to display reconstructed anatomical surface ... to incoming radiation beams is usedto demonst- rate the .... that a "beam's eye" view can be displayed relative.
AN INTERACTIVE THREE-DIMENSIONAL RADIATION TREATMENT PLANNING SYSTEM
D.L. McShan, Ph.D. and A. S. Glicksman, M.D. Department of Radiation Oncology, Rhode Island Hospital Section of Radiation Medicine, Brown University Providence, Rhode Island in the Department of Radiation Oncology utilizing its own computers. This includes a Digital A motion picture film has been produced to Equipment Corporation PDP 11/45 which has 80K words demonstrate a new interactive three-dimensional of core memory, floating point hardware and assoradiation treatment planning system. The new ciated peripherals including a large 116 megabyte system uses computer generated color graphics storage system. The color image display sysdisk to display reconstructed anatomical surface tem, made by DeAnza, Inc. is interfaced directly to data obtained from multi-level tomographic the PDP 11/45. The display system has an image images. Graphic simulation of anatomy relative of 256 x 256 points. Each point can be matrix to incoming radiation beams is used to demonststored with a number from Oto 4095. An intensity rate the potentials of the system for intertransformation unit associated with the hardware actively defining external beam parameters. The provides a "look-up" table for assigning color to systems capabilities for displaying dose disthe pixel values. tributions simultaneously with the contour reA console for interfacing with the program construction are portrayed together with an has been constructed with four potentiosoftware interactive dose readout system. Several four push bottons, and a joy stick. The meters, methods of presenting three-dimensional dose program is designed so that the treatment planner distributions, including dose distributions on uses this console for interaction with the display organ surfaces as well as construction of isosystem. The software for the new planning system dose surfaces, are demonstrated and explored for is written in Fortran and operates in overlays usefulness in evaluating intended radiation requiring 16K words of less of memory. distributions. For 3-D planning, it is first necessary to localize the volume to be treated.' This localization can be achieved by using a set of cross-sectional images obtained with a transverse axial tomograph with a computerized axial tomograph. For the or A motion picture film has been produced to former, the target area and important structures illustrate the interactive computerized threeare identified on the x-ray film and entered into dimensional radiation planning system developed in the computer using a sonic digitizer. For CT data, the Department of Radiation Oncology at Rhode transverse images are retrieved from magnetic Island Hospital. This treatment planning system tapes and the external and internal structures are from conventional sysplanning differs primarily identified through the interactive use of the contems in its ability to deal with complex treatment trol console. volumes, such as the esophagus and pelvis where the The next step is to have the computer display relationship of the tumor volume and the internal these contours. For demonstration, a tumor of the anatomy may vary within the treatment volume. The one-third of the esophagus is planned for upper conventional approach to treatment planning is to treatment. A set of five transverse levels taken make the assumption that there is symmetry throughfrom the thoracic inlet to the mid-thoracic region out and- to use `contours -taken just through the center obtained. The computer can be instructed to was of the tumor volume. Such planning systems are not plot individual levels with the various anatomical easily applied in the regions where these symmetry landmarks distinguished by color. assumptions are not valid. Another type of display that can be selected There are three basic goals which have been with the new planning system is perspective reconset for the development of the new planning system. struction of all five levels. This display allows The first goal is to provide a method of reconstruthe entire set of contours to be examined in their ction and display of tumor volumes in relation to true spatial relationship. The program removes internal anatomy so that both the tumor and the norline segments that are hidden by other planes to mal surrounding tissue can be visualized partimake the orientation more apparent. By expanding cularly structures of critical sensitivity. The the separation of the planes or by changing the second goal is to provide interactive computer hidden areas can be readily inspected. orientation, graphics to simulate irradiation source arrangeThis can also be achi eved byselectively removing some ments. The final goal is to produce graphic disof the transverse planes. However, in so doing, play of dose distributions throughout the volume some of the useful anatomical information may have of interest. been removed from the image. This information can The interactive 3-D system has been developed 72
0195-4210/80/0000-0072$00.75 C 1980 IEEE
cursor can be used to obtain the dose at any point on the plane. In order to evaluate the distribution
be added back by displaying only important anatomical surfaces between the planes. Such reconstruction is achieved by using the full complement of input data. The outlines at each level are fitted with a uniform mesh of points. The points are aligned and connected with a smooth curve. The smooth curve is approximated by a second mesh of points resulting in a matrix of three-dimensional coordinates which map out the surface. These surfaces are shown as solid shaped structures. For more complex regions, the presentation of the surfaces allows easy perception of various anatomical structures. A display of pelvic region showing bone, lymph nodes, rectum and bladder surfaces demonstrates such a region. For these cases, individual contours can be flashed or blinked for easy recognition. Because of the limitation in the computer memory and computational times, at present,several minutes are required to generate a full display of contour surfaces. Time lapsed photography, achieved with a computer controlled camera, is used to show rotation of the reconstructed surfaces. To illustrate treatment planning for external therapy, the case of the esophagus is used. Three methods of interactively setting up proposed fields are available. The first method uses the multilevel display on which can be simulated the beam paths. Three fields are set up: An AP and two laterals. The second method allows anatomical surfaces to be