lm. J Rodrarm Oncology Bml. Phyr Vol. 18, pp. 1485-1494 Printed in the U.S.A. All rights reserved.
Copyright
0360-3016/W $3.00 + .oO 0 1990 Pergamon Press plc
??Technical Innovations and Notes
FULL INTEGRATION OF THE BEAM’S EYE VIEW CONCEPT INTO COMPUTERIZED TREATMENT PLANNING D.
L. MCSHAN,
Department
of Radiation
PH.D.,
B. A. FRAASS,
Oncology,
University
PH.D.
of Michigan
AND
A.
S. LICHTER,
Medical Center,
M.D.
Ann Arbor, MI 48 109
A complete set of beam’s eye view (BEV) and beam portal design features have been integrated into a computerized 3-dimensional radiotherapy treatment planning system. Among the features implemented is the ability to mix BEV graphics with gray-scale images such as simulator and verification radiographs, and digital reconstructed radiographs. Image processing techniques have been developed to both enhance verification images and to detect radiation field boundaries. These portal simulation and presentation techniques are being used clinically to design and verify radiation fields with manual or automatically-designed field shaping blocks. The ability to perform computer dose calculations for planes which are parallel or perpendicular to a specified beam’s central axis is available and this feature has also proven useful for treatment plan evaluation and optimization. Finally, direct comparison of computergenerated portal images with actual simulation and verification radiographs is also possible. These techniques allow the direct integration of “CT-directed treatment planning” with block design, simulator films and port films, and other Beam’s Eye View-type displays. Computerized
treatment planning, Beam’s eye view, Verification, Simulation, 3-D.
INTRODUCTION importance of radiographic imaging of treatment fields is well established for high-energy external beam radiation therapy. Radiographic images from simulations are routinely used to localize the target volume and neighboring anatomy and to simulate proposed treatment ports, while port films from treatment machines are used to verify the actual setup of planned treatment fields. Port films often represent the only direct clinical confirmation of the actual placement of external radiation beams relative to the targeted volume and neighboring anatomy. Simulator and port radiographs are, in fact, “Beams Eye Views” (BEV) of the treatment area. Currently, computerized radiation therapy planning systems make little use of these various BEV images, except to specify the shape of irregular fields so that the equivalent square and dose at a few specific points can be calculated. There is little or no transfer of information between the CT-based treatment plan and the simulator and port films which are used to define field shaping and to verify plan accuracy. Neither simulator and port films, nor the CT treatment plan contain sufficient information about the treatment plan that the evaluation of the plan can be made solely on the basis of any one of these image
sets alone. Some qualitative comparison or “integration” of the two different sets of information is required. Usually this is done by “eyeballing” the CT treatment plan and judging whether the simulator film and port films are consistent with respect to each other. It is our contention that a computerized integration of CT plans with BEV films is critically necessary to fully realize the capabilities of today’s sophisticated treatment planning systems. The present work describes a number of ways in which the BEV radiographs and other information can be quantitatively integrated with the CT information used during treatment planning. Simulator radiographs and portal verification films are video digitized, and used throughout all phases of treatment planning. The images can be correlated with multi-level CT (or MR (4, 19)) image data, and used with computer graphics to produce a BEV representation of the treatment volume (6). This display can then be used to design the treatment fields and shielding blocks. Three-dimensional dose calculations are performed in planes that are tied to the aforementioned BEV views. This allows the direct comparison of the dose distribution with the planned fields (as illustrated by simulator radiographs). The ability to overlay and compare planned simulator films, CT-determined target and normal structure volumes, treatment planning dose calcu-
Presented in part at the 1986 meeting of the American Association of Physicists in Medicine, Lexington, Kentucky, August 3, 1986. Reprint requests to: D. L. McShan, Ph.D., Department of
Radiation Therapy, University of Michigan Hospitals, AGH Room B2C490, Box 0010, 1500 E. Medical Center Dr., Ann Arbor, MI 48 109. Accepted for publication 20 December 1989.
The
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lations, and actual verification him images. leads to an integrated system which brings us improved accuracy and flexibility in treatment planning.
METHODS
AND
MATERIALS
The treatment planning system which is used for this work is a fully 3-dimensional system which has been developed at our institution. The hardware relevant to this work includes a computer* with 64 MB of memory and over 2 gigabytes of disk storage. The computer is coupled to an imaging system’ equipped with four independent workstations each with 5 12 X 5 12 X 32 bits of display memory. The imaging system is also equipped with a video digitizer which is used to digitize radiographic film images, and with a digital video processor which performs arithmetic on an entire image in one frame time ($ second). An electromagnetic digitizer (23) is used to enter block shapes which have been drawn on simulator films into the system, and a computer controlled pen plotter is used to output block shapes when blocks are designed using the BEV features of the planning system. The 3-D treatment planning software (called “UMPlan”) has been developed in-house and runs on any VAX computer running DEC’s VAX/VMS operating system. The planning system allows use of images from a variety of imaging modalities and includes 3-D dose calculations for photons, electrons, and brachytherapy as well as a variety of dose display and evaluation tools. A series of papers has outlined the general features of the planning system (3, 4, 5, 6, 16, 19, 20, 2 I).
ilcquisition qfdigitized radiographs To make direct use of simulator and verification radiographs, films are placed on a shuttered light box equipped with multiple light levels, and then video digitized. The video digitization is performed with a standard TV camera mounted on an optical bench, and the digitization hardware provided on the imaging system. Images are acquired as 5 12 X 5 12 8-bit images. The operator first inputs the geometrical parameters: gantry and collimator angles, field size, and source-film distance for each particular film. The video acquisition system is placed in to a “free-run” mode which allows immediate (real time) viewing of the digitized image on the image display workstation. In addition to the gray scale image of the film, separate computer-generated graphics can be displayed on top of (“overlaying”) the gray scale image of the film. Graphics lines which define the central axis and two orthogonal pairs of symmetric (about the center) lines are used to simulate the expected definition of the field edges. Both the camera and film can be adjusted to align the film to the overlaying graphics lines. The actual image which is saved is an average of 32 sequential frames, so
* VAX 8800, Digital Equipment Corp., Maynard,
MA.
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1990,
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that the effects of noise in the video system and acquisition process are reduced. In practice, the use of an 8 bit video digitizer is adequate when used with manual camera iris adjustments. A higher resolution digitizer would be preferred to handle the dynamic range of the video input without manual adjustments of camera aperature. After acquisition of the image, the graphics overlay of the rectangular beam shape can be scaled and translated to match the image. Provided that the camera optics and electronics are aligned, the graphics lines and the images of the field size wires on the radiograph image should align perfectly. A mismatch in the vertical or horizontal lines indicates an error in either the actual portal setup or possibly in the recording or entry of the parameters. An improvement on this registration process is achieved by fully calibrating the camera setup by first using a standard template (graph paper) to calibrate the field of view. The film is then aligned using just the central axis lines from both the graphics and the simulator field wires. Specification of the source to film distance allows an independent check on the geometrical consistency of the radiographic image field size.
