State-of-the-art of external photon beam radiation treatment planning

0360.3016191 $3.00 + .oO Copynghr 0 1991 Pergamon Press plc

Inr. J. Radrotion Oncology Bioi Phgr Vol. 21, pp. 9-23 Prmted m the U.S.A All rights reserved.

0 Original Contribution

STATE-OF-THE-ART OF EXTERNAL PHOTON BEAM RADIATION TREATMENT PLANNING PHOTON TREATMENT PLANNING COLLABORATIVE WORKING GROUP* University of Pennsylvania School of Medicine and the Fox Chase Cancer Center, Philadelphia, PA 19111; Memorial Sloan-Kettering Cancer Center, New York, NY 10021; Mallinckrodt Institute of Radiology, Washington University School of Medicine, St. Louis, MO 63110; and Massachusetts General Hospital, Department of Radiation Medicine, Boston, MA 02114 and Harvard Medical School A virtual revolution in computer capability has occurred in the last few years, largely based on rapidly decreasing costs and increasing reliability of digital memory and mass-storage capability. These developments have now made it possible to consider the application of both computer and display technologies to a much broader range of problems in radiation therapy, including planning of treatment, dose computation, and treatment verification. Several methods of three-dimensional dose computations in heterogeneous media capable of 3% accuracy are likely to be available, but significant work still remains, particularly for high energy x-rays where electron transport, and possibly pair production, need to be considered. Innovative display and planning techniques, as well as plan evaluation schemes, are emerging and show great promise for the future. No doubt these advances will lead to substantially improved treatment planning systems in the next few years. However, it must be emphasized that for many of these applications a tremendous software and hardware development effort is required. 3-D treatment planning, 3-D display, Imaging, Radiation therapy, Computer graphics. ship to critical organs of the body. In this step, imaging information must be combined with all other relevant information (i . e . , physical examination, pathology, lab reports, etc.). The second step is localization which defines the biological target volume (BTV), the volume of tissue which includes the tumor volume and regions known to have, or considered to be at risk for containing, microscopic extension of disease and organs at risk together with the prescribed dose levels and fractionation schemes. The mobile target volume (MTV) exceeds the BTV to allow for factors such as the lack of reproducibility of patient position and organ motion. The delineation of the target volume is a critical part of the planning process and one in which new imaging technology will play a key role. Better precision in locating a tumor will minimize geometrical miss and may make it possible to increase radiation doses to the target volume, while at the same time minimizing doses to the surrounding dose limiting normal tissues. Higher doses to a tumor can potentially increase survival rates and possibly decrease local recurrence rates. Lower doses to normal tissues can decrease the risk of complications. The next step in the treatment planning process is to determine the treatment approach. If external beam radia-

INTRODUCTION Radiation is an important modality in the treatment of cancer. Quality care requires careful planning in each phase of the treatment process. In this paper, the elements comprising state-of-the-art treatment planning are discussed. This requires a clear understanding of the definition of the term state-of-the-art. A commonly used definition is “that which is commonly available.” In the case of radiation treatment planning, that might mean, for example, the availability of CT scans from the radiology department, a commercial simulator and a commercial treatment planning system. An alternative definition of state-of-the-art is “the best that is currently available.” The availability of such a state-of-the-art system might be limited to a few institutions. In this paper, the latter definition is used. An examination will be made of the latest (end of 1987) innovations for each stage in the treatment planning process either currently or soon to be available so that a model state-of-the-art program can be established. The process of planning treatment consists of several steps. The first step is a thorough evaluation of the patient, including staging procedures to determine the tumor volume, the volume of gross disease, its extent and relation-

page for complete authorship. Reprint requests to: Marc Sontag, Duke University, Oncology, Box 3295, Durham, NC 27710.

Supported in part by NC1 Contracts NO1 CM-47316, NO1 CM-47695, NO1 CM-47696, NO1 CM-47697. *Drs. Marc Sontag and James A. Purdy are the assigned Writing Chairs for this article. The reader is referred to the title 9