displayed together with a proposed therapy beam. Portions of the surface areas involved in the radiation field are indicated. This display is used to demonstrate the requirements for appropriate placement of a lateral field which will irradiate the full volume of the esophagus while minimizing the dose to the lungs and the spinal cord. The third method of simulation is a beam portal simulation. In this mode, the films show that a "beam's eye" view can be displayed relative to the internal anatomy. The individual beam parameters can be interactively adjusted to achieve an acceptable treatment. For the actual external beam calculations, the planner enters other data such as machine type and relative weightings of each field. The dose is computed for each transverse plane and stored on disk for later retrieval and display. For 3-D palnning, the distributions must be evaluated for the entire irradiation volume. Four methods of depicting dose distributions for the 3-D planning are available. The first method of display, the single plan isodose plot, is the most commonly used method in radiation therapy departments, and a typical plot for the midplane is shown. A different form of isodose display for single plane can be produlced by assigning colors to various levels of dose. This is shown by shades of red which are proportional to the dose. With the dose values loaded, an interactive dose readout option can be selected; this allows a band of color to be defined between anytwo dose limits. To illustrate this readout mode, a blue band of 5 percent width is defined. The limits of this band are swept from 1 percent to 110 percent dose value. Adjustments of this band can be used to determine relative dose values and dose gradients. In addition to the band readout, a
throughout the treatment volume, this technique of dose readout can be applied individually to the other calculational planes. The second method of graphic representation of dose uses the multi-level display. The assignment of color to dose values is similar to that done for the single plane. Using the interactive dose readout option, detailed evaluation on all planes is possible. A band of 5 percent width is swept through the range of dose values. The band adjustments are highly interactive and simultaneous for all planes. The third technique for dose evaluation displays isodose surfaces and their intersections with the anatomy. In this mode, the anatomical surfaces are plotted using shades of white so that the isodoes surfaces can be displayed in color. The computer is instructed to plot the back, (normally hidden) surface, of the 90 percent isodose volume in red and then the back surface of the 70 percent isodose volume in green. These are generated in the same manner as the anatomy surfaces except isodose curves are used instead of contour lines. The image is rotated to show a different perspective. Finally, the front of 90 percent isodose surface is plotted as well as the front of the 70 percent isodose surface. For the 70 percent surface, only a random number of surface points are plotted in order to have the 90 percent surface still visible. This image is again rotated. The final method for dose evaluation is a display of dose distribution on the anatomical surfaces. Currently, this is possible only on our intracavitary-interstitial calculational program. In fact, this method is probably best suited for this type of treatment, since the minimum and maximum doses to neighboring anatomy lie on the surfaces of these volumes. The case of concern here is cancer of the cervix treated by an intracavitary application of radioactive sources. The display illustrates a single transverse level which passes through the base of the cervix. The bladder and rectal surfaces as well as the radiation sources are plotted. The dose is calculated on a small section of the transverse plane and on a sagittal plane. Both of these planes pass through the base of the cervix. The dose on the bladder and rectal surfaces is also displayed. With the interactive dose readout mode the dose distribution can be evaluated throughout the volume. This is demonstrated with a blue band which is swept through the range of dose values. The cursor is also used to readout dose at any point on these surfaces. High radiation areas can be identified on the bladder and rectal surface. These areas of high dose are not appreciated on the sagittal and transverse planes and would have been missed using these planes alone. The surface dose readout gives this information readily. Many components of the treatment planning system, which are demonstrated in the film have been in use over the past three years at Rhode Island Hospital. This use has pointed to several areas of the treatment planning process which can be expanded or improved. For instance, the speed-with which anatomical
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data is acquired and entered into the computer can be a limiting factor to the total treatment planning time; therefore, improved handling of the localization process is required. We believe that increased use of CAT scan data with automated pattern recognition of contour boundaries provides a new solution to this problem. What is needed throughout is improved computational speeds, which is essential to increase the interactive mode of the planning system. Of equal importance, is correcting the dose computations for imhomogeneities. CAT scans with derived electron density data provide a solution to achieving accurate dose computations. The ability to graphically simulate beam portals and therefore better define the exposed areas leads to several directions for additional work. The beam simulation mode could allow the user to interactively design beam shaping blocks. In addition, the anatomical surface data derived in the planning process could be used to design tissue compensators. We are now acquiring more experience with the 3-D system for complex treatment areas in the head and neck and pelvis and developing computational techniques for the display of combination of external radiation plus intracavitary or interstitial sources. Finally, the three dimensional presentation of tumor volume and normal anatomy allows a lucid appreciation of the requirements for appropriate local and regional tumor irradiation in a way that By single plane planning cannot achieve. doing this in an interactive mode, the rapid review of a large number of alternative plans is possible until the most suitable is achieved. As an educational tool, it is exceptional. But most important, we believe this will enhance our effective use of radiotherapy in the management of cancer.
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