The most crucial aspect in obtaining the digitized radiographic images is geometrical registration, since the radiographic images or data abstracted from this image are to be correlated with data from other sources such as manual contours, CT, or MRI. The geometry used to take the radiographs must be well defined with a clearly designated set of landmarks fixed to the patient. In our clinic, the initial localization session on the treatment simulator is used to place the patient in the treatment position, with immobilization devices, and to define a reference center (typically, the center of tumor). The lateral and vertical alignment lasers are used to place external tatoo marks on the patient and orthogonal radiographs are taken. Subsequent imaging studies are taken with the patient in the same position using the tatoos for alignment. For cross-sectional imaging studies, markers are placed on the tatoo marks. Within the treatment planning system, the reference system for the imaging study can be defined using the position of these markers, thus effectively rotating the cross-sectional image study to match the initial localization geometry. To fully use the digitized images for planning and evaluation, the data stored with each image includes the geometrical transformation between the treatment reference system and the beam coordinate system. The transformation is stored in the form of a 4 X 4 homogenous matrix which represents a composite transformation matrix derived from a series of rotations and translations ( 12). Perspective is also included in the transformation matrix to account for beam divergence. This single matrix can, for
+ Gould IP8500, Gould Inc., San Jose, CA.
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example, be used to transform a contour defined in the treatment reference system into a set of coordinates in beam geometry which can be used to plot the contours in a geometrically-correct way directly over the radiographic image. Figure 1 shows an example of multiple contours, obtained from a series of axial CT images, plotted over an AP radiographic image. The inverse of this matrix yields the reverse transformation, thereby allowing data defined on a radiographic image to be projected across a cross-sectional image. The simplicity of this method makes many interesting display capabilities possible. The projections of field and block edges across any 2-D image plane are correctly handled for any arbitrary orientation of beam and plane. The positions of these projected lines are computed by finding the intersection of the particular cross-sectional cut with the planes formed by the ray lines passing through the vertices of the line defined for the radiographic image. Figure 2 further illustrates this geometry. Shown is an axonometric view of three axial CT images, the placement of an AP radiation field, and the resulting radiographic image. The intersections of the beam edges are plotted on each of the axial images. Radiographic enhancement
Verification film images can be acquired using the above-described method, assuming a dot graticule (29) is used to indicate the position of the central axis. Unfortunately, image contrast is poor for these images due to the similarities in x-ray mass attenuation coefficients for different materials and high energy radiation. After digi-
Fig. 1. Correlation AP radiograph.
of cross-sectional
contours
abstracted
tization of the image, however, contrast enhancement and other image processing techniques are easily applied. Generally, a verification film is taken with two exposures. One exposure is taken with the collimators and shielding blocks in treatment position and the second with the shielding blocks removed and the collimators opened. Because of the different levels of exposure given, contrast enhancement using a single linear gray scale adjustment is not sufficient for enhancing the two regions simultaneously. A number of image processing enhancement techniques are, however, possible. One technique is to use an adaptive histogram equalization enhancement technique which samples and enhances contrast variations on a local basis (8). An alternative technique, illustrated in Figure 3, relies on a relatively simple renormalization process which effectively corrects for the differences in exposures. Two techniques have been attempted to identify the double exposed region that is the actual treatment field shape. The first technique was to first convolve a high emphasis filter through the image to obtain an edgeenhanced image. The kernel used for this convolution operation is a 9 X 9 matrix with a value of +80 defined for the central element and -1 defined for all other elements. Application of this kernel has the effect of subtracting a smoothed image from the original image thereby enhancing the high frequency components of the image. This edge-enhanced image can then, through the use of simple thresholding and automatic boundary tracking techniques, be used to define the edge of the beam portal and thus outline the higher exposure region. Use of this boundary for verification is described in the results section.
from axial CT images projected
over video-digitized
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Fig. 2. Axonometric view of multiple axial CT cross-sections and a radiographic image produced by the radiation field shown graphically.
A second technique for renormalizing the double exposed port film is to use the computer prescribed field shape to directly determine the “intended” treatment field region and to rescale image values within this region to match the bordering regions. An example of this technique is shown in Figure 3a, the original port film, and Figure 3b. the renormalized image.
RESULTS Uw qf beam ‘s eye vienj graphics
Interactive graphical display techniques for generating of the beam’s eye view for external beams have been demonstrated in earlier work by a number of researchers (8, 13, 22, 24, 26, 27). It has been suggested simulations
Fig. 3. (A) Digitized verification portal image. (B) Shows result of renormalization
of double exposed region.
Computerized treatment planning 0 D. L. MCSHAN et al.
Fig. 4. Standard contour BEV display. Different defined with different colors.
structures
are
that BEV graphics would be a valuable tool in treatment planning, but despite the earlier work, these concepts have only recently been integrated into a few commercial treatment planning systems. While there was considerable
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interest in the earlier demonstrations (24), there have been several reasons for the slow introduction of this seemingly useful tool for radiotherapy treatment planning. Foremost among these reasons is that few commercial planning systems have enough 3-dimensionality (2) to allow the BEV features described here to be reasonably implemented. Computer generated BEV simulations greatly enhance the ease with which one can plan treatments which ensure coverage of the entire treatment volume. In this type of simulation, localization data (target and important anatomical landmarks) are transformed into the projective geometry defined by the beam, and then displayed with additional graphics which show the planned beam edges (Fig. 4). In addition to contours, computer generated surface reconstructions (obtained from serial contours) may also be displayed to enhance the interpretation of the structural data (Fig. 5). Different structures can be added or removed from the display or rendered semi-transparent to fully appreciate the spatial relationships between the target volume and the surrounding anatomy as seen from the perspective of the planned beam. Contour display is the standard mode used for BEV displays, since it is created very quickly. If the display gets too complex to understand easily, then solid surface graphics are used to make the situation clearer to the planner. A significant amount of additional information can be transmitted to the treatment planner through further use of this BEV display. The BEV graphics may be underlayed with any appropriate gray scale image, including digitized
Fig. 5. Autoblock design using BEV view of prostate (pink), bladder (yellow), and margin is used to design a conformational portal for the prostate volume.
rectum (green), A one centimeter
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simulator and verification radiographs, or digital reconstructed radiographs (DRR). Shielding block designs, which are discussed below, are also included in the BEV display so that the relationship between the beam and the CT-derived target and normal structures can be directly examined for geometrical coverage. Figure 6 is an example of this simulation showing the target volume (an esophageal tumor), the spinal cord. and shielding blocks overlaying a digital reconstructed radiograph. To use the displayed information for treatment planning, changes in beam position and orientation are simulated by change in the BEV display. Changes in field size, blocking, and collimator angle are displayed interactively by redrawing beam outlines without redrawing the BEV display of contour information. A change in gantry angle, however, requires redrawing the entire display, requiring a few seconds for each new view. We typically use a display of a 2-D image (for example, an axial CT image, although any arbitrary orientation is allowed) to make the gantry angle adjustments interactively. There are certain cases where the BEV display is the only reasonable way to determine the gantry angle to be used. A “movie mode” has been implemented to allow the generation of a series of BEV views for a range of gantry angles (or other parameters) which are then stored on disk. Due to the complexity of image generation (image 1 generation time for a shaded surface is approximately second per structure on the VAX SSOO), the total time to generate a movie depends on the number of structures and views selected for display. With the real time digital display disk available with our imaging system. the ro-
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tation sequence can be played back at real time frame rates, allowing the user to use the joystick to quickly select the most appropriate beam entrance angle.