Radiation

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tion therapy is selected, a beam arrangement (number, energy, type, sizes, shapes, directions and modifiers) is chosen and doses are calculated. The resulting plan is evaluated by examining the dose level and homogeneity within the target volume, and the dose to normal tissues outside the target volume. Finally, the accuracy of treatment can be verified by using port films and/or other dosimetric techniques. Although the steps involved can be arranged in the categories given above, general agreement as to the exact procedure to be followed does not exist. Different institutions have different capabilities, and the procedures and tools used to accomplish the various tasks can vary widely. Rapid development of new technologies, e.g., beam’s eye view display, 3-D dose calculation, 3-D dose surface display and dose-volume histograms, have the potential to change the practice of radiation oncology, particularly in the way treatments are planned and evaluated, and to result in the use of complex (possibly noncoplanar) beam configurations which are carefully tailored to encompass the target volume and spare normal tissues. IMAGING AND TARGET LOCALIZATION Improvements in imaging technology have significantly enhanced the ability of the radiation oncologist to stage and evaluate the response of tumors during and after treatment. In the coming years one can expect computed tomography, magnetic resonance imaging, single photon emission computed tomography, positron emission tomography, digital ultrasound, angiography, and lymphoscintigraphy to play an increasing role in the treatment planning process. The planning system will have to be able to integrate these data into a unified data set. In this section, a brief review is presented on the application of the various imaging modalities to the treatment planning process. Computed tomography The use of CT images for planning radiation therapy became popular when this technology was first introduced nearly 15 years ago, providing data for both target and normal tissues volume definition and quantification of tissue density. These images do not have the superimposition of structures found on planar radiographs. Thus CT information is easier to interpret. In addition, CT scans present the anatomy in cross-section, the projection most often used for treatment planning. A number of studies have demonstrated the advantage of using CT cross-sections as part of the treatment planning process (2, 7, 18, 28, 36,48, 50, 51, 55, 56, 59,72). Such studies show that the number of geographic misses is significantly decreased when this information is used. Each picture element (voxel) of a CT image has associated with it a Hounsfield number which is a measure of the linear attenuation coefficient for that voxel. This can be related to the electron density which is a parameter needed for dose calculation in a heterogeneous medium. Simple methods

for bringing

CT information

into the

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planning process were devised as soon as the images became readily available: photographic enlargers were used for projecting the cross-sectional views onto contours obtained during simulation and the combined information was used as input to existing treatment planning programs. At present, CT data are routinely read directly by treatment planning computers via magnetic tape. It is imperative that the patient be scanned in the treatment position (68). This requires good immobilization and a flat insert for the CT scanner couch. The axial spatial resolution required for treatment planning has long been an issue. Commercial CT scanners typically have spatial resolution of better than 5 lp/mm which is adequate for planning radiation therapy. The slice thickness and inter-slice spacing for CT sections impacts on nearly every phase of the treatment planning process. It affects definition of structures for localization, the quality of reconstructed images (sagittal, coronal, arbitrary planes and digital radiographs) used for both localization and dose display, the resolution of the beam’s eye view image used for beam arrangement, and the definition of volumes for plan evaluation using dose-volume histograms. The requirements for scan spacing are different for each application and are often determined by the combination of features employed for a particular treatment planning situation. Accurate definition of the inferior and superior borders of the treatment volume for localization requires a close spacing between sections. Sagittal, coronal or arbitrary plane reconstructions may be better suited than crosssectional CT images for monitoring changes near these borders. Increasing the number of slices increases the time that the patient spends in the CT scanner and the increased data decreases throughput on the planning computer as well. Contiguous CT slices with thicknesses in the range of 0.3-0.5 cm for the head and 0.5-1.0 cm for the body appears to be a reasonable compromise between resolution and throughput; further study in this area is required. Some commercial simulators now have a CT option in which images are produced using its fluoroscope as a detector. These images have less resolution than CT scanners and the scan time is long (typically 60 s per scan). Further investigation is required as to its acceptability. A sequence of CT images for localization does not provide information about motion of structures due to either cardiac motion or respiration. Although CT localization can sometimes be combined with a localization procedure in which a fluoroscope is used to assess the movement of organs during treatment, there are no methods to handle those cases where the anatomy cannot be easily visualized with fluoroscopy (when overlaying structures obscure each other). At present, standard practice has the patient either hold his/her breath during the process of accumulating the CT data or more commonly, breathe in a shallow manner. Magnetic resonance imaging The use of magnetic resonance (MR) imaging continues to show promise as an imaging modality for the assessment

State-of-the-art 0 COLLABORATIVE WORKINGGROW

of cancer (8, 14, 16, 24-27, 66, 69). Studies have shown that the central nervous system is especially well seen by MR imaging. Sarcoma, head and neck cancer, prostate cancer and lymph nodes at various sites are also visualized and MR imaging appears suited for their workup. This enhanced ability to visualize the gross tumor and nodal involvement is probably the most significant contribution MR images provide radiation oncologists for treatment planning purposes. The ability to obtain MR images in any arbitrary plane without loss of spatial resolution may also be significant for treatment planning. Tumor localization is not the only area in which MR may prove significant. It may also be useful in predicting and following tumor response after radiation therapy. Ng (53) has shown that changes in phosphorus metabolism may be measured by MR for a number of tumors after treatment, thus serving as a means of monitoring tumor response. Also, Schwartz (62) has postulated that it may be feasible to detect tumor hypoxia in vivo with MR studies. Thus MR may play a key role in monitoring radiation therapy treatments and also predict which tumors may be treated effectively with either conventional x-ray or perhaps high LET radiotherapy. MR images may be subject to geometric distortion (34). Long scan times may introduce artifacts due to patient and respiratory motion. Bone is only imaged negatively, through an absence of signal, and may not be distinguished from air spaces. As yet, no method has been found to derive electron densities from MR image data, making the use of MR for dose calculations suspect. The complementary information that CT and MR sometimes provide, and the fact that sometimes MR is the preferred modality for tumor and/or normal tissue localization, while CT data are needed for dosimetry, make it important to be able to correlate the two modalities. This is a difficult task, although solutions are being investigated (38, 39). Ultrasound scanning Progress in the use of ultrasound imaging in radiation treatment planning has been slower and less dramatic than that of CT and MR. Ultrasound has been used primarily for determining chest wall thickness and for determining external skin and lung contours. In addition, it has also been used to obtain the depth of the lens for treatment of retinoblastoma (24). Research into the measurement of the amplitude of the transmitted ultrasound pulse as analyzed through a 2-D portrayal of attenuation coefficients may allow differentiation between normal and malignant tissues. Also, time of flight or speed of propagation of ultrasound waves through tissue may enable one to map temperature distributions in tissues which could be of value in the use of hyperthermia for therapy (33). Single photon emission tomography Antibodies labelled with radionuclides are potentially of great diagnostic interest and may one day be a major way

