower workstations, but the sluggish interaction inhibits wide exploration of the various possible beam geometries. Use of a more powerful machine improves interactivity and encourages the user to explore alternative treatment geometries, thus taking full advantage of 3-dimensional treatment design. Other factors affecting usefulness are the quality of the wire-loop displays of patient anatomy and the ability of the user to control display properties such as color and density. However, there is need for significant improvement. In particular, the development of a 3-dimensional graphics standard compatible with X will allow use of shaded surface displays (14, 16, 20, 27) and/or volume displays (4, 9, 1 1). Another shortcoming of the current implementation, relative to physical simulation, is the lack of a radiographlike image as a backdrop to the BEV graphical display during portal design. In physical simulation it is standard practice to draw ports on simulator films. In our current implementation, DRR’s are computed only after ports are drawn on the BEV graphical display, primarily because the computation of DRR’s is compute intensive and would compromise the interactivity of the Virtual Simulator. The additional context provided by a greyscale image at the time of portal definition is important, however, and we are investigating methods for computing DRR’s at interactive or near-interactive speeds.
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5. CONCLUSIONS We have implemented a software tool for 3-dimensional radiotherapy beam placement which, to as great an extent as possible, imitates a physical treatment simulator. The Virtual Simulator has been constructed with two primary goals in mind: (a) to provide an intuitive and efficient user interface by using physical simulation as a paradigm for the actions of the software, and (b) to emphasize portability of the software to other computers and other clinics. Our software design incorporates effective mechanisms for achieving both of these design goals. Use of the physical
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simulator model in the Virtual Simulator has proven to be valuable in eliminating user discomfort with the new technology of computer-based 3-dimensional treatment design. As for portability, the current software is portable to a number of commercial UNIX workstations. This portability is enhanced by the decision to eliminate physical input devices such as knob boxes, and to implement those devices virtually, using graphic icons and mouse actions. The portability of the Virtual Simulator and of its virtual devices derives directly from the choice of the industry standard X Window System as part of the software platform.
REFERENCES I. Chen, G. T. Y.; Castro, J. R. Charged particle treatment planning. In: Syllabus: a categorical course in radiation therapy treatment planning. Oak Brook, IL: RSNA; 1986: 101-109. 2. Chen, G. T. Y.; Goitein, M. Treatment planning for heavy charged particle beams. In: Advances in radiation therapy treatment planning, AAPM monograph no. 9. New York: American Institute of Physics; 19835 14-54 1. 3. Chen, G. T. Y.; Verhy, L. J. Charged particle dosimetry. In: Radiation oncology physics- 1986, AAPM monograph no. 15. New York: American Institute of Physics; 1987: 393-426. 4. Drebin, R.: Carpenter, L.: Hanrahan, P. Volume rendering. Comp. Graph. 22(4):65-74: 1988. 5. Fraas, B. A.; M&hart, D. L. 3-D treatment planning: I. Overview of a clinical planning system. In: Proceedings of 9th International Conference of the Use of Computers in Radiation Therapy (ICCR). Scheveningen, The Netherlands: North Holland Publishing Co.; 1987:273-276. 6. Fraas, B. A.; McShan, D. L.; Weeks, K. J. 3-D treatment planning: III. Complete beam’s-eye view planning capabilities. In: Proceedings of 9th International Conference of the Use of Computers in Radiation Therapy (ICCR). Scheveningen, The Netherlands: North Holland Publishing Co.: 1987:193-196. 7. Goitein, M.; Abrams, M. Multi-dimensional treatment planning: I. Delineation of anatomy. Int. J. Radiat. Oncol. Biol. Phys. 9:777-787; 1983. 8. Goitein. M.: Abrams, M.: Rowe& D.; Pollari, H.; Wiles, J. Multi-dimensional treatment planning: II. Beam’s eye-view, back projection, and projection through CT sections. Int. J. Radiat. Oncol. Biol. Phys. 9(6):789-797; 1983. 9. Hoehne, K.-H.; Bomans, M.; Tiede, U.; Riemer, M. Display of multiple 3D objects using the generalized voxel-model. Med. Imaging II, Proc. SPIE 914:850-854; 1988. 10. Kalet, I. J.; Jacky, J. P. A research-oriented treatment planning program system. Comput. Prog. Biomed. 14:85-98; 1982. 11. Levoy, M. Display of surfaces from volume data. IEEE Comp. Graph. Appl. 8(3):29-37; 1988. 12. McShan, D. L.; Silverman, A.; Lanza, D. M.; Reinstein, L. E.; Glicksman, A. S. A computerized three-dimensional treatment planning system utilizing interactive color graphics. Br. J. Radiol. 52(618):478-481; 1979. 13. Mohan, R.; Barest, G.; Brewster, L.; Chui, C.; Kutcher, G.; Laughlin, J.; Fuks, Z. A comprehensive three-dimensional radiation treatment planning system. Int. J. Radiat. Oncol. Biol. Phys. 15(2):48 l-495: 1988.