Shielding block design and display are an important part of the BEV display. If the physician draws the block outline on a simulator radiograph, then the block shape may be directly digitized into the system by using a digitizer tablet to trace the outlines of the desired blocks. With our planning system, the block outlines always appear on the BEV displays and allow (force) a check of the block shape with respect to the CT-derived contour data. Alternatively, the radiographic films can be video digitized and used to provide a direct comparison with the BEV information. With or without the digitized radiograph, the joystick may be used to move a cursor around the field, defining block outlines directly on the display. The cursor can be changed into a circle with a particular radius to assist the planner in maintaining the chosen margin around structures. The third option used to design blocks is to let the computer design the beam shaping based on the outer limits of the projected target shape. A specified margin can be added if desired. Figure 5 shows blocking which has been designed automatically for a prostate tumor volume allowing a 1 cm margin around the tumor volume. The computer-designed block shape can be plotted on a hard copy device for a specified source to film distance so that it can be used to overlay simulator and verification films. Besides indicating the block shapes, this plot also
Fig. 6. Graphical display of surface structures (target and cord) abstracted from axial CT images shown overlaying digital reconstructed
radiographic
portal. Planned
portal and blocking are indicated.
Computerized treatment planning 0 D. L.
includes the BEV projected outlines of the target and other important anatomy. The same plot can be used as a template for cutting the blocks manually using a hot-wire block cutter. Alternatively, the information can be sent directly to a computer-controlled block cutter (30). The ability to directly compare radiographic images against overlaying beam’s eye view graphics is the critical link between conventional treatment planning and beam’s eye view planning. It allows the radiotherapist to resolve past experiences in using only radiographic images for portal design against the information derived from crosssectional CT (or other modality) images. We have begun a formal study of the impact of these capabilities; our preliminary observations are that blocking based on radiographic images alone frequently proves to be less than optimal when evaluated against the information derived from CT images. This experience is similar to the conclusion reached by Goitein et al. ( 10) when analyzing the impact of CT imaging on radiotherapy treatment planning. Digital reconstructed radiographs While images such as Figures 4-6 dramatically increase the ability to accurately design a conformational beam portal, in practice it is sometimes difficult to verify the accuracy of the actual treatment setup and delivery due to the poor contrast on megavoltage verification films when compared to CT imaging. Edges identified on the cross-sectional images used for planning are not always seen on radiographic images, especially for soft tissue boundaries. A digital reconstructed radiograph (DRR), first demonstrated by Goitein et al. (9), is used to make the link between the actual portal image and the image that is expected by calculating ray line projections through the CT data for the particular field being investigated. Figure 7a and b compare the image-processed x-ray port
Fig. 7. (A) Video digitized verification generated from serial CT data.
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film to a DRR to illustrate the differences between the two. The geometric and structural comparisons between DRR images and radiographic images represents a quality assurance check which ties together localization data, planned field design, and the actual treatment setup. The DRR is especially beneficial when performing many noncoplanar treatment plans, since some field arrangements cannot be filmed on the simulator because the image intensifier system collides with the simulator table at extremes of gantry angle combined with table angle. Beam’s eye view dose evaluations The ability to calculate and assess the dose to be delivered is essential in assuring that the desired therapeutic value is achieved. The conventional dose evaluation tool is a display of isodose curves on an axial cross-sectional plane, which is normally coplanar with the isocentric beams. Attempting to understand the variation of the dose across the entire field while looking only at the dose distribution displayed on a few transverse CT images is hazardous. Calculational planes which utilize the beam symmetry, especially planes perpendicular to the axis of the beam (BEV planes), can be extraordinarily useful in transmitting to the physician the 3-dimensionality of the dose distribution, and its relationship to shielding blocks. Decisions regarding penumbra and scattering effects, beam shaping effects, and effects of inhomogeneities in the path of the field are easily assessed in these beam symmetric planes. For these views to be useful for dose evaluations, anatomical landmark identification data is also needed. In the present work, any anatomical information (such as target volume, cord, kidneys) which has been contoured on the axial CT data may be displayed on these views by determining the intersections of their derived surfaces with the beam symmetric planes (18, 20). More detailed ana-
radiograph with renormalization. (B) Digital reconstruction
radiograph
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tomical structural data is obtained by reconstructing and displaying CT cross-sectional images on the desired viewing planes. Figure 8 illustrates the dose distribution on an axial and orthogonal BEV plane for an oblique set of opposed fields. The dose distribution is shown with a “colorwash” display which uses color to highlight different dose levels (18). This display clearly shows the extent of the irradiation due to these fields and the effects of the blocking. This capability to generate and compare calculational results in cross-sections which are parallel or perpendicular to a beam’s central axis also provides a powerful tool for comparing measured data to calculated data, for comparing the effects of different calculational methods, and for quality assurance checks. Obviously, to make these types of comparisons, correct 3-D beam geometry (divergence) and 3-D dose calculations are required. especially when dealing with blocked fields.
Integrated ver$cation techniques The use of image processing techniques to attempt to increase the amount of information which can be gleaned from megavoltage verification (port) films has been described by a number of workers (1, 15, 17, 25. 28). We here describe the use of two techniques which we have found useful to integrate into the planning system. Port films are digitized into the planning system as described in the methods section. The simple edge enhancement routine is then performed to determine the boundary of the double-exposed region of the verification film. This edge can then be directly compared to the edge of the
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radiation field that was planned using the computer system by plotting the two outlines. However, we have found another technique to be much more effective in transmitting the overall results of the comparison. If the port film image has been correctly entered into the system using its dot graticule to determine the center of the field and its alignment, then the planned treatment field edges and blocks can be overlaid on the digitized image. The area inside and outside the “planned” portal can be analyzed; and the interior region of the portal, corresponding (ideally) to the double exposed region, can be resealed so that the gray scale variation inside the “planned” portal is similar to that outside the field. This makes several things possible. The first is that it is possible to use standard contrast enhancement techniques to enhance the entire image, since all parts of the image are now of similar intensity ranges. An important quality assurance tool is also available: graphics illustrating the location of anatomy defined from the CT data can be overlaid and compared to the anatomy which is visible on the verification film. The most striking effect, however, is that this procedure highlights areas of disagreement between the planned portal and the area actually irradiated. When the radiated area extends outside the planned portal, then it is not resealed. and so appears dark on the image. Areas which should have been irradiated, but were not, appear white, since they were inside the planned portal and were rescaled. As illustrated in Figure 7A, it is clear at a glance which areas were incorrectly over-irradiated or under-irradiated. As these images are incorporated in the planning
Fig. 8. Dose calculation on an axial plane and on a BEV plane perpendicular distribution is shown using a color-wash display.
to an oblique pair of fields. Dose
Computerized treatment planning 0 D. L. MCSHAN et al.