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of localizing tumors; they are also of interest therapeutically (15, 33, 41, 54). Single photon emission computed tomography (SPECT) can be used to determine the quantitative spatial distribution of the radionuclide. The resident times for radionuclides in tumor sites and critical organs are estimated by linking the serial quantitative images through connective models. This information provides the cumulated activity and also the volume or mass of tissue within which it is distributed which, in therapeutic applications, allows one to calculate the dose distribution. Positron emission tomography Positron emission tomography (PET) compliments CT or MRI scanning in the anatomical localization of a tumor by providing information on regional tissue physiology (33). Specifically, it is now possible with PET to measure regional blood flow noninvasively in a variety of metabolic processes. Thus far, the organ most extensively studied with PET has been the brain (4). Measurement of tumor blood flow may also be achieved using PET imaging techniques following high energy radiotherapy. High energy photons produce oxygen- 15 which may be imaged experimentally to determine the blood flow (71). However, this technique is in need of further research to fully develop its usefulness. As mentioned above, antibodies labelled with radionuelides are being used therapeutically. Labelling the antibodies with suitable positron emitting radionuclides such as Co-55 and Ga-68 would allow PET to be used to measure the distribution and uptake within the patient. Hyperthermia alone or in combination with radiotherapy is another treatment modality for cancer in which PET could play a significant role. At this time there is an insufficient knowledge of the tumor microvasculature and its physiological response to heat. With growing clinical interest being shown in hyperthermia, it is important to measure changes in regional blood flow in microvascular volume of patients undergoing treatment. PET appears well suited to study these changes in deep seated tumors. Lymphoscintigraphy Radiation therapy of patients with breast carcinoma sometimes involves irradiating the ipsilateral internal mammary lymph nodes by a separate anterior field or by opposing tangential fields, which also treat the breast and chest wall. The usefulness of lymphoscintigraphy for the localization of the internal mammary lymph nodes has been demonstrated (17). For the anterior field technique, the planning process is straightforward in that the treatment field is superimposed onto an anterior lymphoscintigram. For treatment by opposing tangential fields, the problem is more complex. Siddon (67) describes a method by which the 3-D lymph node positions obtained by a stereo-lymphoscintigraphic procedure are projected onto the simulation tangential radiographs. Jones (37) points out that one of the problems in introducing lymphoscintigraphy into the treatment planning process has been its use only in verifying

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that the treatment ports encompass the nodes. They recommend that lymphoscintigraphy be used to look at the nodes prospectively to facilitate the design of the field arrangement to encompass the nodes in a satisfactory manner. Fluoroscopy

Fluoroscopy plays a key role in the simulation process in determining field arrangement and will be discussed in a later section of this report. Fluoroscopy also plays a special role not easily handled by other imaging modalities in that it demonstrates the effects of physiologic movement. The motion of structures due to either cardiac motion or respiration can be quantified by viewing the image from an intensifier tube. Image correlation

Each of the imaging modalities described above may contribute to the localization procedure. It is necessary to be able to combine these into a single, unified data set using image correlation techniques. Historically, the burden of integrating this data has fallen primarily on the physician. Typically, films of the multiple imaging studies are viewed on light boxes and anatomic structures are transferred from one study to another by wax pencil. Complicating factors include differences in a) patient position; b) section thickness or pixel size; c) anatomy visible in the different modalities (e.g., bone/air interfaces are clearly seen on CT scans but are not visible on MR images); d) spatial resolution; and e) image distortion characteristics of the imaging modality (e.g., magnetic field nonuniformity). The accuracy with which image correlation can be performed under these conditions is questionable. Yet there are clinical situations in which accuracy on the order of a few millimeters is critical. In such situations, it is desirable to be able to transfer a 3-D volume of interest defined in one study into the frame of reference of a second study, with an accuracy of a few millimeters. The manual system of correlation must be replaced. Several techniques have been developed to aid in image correlation. The use of fiducial markers on the patient makes the transformation easier. Matching of surfaces such as the inner surface of the skull has proved to be a promising technique (38, 39). Highly interactive graphics systems may allow visual matching of the images. Clearly, work remains to be done in this area and considerable research opportunities exist. SIMULATION