14. Mosher. C.; Sherouse, G. W.; Chaney, E.; Rosenman, J. 3D Displays and user interface design for a radiation therapy treatment planning CAD tool. Proc. 1987 SPIE 902:64-7 I ; 1988. 15. Mosher. C.; Sherouse, G. W.: Mills, P.; Novins, K.: Pizer, S.; Rosenman, J.; Chaney, E. The Virtual Simulator. In: Proc. 1986 Workshop on Interactive 3D Graphics. Association for Computing Machinery; 1987:37-42. 16. Pizer, S.; Fuchs, H.; Mosher, C.; Lifshitz, L.: Abram, G.; Ramanathan, S.; Whitney, B.: Rosenman, J.; Staab, E.; Chaney, E.; Sherouse, G. W. 3D shaded graphics in radiotherapy and diagnostic imaging. In: National Computer Graphics Association Technical Sessions Proceedings, Anaheim, CA, Vol. III; 1986: 107-I 13. 17. Purdy, J. A.; Wong, J. W.; Harms, W. B.; Drzymala, R. E.; Emami, B.; Matthews, J. W.; Krippner, K.; Ramchander, P. K. Three dimensional radiation treatment planning system. In: Proceedings of 9th International Conference of the Use of Computers in Radiation Therapy (ICCR), Scheveningen, The Netherlands: North Holland Publishing Co.; 1987:277-279. 18. Reinstein, L. E.; McShan, D.; Webber, B. M.; Glicksman, A. S. A computer-assisted three-dimensional treatment planning system. Radiology 127( 1):259-264; 1978. 19. Rosenman. J.; Sailer, S. L.; Sherouse, G. W.; Chaney. E. L.: Tepper, J. E. Virtual simulation: Initial clinical results. Int. J. Radiat. Oncol. Biol. Phys. 20(4):843-85 1: 199 1. 20. Rosenman. J.; Sherouse, G. W.; Fuchs. H.; Pizer. S.; Skinner, A.: Mosher, C.; Novins, K.: Tepper, J. Three-dimensional display techniques in radiation therapy treatment planning. Int. J. Radiat. Oncol. Biol. Phys. 16:263-269: 1989. 2 I. Scheifler, R. W.; Gettys, J. The X Window system. ACM Trans. Graph. 5(2):79- 109; 1986. 22. Sherouse. G. W. Coordinate transformation as a primary representation of radiotherapy beam geometry. Med. Phys. (In press). 23. Sherouse, G. W.; Bourland, D. J.; Reynolds, K.: McMurry, H. L.; Mitchell, T.; Chaney, E. L. Virtual simulation in the clinical setting: some practical considerations. Int. J. Radiat. Oncol. Biol. Phys. 19: 1059- 1065: 1990. 24. Sherouse, G. W.; Mosher, C. User interface issues in radiotherapy CAD software. In: Proceedings of 9th International Conference of the Use of Computers in Radiation Therapy (ICCR). Scheveningen, The Netherlands: North Holland Publishing Co.; 1987:429-432. 25. Sherouse, G. W.; Mosher, C.; Novins, K.; Rosenman, J.; Chaney, E. Virtual simulation: concept and implementation. In: Proceedings of 9th International Conference of the Use
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of Computers in Radiation Therapy (ICCR). Scheveningen, The Netherlands: North Holland Publishing Co.; 1987:433436. 26. Sherouse, G. W.; Novins, K.; Chaney, E. L. Computation of digitally reconstructed radiographs for use in radiotherapy treatment design. Int. J. Radiat. Oncol. Biol. Phys. 18(3): 651-658; 1990. 27. Sontag, M. R.; Reynolds, R. A.; Talton, G. K.; Waxler, G. K.; Wallace, R. E.; Cheng, E. C. 3D graphics in radiation therapy treatment planning. Med. Phys. 13(4):574; 1986.
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28. Sontag, M. R.; Altshuler, M. D.; Bloch, P.; Reynolds, R. A.; Wallace, R. E. Design and clinical implementation of a second generation three-dimensional treatment planning system. In: Proceedings of 9th International Conference of the Use of Computers in Radiation Therapy (ICCR). Scheveningen, The Netherlands: North Holland Publishing Co.; 1987:285-288. 29. Viitanen, J. Development and evaluation of a dose planning system for radiation therapy. Technical Research Center of Finland, Publication no. 52; Espoo, Finland; 1989.