system, they can then be directly overlaid with BEV graphics which illustrate the importance of the various misalignments between the planned and actual irradiated fields with respect to the CT-derived target and normal structures. DISCUSSION It has been recognized over the last few years (11, 13) that making a connection between treatment planning information (typically displayed on transverse CT images) and portal views (simulator and verification radiographs) is very important. A large amount of sophistication in imaging, display techniques, and dose calculations is being routinely used in many treatment planning systems. However, when it becomes time to design shielding blocks, for example, there is little or no communication of information between the simulator radiographs on which the block shapes are usually drawn with a grease pencil, and the CT-based treatment plan and dose calculations. In addition, there is virtually no way to verify the accuracy of patient set-up and treatment, since the verification radiograph is typically compared only to the simulator radiograph, and not to any information from the planning system. In this paper, we present a comprehensive set of treatment planning system capabilities which allow the direct integration of all the portal information which is available with the computerized treatment planning information. Although various specific capabilities have been reported by others in the past (9, 11, 13, 24, 27) a systematic approach to this integration has not been reported. We have found that to attempt to use CT-type information, dose information, and BEV information, this integrated set of features is essential. The clinical utility of the BEV features described here is undeniable. The BEV review of block shapes with respect to the CT-derived target volume is mandatory in our clinic for all patients, and one is severely limited in the development of complex treatment plans without this ability. Solid surface graphics are increasingly used, especially for complicated and non-coplanar treatment plans in which the simple line representations of structures are confusing. Most of our shielding blocks are now designed on the planning system, using either the joystick with the BEV display, or the “autoblock” option. Finally, the digitization and use of simulator radiographs and DRRs has grown quickly. In many cases, especially those in which multiple imaging modalities are involved (for example MRI (4)), the digitized radiographs and other associated simulator data (mechanical contours) are critical during the alignment and correlation procedures. We continue to explore the capabilities which can be applied to the verification of treatment accuracy, including port film enhancement and integration with other treatment planning BEV information, and the use of DRRs for comparison to port or simulator films. The identification
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of landmark information either manually or automatically on separate films provides one mechanism for comparing separate images. Direct overlays of images with the ability to interactively (or automatically) translate and rotate one image relative to the other is also possible. The full use of these features can give a large amount of information about the accuracy of treatment planning and treatment delivery. The ability to integrate computerized portal imaging techniques into routine clinical treatment planning practice leads one rather directly to the need for integration of an on-line verification system which directly (digitally) acquires images from the therapeutic radiation beam. Investigations into possible real-time acquisition systems are being pursued ( 1, 14, 3 1). Several scenarios for the use of these images are possible, ranging from simple comparisons for initial set-up checks to systems which would either inhibit or possibly even adjust beam placements during a given treatment session based on the comparisons between the planned and actual portal. Both scenarios require a reliable method of correlating the images and the placement of the exposure with respect to the targeted volume. In combination with this analysis, an understanding of the degree of “acceptable” variation is required. The need for this error analysis has been emphasized by Goitien and Abrams (7). CONCLUSIONS Use of digital radiographic images and computer generated BEV simulations has proven to be a very useful tool in our clinical treatment planning system. Computerized BEV simulation assists in 3-dimensional treatment planning by providing localization data on tumor extent from the view point of an incoming beam of radiation and, therefore, assists in the specifying the shape and design of proposed treatment fields. This computerized facility helps to reduce the actual simulation and setup time for the patient. In effect, most of the simulation checks needed are done with the computer simulation using the patient’s localization data, but without the patient’s physical presence. Furthermore, developments on the integration of digitized radiographs within the computerized treatment planning system will ease the future transition to the use of true digital radiographic imaging modalities. The full specification of the treatment geometry within the computerized planning system facilitates the integration with computerized radiotherapy monitoring systems and provides valuable quality assurance checks for accurate and consistent radiation therapy delivery. Finally, computerized beam’s eye view simulation effectively links together the two previously separate but important processes of treatment planning: cross-sectional treatment planning and radiographic portal simulation and verification. This link provides a vital and necessary component in bringing about true 3-dimensional treatment planning in routine clinical practice.
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REFERENCES 1. Baily, N. A.; Horn, R. A.; Kamp, T. D. Fluoroscopic visualization of megavoltage therapeutic x-ray beams. Int. J. Radiat. Oncol. Biol. Phys. 6:935-939; 1980. 2. Fraass, B. A. Practical implications of three-dimensional radiation therapy treatment planning. In: Paliwal, B. R., Griem, M. L., eds. Radiation therapy treatment planning. Chicago: RSNA; 1986: 13-22. 3. Fraass, B. A.; McShan, D. L. 3-D treatment planning: I. Overview of a clinical planning system. In: Bruinvis, I. A. D., et al., eds. The use of computers in radiation therapy. North Holland: Elsevier Science Publishers B.V.; 1987: 273-276. 4 Fraass, B. A.; McShan. D. L.; Diaz, R. F.; Lichter, A. S.; Ten Haken, R. K.; Aisen, A.; Glazer, G. Integration of MRI into radiation therapy treatment planning. Int. J. Radiat. Oncol. Biol. Phys. 13: 1897-l 908; 1987. 5. Fraass, B. A.; McShan, D. L.; Ten Haken, R. K.; Hutchins, K. M. 3-D treatment planning: V. A fast 3-D photon calculation model. In: Bruinvis, I. A. D., et al., eds. The use of computers in radiation therapy. North-Holland: Elsevier Science Publishers B.V.; 1987:521-525. 6. Fraass, B. A.; McShan, D. L.; Weeks, K. J. 3-D treatment planning: III. Complete beam’s-eye-view planning capabilities. In: Bruinvis, I. A. D., et al., eds. The use of computers in radiation therapy. North Holland: Elsevier Science Publishers B.V.; 1987:193-196. 7. Goitein, M. Limitations of two-dimensional treatment planning programs. Med. Phys. 9:580-586; 1982. 8. Goitein, M.; Abrams, M. Multi-dimensional treatment planning: I. Delineation of anatomy. Int. J. Radiat. Oncol. Biol. Phys. 9:777-787; 1983. 9. Goitein, M.; Abrams, M.; Rowell, 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. 9:789-797: 1983. 10. Goitein, M.; Wittenberg, J.: Mendiondo, M.; Doucette, J.; Friedberg, C. The value of CT scanning in radiation therapy treatment planning: a prospective study. Int. J. Radiat. Oncol. Biol. Phys. 5: 1787- 1792; 1979. Il. Griffin, B. R.; Shuman, W. P.; Luk, K. H.; Tong, D. Locate: an application of computer tomography in radiation therapy treatment planning with emphasis on tumor localization. Int. J. Radiat. Oncol. Biol. Phys. 10:555-559; 1984. 12. Hall, E. Computer image processing and recognition. NY: Academic Press; 1986:76-88. 13. Haynor, D.; Borning, A.; Griffin, B.; Jacky, J.; Kalet, I.; and Shuman, W. Radiotherapy planning: direct tumor location on simulation and port films using CT. Radiology 158:537540; 1986. M.; Lam, W. An on-line electronic 14. Lam, K.; Partowmah, portal imaging system for external beam radiotherapy. Br. J. Radiol. 59:1007-1013; 1986. 15. Leong, J. A digital image processing system for high energy x-ray portal images. Phys. Med. Biol. 29: 1527- 1535; 1984. 16. Lichter, A. S. Clinical practice of modern radiation therapy treatment planning. In: Paliwal, B., Greim, M., eds. Radiation therapy treatment planning. Chicago, IL: RSNA; 1986: 7-12.