There are many different approaches to the use of radiation therapy simulators. Thus simulation has never been a well-defined procedure and the role of the simulator in the treatment planning process is becoming more confused as new treatment planning tools are introduced. In the past, because the simulator unit played a role in virtually every phase of treatment planning, defining the

May 15, 1991, Volume 21, Number

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various treatment planning steps also defined the steps in the simulation process. Today, however, much of the treatment planning information is taken from CT data and use of the simulator has changed considerably. The following is a list of the steps in the treatment planning process where simulation has classically played a role: 1. Localization-The simulator is used as a reference device to index available diagnostic studies to marks on the patient’s skin or to provide primary information on the position of anatomic structures relative to skin marks. An orthogonal set of radiographs is used for this purpose. Also, a simulator with a fluoroscopic attachment can be used to show how structures move with respiration or cardiac movement. 2. Beam arrangement-The simulator is used to determine the relative position of structures at different gantry angles. This is typically done dynamically using the fluoroscope but can be done statically by taking multiple radiographs. 3. Field shaping-Radiographs of the selected fields with areas to be protected indicated are used as templates for block fabrication. 4. Field marking-The light field is used to mark outlines of treatment and set-up fields on the patient. 5. Block verification-The shape and position of shielding blocks are verified by placing the patient on the simulator and obtaining a radiograph with the blocks in position. 6. Field verification-The treatment field radiographs taken on the simulator are compared to the treatment machine portal films to verify correct field placement. Initially, the introduction of CT data had little effect on this list, This new diagnostic modality was brought into treatment planning as an additional procedure introduced at time of localization. However, as radiation therapy departments gained more procedural control over CT scanning of their patients (using radiation therapy personnel to monitor patient positioning during scanning and to mark reference points on the patient’s skin as part of the procedure), the importance of the simulator for localization, at least for complex CT aided plans, has diminished. In addition, it is no longer necessary to use simulator information to map outlines of structures onto outer skin contours. It is now routine to transfer CT data directly from the CT computer to the treatment planning computer so that geometric relationships are exactly defined. In addition, beam’s eye view displays which use the CT data allow examination of different field arrangements before the time the patient enters the simulation room. These developments, for complex planning cases, have relegated the simulator to the role of generating the field verification films used for field shaping and comparison to portal films, and marking field outlines on the patient’s skin. In fact, it has recently been suggested (22, 52, 58, 64) that these functions also be handled using the digital data available from the CT data

State-of-the-art

set. This approach has been called CT simulation simulation.

0 COLLABORATIVE WORKING GROUP

or digital

Digital simulation Digital information has been used to produce a template for block fabrication by a number of investigators (21) and is now routine in some centers. This technique uses the beam’s eye view (BEV) image to determine the field shape and generates a plot of this outline which is used as a template for block fabrication. The data can also be used for automatic block fabrication using numerically controlled milling machines. Making shielding blocks from CT data has the advantage of separating structures which are not as easily visualized on simulator radiographs. The direct marking of fields on a patient’s skin, using CT data to define the intersect points, has been discussed by a number of authors (19, 29). This technique is implemented on commercially available equipment and has been tested in a clinical setting (23). This method uses the outer skin surface information as determined from the CT skin surface contours and defines skin intersect points using the field geometry. These points are then located with a computer directed laser and the field marked on the patient’s skin. Rectangular fields can be so marked or additional points used to outline blocked fields. Marking the blocks on the patient gives a check for block positioning. The step of mounting the blocks on the simulator for verification of position is not possible with the digital simulation approach.

OUTLINING

OF STRUCTURES

Target volumes and critical organs need to be outlined to allow proper beam placement and plan evaluation. Usually outlines are obtained from a series of axial CT scans although, in principle, arbitrary planes of any imaging modality could be used. Outlines may be obtained manually or automatically by the computer. Manual contouring is accomplished interactively using a light pen, track ball, digitizer pad, or joy stick. This is very labor intensive for even the most experienced user. A high quality display of the CT image must be available with the ability to change the mean and window interactively. Automatic contouring has the potential to speed the contouring process. “Isodensity” lines are drawn in regions of high density gradients. This is very effective for drawing the skin surface and internal structures with densities substantially different than soft tissue. However, this approach does not work very well for structures with water-like densities and for target volumes. The use of an expert system may further speed this process. Structures are drawn by the computer on the basis of past experience with other patients. Properties such as density, location and proximity to other structures might be used. For example, the spinal cord is a posterior, mid-line structure of waterlike density bounded with a variable margin by bone of