17. McMurry, H.; Chaney, E.; Sherouse, G.; Rosenman, J.; Varia, V. Contrast limited adaptive histogram equalization of radiotherapy films. Med. Phys. 13(4):598; 1986. 18. McShan, D. L. Treatment plan evaluation and optimization. in Radiation Therapy Treatment Planning. In: Paliwal, B., Greim, M., eds. Radiation therapy treatment planning. Chicago, IL: RSNA; 1986:33-40. 19. McShan, D. L.; Fraass, B. A. Integration of multi-modality imaging for use in radiation therapy treatment planning. In: Lemke, H. U., Rhodes, M. L., Jaffee, C. C., Felix, R., eds. Computer assisted radiology. Berlin: Springer Verlag; 1987:300-304. 20. McShan, D. L.; Fraass, B. A. 3-D treatment planning: II. integration of gray scale images and solid surface graphics. In: Bruinvis, I. A. D., et al., eds. The use of computers in radiation therapy. North Holland: Elsevier Science Publishers B.V.; 1987:4 l-44. 21. McShan, D. L.; Fraass, B. A.; Ten Haken, R. K.; Jost, R. Three-dimensional electron beam dose calculations and dosimetric evaluations. Med. Phys. 12(4):507; 1985. 22 McShan, D. L.: Glicksman, A. S. Graphical simulation and design of beam portal blocking. In: Proceeding of the Eighth International Conference on the Use of Computers in Radiation Therapy. Toronto. Canada: IEEE Computer Society; 1984:114-l 18. 23. McShan, D. L.; Matrone, G. M.; Fraass, B. A.; Lichter, A. S. A large screen digitizer system for radiation therapy planning. Med. Phys. 14(3); 1987. 24. McShan, D. L.; Silverman, A.: Lanza, D.; Reinstien, L.: Glicksman, A. Three-dimensional radiation treatment planning and dose display utilizing interactive color graphics. Br. J. Radiol. 52:478-481; 1979. 25. Meertens, H. Digital processing of high-energy photon beam films. Med. Phys. 12:ll l-l 13; 1985. 26. Reinstein, L.; McShan, D. L.; Lang, R.; Glicksman, A. S. Three-dimensional reconstruction of CT images for treatment planning in carcinoma of the lung. In: Ling, C. C.; Rogers, C.; Morton, R. J., eds. Computer tomograph in radiation therapy. New York: Raven Press; 1983: 155-165. 27. Reinstein, L.: McShan, D. L.; Webber, B.; Glicksman, 4. S. A computer assisted three-dimensional treatment planning system. Radiology 137( 1):259-264; 1978. J.; McMurry. H. L.; Pizer, 28. Sherouse, G. W.; Rosenman, S. M.; Chaney, E. L. Automatic digital contrast enhancement of radiotherapy films. Int. J. Radiat. Oncol. Biol. Phys. 13:801-806; 1987. 29. van de Geijn, J.; Harrington, F.; Fraass, B. A. A graticule for evaluation of megavolt x-ray port films. Int. J. Radiat. Oncol. Biol. Phys. 81999-2000; 1982. 30. Weeks, K. J.; Fraass, B. A.: McShan, D. L.; Hardybala, S. S.: Hargreaves, E. A.; Lichter, A. S. Comparison of automated and manual shielding block fabrication. Int. J. Radiat. Oncol. Biol. Phys. 1650 l-504; 1989. 31. Wong, J.; Binns, W. R.; Epstein, J. W.; Karlmann, M. H. Plastic scintillator sheet as area1 dosimeter therapy. Med. Phys. 13(4):609; 1986.
J.; Israel, in radio-
Copyright
0360-3016/W $3.00 + .oO 0 1990 Pergamon Press plc
??Technical Innovations and Notes
FULL INTEGRATION OF THE BEAM’S EYE VIEW CONCEPT INTO COMPUTERIZED TREATMENT PLANNING D.
L. MCSHAN,
Department
of Radiation
PH.D.,
B. A. FRAASS,
Oncology,
University
PH.D.
of Michigan
AND
A.
S. LICHTER,
Medical Center,
M.D.
Ann Arbor, MI 48 109
A complete set of beam’s eye view (BEV) and beam portal design features have been integrated into a computerized 3-dimensional radiotherapy treatment planning system. Among the features implemented is the ability to mix BEV graphics with gray-scale images such as simulator and verification radiographs, and digital reconstructed radiographs. Image processing techniques have been developed to both enhance verification images and to detect radiation field boundaries. These portal simulation and presentation techniques are being used clinically to design and verify radiation fields with manual or automatically-designed field shaping blocks. The ability to perform computer dose calculations for planes which are parallel or perpendicular to a specified beam’s central axis is available and this feature has also proven useful for treatment plan evaluation and optimization. Finally, direct comparison of computergenerated portal images with actual simulation and verification radiographs is also possible. These techniques allow the direct integration of “CT-directed treatment planning” with block design, simulator films and port films, and other Beam’s Eye View-type displays. Computerized
treatment planning, Beam’s eye view, Verification, Simulation, 3-D.
INTRODUCTION importance of radiographic imaging of treatment fields is well established for high-energy external beam radiation therapy. Radiographic images from simulations are routinely used to localize the target volume and neighboring anatomy and to simulate proposed treatment ports, while port films from treatment machines are used to verify the actual setup of planned treatment fields. Port films often represent the only direct clinical confirmation of the actual placement of external radiation beams relative to the targeted volume and neighboring anatomy. Simulator and port radiographs are, in fact, “Beams Eye Views” (BEV) of the treatment area. Currently, computerized radiation therapy planning systems make little use of these various BEV images, except to specify the shape of irregular fields so that the equivalent square and dose at a few specific points can be calculated. There is little or no transfer of information between the CT-based treatment plan and the simulator and port films which are used to define field shaping and to verify plan accuracy. Neither simulator and port films, nor the CT treatment plan contain sufficient information about the treatment plan that the evaluation of the plan can be made solely on the basis of any one of these image
sets alone. Some qualitative comparison or “integration” of the two different sets of information is required. Usually this is done by “eyeballing” the CT treatment plan and judging whether the simulator film and port films are consistent with respect to each other. It is our contention that a computerized integration of CT plans with BEV films is critically necessary to fully realize the capabilities of today’s sophisticated treatment planning systems. The present work describes a number of ways in which the BEV radiographs and other information can be quantitatively integrated with the CT information used during treatment planning. Simulator radiographs and portal verification films are video digitized, and used throughout all phases of treatment planning. The images can be correlated with multi-level CT (or MR (4, 19)) image data, and used with computer graphics to produce a BEV representation of the treatment volume (6). This display can then be used to design the treatment fields and shielding blocks. Three-dimensional dose calculations are performed in planes that are tied to the aforementioned BEV views. This allows the direct comparison of the dose distribution with the planned fields (as illustrated by simulator radiographs). The ability to overlay and compare planned simulator films, CT-determined target and normal structure volumes, treatment planning dose calcu-
Presented in part at the 1986 meeting of the American Association of Physicists in Medicine, Lexington, Kentucky, August 3, 1986. Reprint requests to: D. L. McShan, Ph.D., Department of
Radiation Therapy, University of Michigan Hospitals, AGH Room B2C490, Box 0010, 1500 E. Medical Center Dr., Ann Arbor, MI 48 109. Accepted for publication 20 December 1989.
The
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lations, and actual verification him images. leads to an integrated system which brings us improved accuracy and flexibility in treatment planning.
METHODS
AND
MATERIALS
The treatment planning system which is used for this work is a fully 3-dimensional system which has been developed at our institution. The hardware relevant to this work includes a computer* with 64 MB of memory and over 2 gigabytes of disk storage. The computer is coupled to an imaging system’ equipped with four independent workstations each with 5 12 X 5 12 X 32 bits of display memory. The imaging system is also equipped with a video digitizer which is used to digitize radiographic film images, and with a digital video processor which performs arithmetic on an entire image in one frame time ($ second). An electromagnetic digitizer (23) is used to enter block shapes which have been drawn on simulator films into the system, and a computer controlled pen plotter is used to output block shapes when blocks are designed using the BEV features of the planning system. The 3-D treatment planning software (called “UMPlan”) has been developed in-house and runs on any VAX computer running DEC’s VAX/VMS operating system. The planning system allows use of images from a variety of imaging modalities and includes 3-D dose calculations for photons, electrons, and brachytherapy as well as a variety of dose display and evaluation tools. A series of papers has outlined the general features of the planning system (3, 4, 5, 6, 16, 19, 20, 2 I).
ilcquisition qfdigitized radiographs To make direct use of simulator and verification radiographs, films are placed on a shuttered light box equipped with multiple light levels, and then video digitized. The video digitization is performed with a standard TV camera mounted on an optical bench, and the digitization hardware provided on the imaging system. Images are acquired as 5 12 X 5 12 8-bit images. The operator first inputs the geometrical parameters: gantry and collimator angles, field size, and source-film distance for each particular film. The video acquisition system is placed in to a “free-run” mode which allows immediate (real time) viewing of the digitized image on the image display workstation. In addition to the gray scale image of the film, separate computer-generated graphics can be displayed on top of (“overlaying”) the gray scale image of the film. Graphics lines which define the central axis and two orthogonal pairs of symmetric (about the center) lines are used to simulate the expected definition of the field edges. Both the camera and film can be adjusted to align the film to the overlaying graphics lines. The actual image which is saved is an average of 32 sequential frames, so
* VAX 8800, Digital Equipment Corp., Maynard,
MA.
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1990,
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that the effects of noise in the video system and acquisition process are reduced. In practice, the use of an 8 bit video digitizer is adequate when used with manual camera iris adjustments. A higher resolution digitizer would be preferred to handle the dynamic range of the video input without manual adjustments of camera aperature. After acquisition of the image, the graphics overlay of the rectangular beam shape can be scaled and translated to match the image. Provided that the camera optics and electronics are aligned, the graphics lines and the images of the field size wires on the radiograph image should align perfectly. A mismatch in the vertical or horizontal lines indicates an error in either the actual portal setup or possibly in the recording or entry of the parameters. An improvement on this registration process is achieved by fully calibrating the camera setup by first using a standard template (graph paper) to calibrate the field of view. The film is then aligned using just the central axis lines from both the graphics and the simulator field wires. Specification of the source to film distance allows an independent check on the geometrical consistency of the radiographic image field size.
The most crucial aspect in obtaining the digitized radiographic images is geometrical registration, since the radiographic images or data abstracted from this image are to be correlated with data from other sources such as manual contours, CT, or MRI. The geometry used to take the radiographs must be well defined with a clearly designated set of landmarks fixed to the patient. In our clinic, the initial localization session on the treatment simulator is used to place the patient in the treatment position, with immobilization devices, and to define a reference center (typically, the center of tumor). The lateral and vertical alignment lasers are used to place external tatoo marks on the patient and orthogonal radiographs are taken. Subsequent imaging studies are taken with the patient in the same position using the tatoos for alignment. For cross-sectional imaging studies, markers are placed on the tatoo marks. Within the treatment planning system, the reference system for the imaging study can be defined using the position of these markers, thus effectively rotating the cross-sectional image study to match the initial localization geometry. To fully use the digitized images for planning and evaluation, the data stored with each image includes the geometrical transformation between the treatment reference system and the beam coordinate system. The transformation is stored in the form of a 4 X 4 homogenous matrix which represents a composite transformation matrix derived from a series of rotations and translations ( 12). Perspective is also included in the transformation matrix to account for beam divergence. This single matrix can, for
+ Gould IP8500, Gould Inc., San Jose, CA.
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example, be used to transform a contour defined in the treatment reference system into a set of coordinates in beam geometry which can be used to plot the contours in a geometrically-correct way directly over the radiographic image. Figure 1 shows an example of multiple contours, obtained from a series of axial CT images, plotted over an AP radiographic image. The inverse of this matrix yields the reverse transformation, thereby allowing data defined on a radiographic image to be projected across a cross-sectional image. The simplicity of this method makes many interesting display capabilities possible. The projections of field and block edges across any 2-D image plane are correctly handled for any arbitrary orientation of beam and plane. The positions of these projected lines are computed by finding the intersection of the particular cross-sectional cut with the planes formed by the ray lines passing through the vertices of the line defined for the radiographic image. Figure 2 further illustrates this geometry. Shown is an axonometric view of three axial CT images, the placement of an AP radiation field, and the resulting radiographic image. The intersections of the beam edges are plotted on each of the axial images. Radiographic enhancement
Verification film images can be acquired using the above-described method, assuming a dot graticule (29) is used to indicate the position of the central axis. Unfortunately, image contrast is poor for these images due to the similarities in x-ray mass attenuation coefficients for different materials and high energy radiation. After digi-
Fig. 1. Correlation AP radiograph.
of cross-sectional
contours
abstracted
tization of the image, however, contrast enhancement and other image processing techniques are easily applied. Generally, a verification film is taken with two exposures. One exposure is taken with the collimators and shielding blocks in treatment position and the second with the shielding blocks removed and the collimators opened. Because of the different levels of exposure given, contrast enhancement using a single linear gray scale adjustment is not sufficient for enhancing the two regions simultaneously. A number of image processing enhancement techniques are, however, possible. One technique is to use an adaptive histogram equalization enhancement technique which samples and enhances contrast variations on a local basis (8). An alternative technique, illustrated in Figure 3, relies on a relatively simple renormalization process which effectively corrects for the differences in exposures. Two techniques have been attempted to identify the double exposed region that is the actual treatment field shape. The first technique was to first convolve a high emphasis filter through the image to obtain an edgeenhanced image. The kernel used for this convolution operation is a 9 X 9 matrix with a value of +80 defined for the central element and -1 defined for all other elements. Application of this kernel has the effect of subtracting a smoothed image from the original image thereby enhancing the high frequency components of the image. This edge-enhanced image can then, through the use of simple thresholding and automatic boundary tracking techniques, be used to define the edge of the beam portal and thus outline the higher exposure region. Use of this boundary for verification is described in the results section.
from axial CT images projected
over video-digitized
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Fig. 2. Axonometric view of multiple axial CT cross-sections and a radiographic image produced by the radiation field shown graphically.
A second technique for renormalizing the double exposed port film is to use the computer prescribed field shape to directly determine the “intended” treatment field region and to rescale image values within this region to match the bordering regions. An example of this technique is shown in Figure 3a, the original port film, and Figure 3b. the renormalized image.