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higher density. Again, this is an area where considerable research opportunities exist. BEAM ARRANGEMENT To develop a good treatment plan efficiently, one must be able to understand the complex geometrical relationships among the beams, target volumes and internal structures. Often plans need to be recalculated because the treatment planner team did not implement what was intended. For example, the target volume was not encompassed by all of the beams. Visual optimization and verification of the plan prior to dose calculation will minimize the need for repetitive calculations. This requires a highly interactive graphics workstation. Placement of fields The traditional method of arranging the treatment fields for a plan is to use a combination of simulation films and cross-sectional representations of the anatomy. Often the arrangement of the fields is accomplished using only a single or at most a few patient cross-sections. Optimum plan design may not be obtained, since the relationship of important structures within and outside the 3-D treatment volume is often difficult to perceive. This is particularly true when the central axis of one of the fields is not parallel to one of the major patient’s planes (axial, sagittal). The situation is further complicated if the central axes of three or more fields do not form a plane. A significant improvement in field placement may be obtained using the beam’s eye view (BEV) as illustrated in Figure 1 (29, 47). In this representation the patient’s anatomy is viewed from the therapy machine’s source position. The patient’s anatomical structures are projected along ray lines from the source onto a plane perpendicular to the central axis of the field. The position of this BEV plane is arbitrary; it may be at the isocenter, or at the position where beam defining apertures will be defined. In order to obtain a feeling of perspective, structures in front of and behind the BEV plane may be displayed with, respectively, greater and lesser intensities. The common practice, for speed and visibility, is to represent the patient’s anatomy by a series of closely spaced axial contours of the internal structures and outer surfaces. The structures appear in the BEV as a ring stack projected onto the BEV viewing plane. Sometimes the outer contour lines are projected as dashed lines in order to give the illusion of peering through the surface of the patient. With this capability, the orientation of a field with minimum normal tissue overlap is much easier to obtain than with traditional axial planning. Some ambiguity exists in the representation of BEV; the “eye,” which can be considered to be attached to the source, may either rotate with the collimators or not. Thus a collimator rotation may be viewed in the BEV plane as either a rotation of the collimator or of the patient. With the former mode, it is easier to appreciate the patient, treatment machine and couch positions for unusual field orientations.

I. J. Radiation Oncology 0 Biology 0 Physics

Fig. 1. Beam’s eye view (BEV) display in which the contours of critical structures and target volumes that have been outlined on the patient’s serial CT sections are seen in perspective as though the observer’s eye were placed at the source of radiation looking out along the axis of the radiation beam. The outline of the beam shaping blocks is displayed. The BEV approach is useful in identifying the best beam angles at which to irradiate the target and avoid irradiating adjacent normal structures by interactively moving the patient and the treatment beam. [MIR System.]

will be seen in the BEV plane, it is important to show them in color. It is also imperative to be able to include, exclude, highlight, and otherwise manipulate the visibility of individual structures to aid field placement. Structure manipulation must be interactive since it is necessary to almost continuously display and suppress display of structures as the beam parameters are adjusted. Other cues are important for field placement. In order to appreciate the position of the field a visual and quantitative representation of the machine and couch position and parameters is valuable. This set should include: collimator size and angle; gantry angle; couch longitudinal, vertical and lateral positions; and couch angle. An indication of whether the beam aperture is from the patient’s left, right, anterior, posterior, oblique, etc., is needed. In addition, the projection of the field onto standard patient sections is useful. The BEV representation has certain important limitations. It is a one field presentation. When there are additional fields, as is almost always the case in external photon beam therapy, the planner must remember the placement of the previous fields. Thus field overlap at critical structures may not be fully realized until after the dose distributions are calculated. Even with a single field, the distance of structures proximal or distal from the tumor may not be adequately appreciated since there is little depth information in the BEV. A further improvement could be obtained by adding a second observer’s or physician’s eye view (PEV) as shown in Figure 2 (58). Such a representaSince many structures

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Fig. 2. Physician’s eye view (PEV) which is as if the observer’s eye is in the treatment room at any arbitrary position showing the patient and all of the radiation beams. PEV is ideal for demonstrating proper handling of multi-beam arrangements, field abutments and gaps. Real-time interactivity greatly enhances this type display. [MIR System.]

tion literally permits the viewer to walk around the patient and view the field attached to and passing through the patient. In fact, multiple fields can be seen in this manner. A variation on this approach is shown in Figure 3 (60), in which each beam is projected on the anatomy, allowing the intersection of all of the beams to be viewed simultaneously. The original gray scale data may be used, allowing good anatomic detail to be maintained with some sacrifice in speed. The relative merits of BEV and PEV need to be studied further. It is probable that they will be shown to be complementary rather than competing methods of display. Beam modification The power of the BEV is greatly enhanced when apertures can be outlined or blocks positioned in the BEV viewing plane. It is necessary when drawing an aperture to leave an adequate margin. When the aperture is drawn interactively it is important to have an indication of the margin, for which any number of techniques could be used. For example, a cursor point can be shown surrounded by a circle with a radius equal to the margin. If a point on the circumference of the circle is used to trace the target volume, the cursor point traces the aperture. It is valuable after designing a beam aperture to show its projection onto a digitally reconstructed radiograph (DRR) simulated from CT data. The DRR is produced by projecting the CT numbers of the patient along ray lines from the source of the field onto a plane. A gray scale representation of that projection is similar, but not identical, to a x-ray radiograph. It is possible to produce video and a hard copy of this simulated radiograph, with a superimposed aperture, at the correct film magnification. It is also possible to

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COLLABORATIVE WORKINGGROUP

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plane. The BEV is not capable of graphically aligning the beams since only one field representation is available at a time. However, since the size of the fields in the match plane are known, it is trivial to obtain the correct separation of the central axis for alignment. This is a very restricted solution, since it is often necessary to match noncoplanar fields. Examples are the matching of lateral whole brain field with a posterior spinal cord field for medulloblastoma, and the matching of tangential fields with the supraclavicular field for breast cancer treatment. For the situation where these fields are matched in a plane, e.g., a vertical plane for the supraclavicular and tangent match, it is possible and would be helpful to have a graphical representation of the multiple beam intersections in that plane. A more general case is the solution to matching fields on the curved surface of the patient. One solution would be to display the light field intersections of all the treatment fields on a perspective graphical image of the patient’s surface. Better still is a representation of the entrance and exit radiation field intersections. Rotational fields

(b) Fig. 3. A variation of the PEV presentation showing (a) radiation beams for a tangential breast irradiation projected on the skin surface and (b) a cut-away view of this volume rendered data set shows how the beams cover the underlying lung. [UP System.]

adjust the contrast to improve structure visibility. Computerized smoothing and image enhancement techniques may also be used and need to be evaluated. The BEV of the aperture with the viewing plane at the blocking tray distance could be used for shielding block fabrication. When wedges are used, it should be possible to obtain a representation of the wedge with the BEV. Thus the wedge’s orientation with respect to the collimator would be apparent, and should reduce errors in planning. When compensators are designed, there should also be an indication that a compensator is attached to the beam in the BEV. A display of the compensator’s isothickness lines in the BEV is instructive. Matching

of j?elds

It should be possible to design plans with matching or abutting fields. The simplest problem is to match coplanar beams in a plane through the patient, e.g., the mid-coronal

A planning system should facilitate the use of rotational treatment. The process of field placement would involve setting the isocenter and defining the ranges in which the radiation field is on and off. One approach is to view the target coverage and normal structures from the BEV for a series of beam gantry angles. The start and stop angles and optimum isocenter position and collimator settings can be obtained by trial and error. A treatment planning system can be used to carry out more complex conformal calculations. A technique that may prove useful in the future is treatment with multileaf collimators (12). In this method the aperture is adjusted at each gantry position by adjusting a set of vanes or leaves, 30 to 40 in number with a length of perhaps 0.5 cm to 1.O cm as projected at the isocenter. It would be advantageous to have automatic computation of the vane settings (with proper field margin) at a prechosen set of gantry angles once the isocenter and x-ray start-stop gantry angles have been chosen. DOSE CALCULATION Computers have been used to calculate radiation dose distributions since the 1960’s. In the last 25 years, there has been a gradual improvement in dose calculation accuracy. This has been due as much to improvements in computers as to refinements of the calculational methods since the choice of a computer algorithm has often been dictated by limitations in computational speed and memory. For photon dose calculations, the majority of the commercial planning systems still use methods that were developed before the advent of CT when typical RTP computers typically had 16 kbyte of memory and 2 mbyte of disk

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storage. Doses are calculated assuming water density. Dosimetric data measured under well-defined conditions such as transient electronic equilibrium, normal incidence, fixed source-to-axis distances (SAD) are stored in tabular form and the computer manipulates these data. While this is appealing in that the calculations reproduce the measurements exactly, deviations from the defined conditions may produce significant calculational error. The introduction of CT provided an opportunity to make reliable corrections for tissue inhomogeneities. Most commercial systems use some type of effective path length approach in which only the density information along the primary photon path is used for dose calculation and include such methods as the ratio of TAR method, generalized Batho power-law method (3), etc. As noted in many review articles, these methods have deficiencies in the calculational accuracy, particularly when being applied to the highly irregular geometry of the patient (13, 57). Developed in 1977, the semi-empirical equivalent TAR (ETAR) method was the first algorithm specifically designed for CT pixel-based calculation and uses 3-D information (68). In the commercial implementations, the method is modified empirically to a less time-consuming formulation. While considerable improvement over the earlier methods has been achieved, disagreement with measurements has been shown for the ETAR calculations in some stringent geometries due, in part, to this modification. Recently, a modified columnar dSAR method has been introduced commercially. More recently, methods have been developed that use absorbed dose distributions derived from theoretical or Monte Carlo calculations. These data can include information not easily deduced from measurements, such as electron transport. However, theoretical dose values are generated with idealized assumptions and only approximate the therapy beam. These methods include the Delta-Volume method, Differential Pencil Beam (DPB) method, Dose Spread Array (DSA) method and the Fourier Convolution method (5, 6, 46, 49, 75, 76). The first three methods employ full 3-D scatter ray trace calculation to sample density information along the ray path connecting any two volume elements. In addition, the DPB and DSA calculations include electron transport corrections. The accuracy of each of the methods approaches or surpasses the accuracy obtainable with those based on measured dose tables, particularly in regions of electronic nonequilibrium. At present, only the Delta-Volume method has been implemented in a planning system (40). We anticipate that a 3-D dose calculation method that models the physics of radiation transport will be implemented in a clinical planning system in the near future. PLAN EVALUATION Evaluation of a 3-D treatment plan requires that the treatment planning team review a large volume of data.