RESULTS Uw qf beam ‘s eye vienj graphics
Interactive graphical display techniques for generating of the beam’s eye view for external beams have been demonstrated in earlier work by a number of researchers (8, 13, 22, 24, 26, 27). It has been suggested simulations
Fig. 3. (A) Digitized verification portal image. (B) Shows result of renormalization
of double exposed region.
Computerized treatment planning 0 D. L. MCSHAN et al.
Fig. 4. Standard contour BEV display. Different defined with different colors.
structures
are
that BEV graphics would be a valuable tool in treatment planning, but despite the earlier work, these concepts have only recently been integrated into a few commercial treatment planning systems. While there was considerable
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interest in the earlier demonstrations (24), there have been several reasons for the slow introduction of this seemingly useful tool for radiotherapy treatment planning. Foremost among these reasons is that few commercial planning systems have enough 3-dimensionality (2) to allow the BEV features described here to be reasonably implemented. Computer generated BEV simulations greatly enhance the ease with which one can plan treatments which ensure coverage of the entire treatment volume. In this type of simulation, localization data (target and important anatomical landmarks) are transformed into the projective geometry defined by the beam, and then displayed with additional graphics which show the planned beam edges (Fig. 4). In addition to contours, computer generated surface reconstructions (obtained from serial contours) may also be displayed to enhance the interpretation of the structural data (Fig. 5). Different structures can be added or removed from the display or rendered semi-transparent to fully appreciate the spatial relationships between the target volume and the surrounding anatomy as seen from the perspective of the planned beam. Contour display is the standard mode used for BEV displays, since it is created very quickly. If the display gets too complex to understand easily, then solid surface graphics are used to make the situation clearer to the planner. A significant amount of additional information can be transmitted to the treatment planner through further use of this BEV display. The BEV graphics may be underlayed with any appropriate gray scale image, including digitized
Fig. 5. Autoblock design using BEV view of prostate (pink), bladder (yellow), and margin is used to design a conformational portal for the prostate volume.
rectum (green), A one centimeter
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simulator and verification radiographs, or digital reconstructed radiographs (DRR). Shielding block designs, which are discussed below, are also included in the BEV display so that the relationship between the beam and the CT-derived target and normal structures can be directly examined for geometrical coverage. Figure 6 is an example of this simulation showing the target volume (an esophageal tumor), the spinal cord. and shielding blocks overlaying a digital reconstructed radiograph. To use the displayed information for treatment planning, changes in beam position and orientation are simulated by change in the BEV display. Changes in field size, blocking, and collimator angle are displayed interactively by redrawing beam outlines without redrawing the BEV display of contour information. A change in gantry angle, however, requires redrawing the entire display, requiring a few seconds for each new view. We typically use a display of a 2-D image (for example, an axial CT image, although any arbitrary orientation is allowed) to make the gantry angle adjustments interactively. There are certain cases where the BEV display is the only reasonable way to determine the gantry angle to be used. A “movie mode” has been implemented to allow the generation of a series of BEV views for a range of gantry angles (or other parameters) which are then stored on disk. Due to the complexity of image generation (image 1 generation time for a shaded surface is approximately second per structure on the VAX SSOO), the total time to generate a movie depends on the number of structures and views selected for display. With the real time digital display disk available with our imaging system. the ro-
June 1990. Volume 18, Number 6
tation sequence can be played back at real time frame rates, allowing the user to use the joystick to quickly select the most appropriate beam entrance angle.
Shielding block design and display are an important part of the BEV display. If the physician draws the block outline on a simulator radiograph, then the block shape may be directly digitized into the system by using a digitizer tablet to trace the outlines of the desired blocks. With our planning system, the block outlines always appear on the BEV displays and allow (force) a check of the block shape with respect to the CT-derived contour data. Alternatively, the radiographic films can be video digitized and used to provide a direct comparison with the BEV information. With or without the digitized radiograph, the joystick may be used to move a cursor around the field, defining block outlines directly on the display. The cursor can be changed into a circle with a particular radius to assist the planner in maintaining the chosen margin around structures. The third option used to design blocks is to let the computer design the beam shaping based on the outer limits of the projected target shape. A specified margin can be added if desired. Figure 5 shows blocking which has been designed automatically for a prostate tumor volume allowing a 1 cm margin around the tumor volume. The computer-designed block shape can be plotted on a hard copy device for a specified source to film distance so that it can be used to overlay simulator and verification films. Besides indicating the block shapes, this plot also
Fig. 6. Graphical display of surface structures (target and cord) abstracted from axial CT images shown overlaying digital reconstructed
radiographic
portal. Planned
portal and blocking are indicated.
Computerized treatment planning 0 D. L.
includes the BEV projected outlines of the target and other important anatomy. The same plot can be used as a template for cutting the blocks manually using a hot-wire block cutter. Alternatively, the information can be sent directly to a computer-controlled block cutter (30). The ability to directly compare radiographic images against overlaying beam’s eye view graphics is the critical link between conventional treatment planning and beam’s eye view planning. It allows the radiotherapist to resolve past experiences in using only radiographic images for portal design against the information derived from crosssectional CT (or other modality) images. We have begun a formal study of the impact of these capabilities; our preliminary observations are that blocking based on radiographic images alone frequently proves to be less than optimal when evaluated against the information derived from CT images. This experience is similar to the conclusion reached by Goitein et al. ( 10) when analyzing the impact of CT imaging on radiotherapy treatment planning. Digital reconstructed radiographs While images such as Figures 4-6 dramatically increase the ability to accurately design a conformational beam portal, in practice it is sometimes difficult to verify the accuracy of the actual treatment setup and delivery due to the poor contrast on megavoltage verification films when compared to CT imaging. Edges identified on the cross-sectional images used for planning are not always seen on radiographic images, especially for soft tissue boundaries. A digital reconstructed radiograph (DRR), first demonstrated by Goitein et al. (9), is used to make the link between the actual portal image and the image that is expected by calculating ray line projections through the CT data for the particular field being investigated. Figure 7a and b compare the image-processed x-ray port
Fig. 7. (A) Video digitized verification generated from serial CT data.