May 15, 1991, Volume 21, Number 1

Fig. 4. Dose-volume histogram (DVH’s) provide a complete summary of the entire 3-D dose matrix showing the amount of target volume or critical structure receiving more or less than a specified dose level. Shown here is a cumulative dose-volume histogram for the right lung of a patient with lung cancer. The plan was calculated first without correcting for any heterogeneities (Plan A) and then repeated without changing the beam settings but correcting for heterogeneities (Plan B). The DVH provides the evaluator a tool to very quickly assess the impact of the inhomogeneity.

This is both time consuming and opens up the possibility of missing a key detail lost in the mass of data. Several techniques have been developed to improve the evaluation process. Dose-volume

histograms

Dose-volume histograms (DVH) graphically, as the name implies, illustrate the dose that various organs receives (10). It may be plotted as a differential DVH or a cumulative or integral DVH as shown in Figure 4. The information is organ rather than slice based so that the evaluation for the entire volume of a target or critical structure is reduced to a single graph. The price paid for this data reduction is the loss of information regarding the spatial relationships between dose and organ. DVHs are useful for identifying potential trouble spots such as a cold spot in a target volume. Isodose distributions are still required to identify the location of the cold spot. Isodose distribution

display

While DVHs provide information on the existence and magnitude of “hot” and “cold” spots within an irradiated volume, they do not indicate their location. The sheer volume of dose data in 3-D planning makes it difficult to interpret or to assimilate it when presented in a 2-D format such as isodoses displayed on individual 2-D axial, sagittal or coronal gray-scale images of a patient’s anatomy. Displays which present multiple views of reconstructed sections, such as sagittal and coronal sections, on a single screen do appear to be helpful. Arbitrary plane sections may be of some use, but are difficult to interpret.

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(4 Fig. 5. 3-D dose surface display with real-time interactivity enabled the radiation oncologists to view the target volumes or normal tissue volumes with superimposed isodose surfaces or

“dose clouds” from any arbitrary viewing angle. Such capability is a valuable tool for the evaluation of 3-D dose distributions in terms of adequate coverage of the target volumes and sparing of critical structures. [MIR System.]

Real-time display of the dose distributions in the axial, sagittal and coronal planes helps overcome the limitations of a 3-D distribution on a 2-D medium (CRT) (70). This requires special graphics hardware in which all the images are loaded in graphics memory and rapidly accessed. The real-time capability gives the illusion of the user moving through the CRT, thus providing access to the third dimension. Color wash representation displays provide an immediate (but subjective) impression of the dose distribution. This method of display appears most useful when viewing several scans in rapid order or when comparing multiple plans. However, for more quantitative use, a method to distinguish discrete dose values is needed. Isodose surfaces generated from the calculated 3-D dose matrix and displayed as wire frame surfaces on a graphics device capable of real-time manipulation of the images is also an effective means of dose data presentation. Figure 5 shows this type isodose presentation which was used by one of the contractors and proved to be an invaluable aid in plan evaluation (58). Another promising display technique uses volume rendered, rather than surface rendered, images. Volume rendered images retain the entire data set. For example, Figure 6 shows that by changing the density window, the treatment planner can remove the skin surface and soft tissue to reveal the skeletal structures. This capability requires special graphics hardware in which all the image data are loaded in graphics memory and rapidly accessed. Several other display presentations for 3-D planning have been developed by institutions not participating in this

(4 Fig. 6. (a) In contrast to surface rendered images, volume rendered images retain the entire data set. (b) Changing the density window removes the skin surface and soft tissue to reveal the skeletal structures. [UP System.] contract and are shown (Figs. 7, 8, and 9) (9, 20, 47).

planning

here for completeness

Volumes of regret Volumes of regret is a dose reduction technique applied to the dose distribution (63). Portions of organs that receive doses that fall outside of the window of acceptability are highlighted. For critical organs, this is a one-sided test, i.e., regions where the dose lies above an acceptable level are shown. For the target, regions where the dose is either too low or too high to be acceptable are shown. A volume of regret example is shown in Figure 10. This provides the treatment planning team with an immediate appreciation of potential problems while maintaining much of the spatial correlation of dose to anatomy. Tumor control probability and normal tissue complication probability Plan evaluation historically has been based on the subjective comparison of two or more plans by the physi-

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Fig. 7. Display showing four field technique for prostate cancer with axialmetric view of sag&al and axial CT. The bladder and rectum are shown as solid surfaces. Dose is represented as a color wash display on surfaces. [UM System.]

cian. Quantitative methods are being developed to aid in this evaluation, allowing the eventual development of a scoring system (32, 45, 61, 73, 74). Tumor control probabilities may be calculated to predict the likelihood of local control. Dose-volume histograms and the predicted behavior of the cell population in question are used as input data. While the former may be very precise, large uncertainty exists in the biological and clinical data. Similarly, normal tissue complication probabilities may be calculated. While absolute probabilities are not yet attainable, relative probabilities may be used to rank treatment plans.