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film to a DRR to illustrate the differences between the two. The geometric and structural comparisons between DRR images and radiographic images represents a quality assurance check which ties together localization data, planned field design, and the actual treatment setup. The DRR is especially beneficial when performing many noncoplanar treatment plans, since some field arrangements cannot be filmed on the simulator because the image intensifier system collides with the simulator table at extremes of gantry angle combined with table angle. Beam’s eye view dose evaluations The ability to calculate and assess the dose to be delivered is essential in assuring that the desired therapeutic value is achieved. The conventional dose evaluation tool is a display of isodose curves on an axial cross-sectional plane, which is normally coplanar with the isocentric beams. Attempting to understand the variation of the dose across the entire field while looking only at the dose distribution displayed on a few transverse CT images is hazardous. Calculational planes which utilize the beam symmetry, especially planes perpendicular to the axis of the beam (BEV planes), can be extraordinarily useful in transmitting to the physician the 3-dimensionality of the dose distribution, and its relationship to shielding blocks. Decisions regarding penumbra and scattering effects, beam shaping effects, and effects of inhomogeneities in the path of the field are easily assessed in these beam symmetric planes. For these views to be useful for dose evaluations, anatomical landmark identification data is also needed. In the present work, any anatomical information (such as target volume, cord, kidneys) which has been contoured on the axial CT data may be displayed on these views by determining the intersections of their derived surfaces with the beam symmetric planes (18, 20). More detailed ana-
radiograph with renormalization. (B) Digital reconstruction
radiograph
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tomical structural data is obtained by reconstructing and displaying CT cross-sectional images on the desired viewing planes. Figure 8 illustrates the dose distribution on an axial and orthogonal BEV plane for an oblique set of opposed fields. The dose distribution is shown with a “colorwash” display which uses color to highlight different dose levels (18). This display clearly shows the extent of the irradiation due to these fields and the effects of the blocking. This capability to generate and compare calculational results in cross-sections which are parallel or perpendicular to a beam’s central axis also provides a powerful tool for comparing measured data to calculated data, for comparing the effects of different calculational methods, and for quality assurance checks. Obviously, to make these types of comparisons, correct 3-D beam geometry (divergence) and 3-D dose calculations are required. especially when dealing with blocked fields.
Integrated ver$cation techniques The use of image processing techniques to attempt to increase the amount of information which can be gleaned from megavoltage verification (port) films has been described by a number of workers (1, 15, 17, 25. 28). We here describe the use of two techniques which we have found useful to integrate into the planning system. Port films are digitized into the planning system as described in the methods section. The simple edge enhancement routine is then performed to determine the boundary of the double-exposed region of the verification film. This edge can then be directly compared to the edge of the
June 1990. Volume 18, Number 6
radiation field that was planned using the computer system by plotting the two outlines. However, we have found another technique to be much more effective in transmitting the overall results of the comparison. If the port film image has been correctly entered into the system using its dot graticule to determine the center of the field and its alignment, then the planned treatment field edges and blocks can be overlaid on the digitized image. The area inside and outside the “planned” portal can be analyzed; and the interior region of the portal, corresponding (ideally) to the double exposed region, can be resealed so that the gray scale variation inside the “planned” portal is similar to that outside the field. This makes several things possible. The first is that it is possible to use standard contrast enhancement techniques to enhance the entire image, since all parts of the image are now of similar intensity ranges. An important quality assurance tool is also available: graphics illustrating the location of anatomy defined from the CT data can be overlaid and compared to the anatomy which is visible on the verification film. The most striking effect, however, is that this procedure highlights areas of disagreement between the planned portal and the area actually irradiated. When the radiated area extends outside the planned portal, then it is not resealed. and so appears dark on the image. Areas which should have been irradiated, but were not, appear white, since they were inside the planned portal and were rescaled. As illustrated in Figure 7A, it is clear at a glance which areas were incorrectly over-irradiated or under-irradiated. As these images are incorporated in the planning
Fig. 8. Dose calculation on an axial plane and on a BEV plane perpendicular distribution is shown using a color-wash display.
to an oblique pair of fields. Dose
Computerized treatment planning 0 D. L. MCSHAN et al.
system, they can then be directly overlaid with BEV graphics which illustrate the importance of the various misalignments between the planned and actual irradiated fields with respect to the CT-derived target and normal structures. DISCUSSION It has been recognized over the last few years (11, 13) that making a connection between treatment planning information (typically displayed on transverse CT images) and portal views (simulator and verification radiographs) is very important. A large amount of sophistication in imaging, display techniques, and dose calculations is being routinely used in many treatment planning systems. However, when it becomes time to design shielding blocks, for example, there is little or no communication of information between the simulator radiographs on which the block shapes are usually drawn with a grease pencil, and the CT-based treatment plan and dose calculations. In addition, there is virtually no way to verify the accuracy of patient set-up and treatment, since the verification radiograph is typically compared only to the simulator radiograph, and not to any information from the planning system. In this paper, we present a comprehensive set of treatment planning system capabilities which allow the direct integration of all the portal information which is available with the computerized treatment planning information. Although various specific capabilities have been reported by others in the past (9, 11, 13, 24, 27) a systematic approach to this integration has not been reported. We have found that to attempt to use CT-type information, dose information, and BEV information, this integrated set of features is essential. The clinical utility of the BEV features described here is undeniable. The BEV review of block shapes with respect to the CT-derived target volume is mandatory in our clinic for all patients, and one is severely limited in the development of complex treatment plans without this ability. Solid surface graphics are increasingly used, especially for complicated and non-coplanar treatment plans in which the simple line representations of structures are confusing. Most of our shielding blocks are now designed on the planning system, using either the joystick with the BEV display, or the “autoblock” option. Finally, the digitization and use of simulator radiographs and DRRs has grown quickly. In many cases, especially those in which multiple imaging modalities are involved (for example MRI (4)), the digitized radiographs and other associated simulator data (mechanical contours) are critical during the alignment and correlation procedures. We continue to explore the capabilities which can be applied to the verification of treatment accuracy, including port film enhancement and integration with other treatment planning BEV information, and the use of DRRs for comparison to port or simulator films. The identification
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of landmark information either manually or automatically on separate films provides one mechanism for comparing separate images. Direct overlays of images with the ability to interactively (or automatically) translate and rotate one image relative to the other is also possible. The full use of these features can give a large amount of information about the accuracy of treatment planning and treatment delivery. The ability to integrate computerized portal imaging techniques into routine clinical treatment planning practice leads one rather directly to the need for integration of an on-line verification system which directly (digitally) acquires images from the therapeutic radiation beam. Investigations into possible real-time acquisition systems are being pursued ( 1, 14, 3 1). Several scenarios for the use of these images are possible, ranging from simple comparisons for initial set-up checks to systems which would either inhibit or possibly even adjust beam placements during a given treatment session based on the comparisons between the planned and actual portal. Both scenarios require a reliable method of correlating the images and the placement of the exposure with respect to the targeted volume. In combination with this analysis, an understanding of the degree of “acceptable” variation is required. The need for this error analysis has been emphasized by Goitien and Abrams (7). CONCLUSIONS Use of digital radiographic images and computer generated BEV simulations has proven to be a very useful tool in our clinical treatment planning system. Computerized BEV simulation assists in 3-dimensional treatment planning by providing localization data on tumor extent from the view point of an incoming beam of radiation and, therefore, assists in the specifying the shape and design of proposed treatment fields. This computerized facility helps to reduce the actual simulation and setup time for the patient. In effect, most of the simulation checks needed are done with the computer simulation using the patient’s localization data, but without the patient’s physical presence. Furthermore, developments on the integration of digitized radiographs within the computerized treatment planning system will ease the future transition to the use of true digital radiographic imaging modalities. The full specification of the treatment geometry within the computerized planning system facilitates the integration with computerized radiotherapy monitoring systems and provides valuable quality assurance checks for accurate and consistent radiation therapy delivery. Finally, computerized beam’s eye view simulation effectively links together the two previously separate but important processes of treatment planning: cross-sectional treatment planning and radiographic portal simulation and verification. This link provides a vital and necessary component in bringing about true 3-dimensional treatment planning in routine clinical practice.
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I. J. Radiation Oncology 0 Biology 0 Physics
June 1990,Volume 18, Number 6
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