TREATMENT VERIFICATION The verification step in the treatment planning process uses higher quality, higher contrast images obtained during the simulation process to help interpret lower quality, lower contrast portal images obtained with the patient positioned on the treatment machine. The procedure uses reference structures (typically bone, airways, or metallic clips) which can be seen on both images to determine agreement of the field positions. In addition, the high contrast image establishes the relative position of other structures which might not be seen on the low contrast portal image. Thus, assuming that the patient is in the same position for both procedures, the location of structures on the portal image is determined by inference. Obviously, any discussion of treatment verification must address both aspects of the procedure, i.e., obtaining portal information and the gen-

May 15, 1991, Volume 21, Number 1

Fig. 8. Display showing a four panel view: the top two being axial CT slices and the bottom two simulator films with and without BEV display superimposed. [UM System.]

eration of verification information for comparison to the portal image. Currently, considerable attention is being paid to improving the quality of radiographs of the treatment field and developing on line imaging capabilities (11, 42, 44). Images can be optimized by using various film/screen combination and can be enhanced to improve contrast (65). Attempts at using detectors other than film are also showing promise (35). This research offers the possibility of viewing the portal position during treatment and allows easy enhancement of the digital information. Also, the information is captured by computer and can easily be compared with other stored data, e.g., the BEV. Another interesting attempt at improving the quality of the portal image uses a diagnostic x-ray tube mounted on the gantry of the treatment unit to produce duplicate images of the treatment portal. Obviously, these devices must be carefully calibrated to produce exactly the same geometry as obtained with the treatment head and must include a facility for holding field shaping blocks. Portal information, either in the form of a radiograph or a digital image, is usually compared to radiographs taken from the treatment unit simulator. However, this situation is also changing. There is considerable interest in making the comparison directly with CT information. This can be accomplished in two different ways. In one approach, the CT data can be reformatted to produce a digitally reconstructed radiograph, which includes all effects of perspective distortion and which the portal film should match. Field outlines and center marks or cross-hairs can be

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(4

(4

(b) Fig. 10. Display showing “volumes-of-regret” which indicate regions of potential problem. (a) Using a soft tissue density window, a volume of potential underdose is indicated in red for the breast irradiation shown. The green in the target and the cyan in the heart and contralateral breast indicate doses within acceptable limits. (b) Lowering the density window shows the high dose volume of lung in red. [UP System.]

(b) Fig. 9. (a) Shaded graphic display with opaque surfaces showing laser alignment lights and light field projected onto the anterior chest wall. The patient is laying supine with arms extended above the head. External anatomical features and alignment landmarks are easily evaluated but underlying structures are hidden. (b) Shaded graphic display of (a) with transparent surfaces. Important underlying structures are visible. In this case the esophagus, which is the target volume, and the stomach and spinal cord are seen beneath the skin surface. The light field is projected through the patient and intersections with the cord and esophagus are clearly visible. [NC System.]

superimposed to make an image similar to the simulator radiograph. A second approach uses the ring stack or wire frame outlines of a BEV image for comparison. When the portal information is available as digitized data, the BEV can be superimposed as a computer step. Another approach plots the BEV outlines with the appropriate magnification onto transparent sheets to be used as overlays on port films. The correlation of digitally reconstructed radiographs with port films is complicated because it is difficult to

obtain the required resolution with the DRR images. The resolution in the lateral direction is determined by the pixel size of the transverse CT image. The resolution in the cephalad/caudad direction is determined by the slice thickness and spacing. In many cases it is difficult to obtain the number of slices needed to produce resolution along the patient’s axis which is even close to that measured perpendicular to this direction. For example, to cover 25 cm of anatomy with a i cm slice spacing requires 75 images. This is a formidable task for even the fastest CT units available and the patient must remain motionless for a long time. In addition, respiratory, cardiac and bowel motion are a problem for such long studies. The time needed to reformat such large data sets is an additional problem. Given these problems, it is not clear that reformatted images can take the place of simulator radiographs. Additional research is needed in this area. The use of thermoluminescent dosimetry (TLD) or diode detectors to verify the treatment plan is also important. TLD has proven extremely valuable for determining the dose within cavities such as the mouth, esophagus, bladder or rectum. TLD dosimeters have also been used to determine the build up dose at the patient’s skin surface.

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Table 1. 3-D radiation treatment planning system specifications

(1) (2) (3) (4) (5) (6) (7) (8) (9) (IO) (11) (12)

(13) (I4 (15)

(16) (17) (18) (19) (20) (21) (22)

32 bit central processing unit (CPU) and data bus capable of at least 3 million instructions per section (MIPS) Optional coprocessor capable of increasing speed to 20 MIPS 20 mbyte of memory 0.5-l .O gbyte of disk storage 6250 bits per inch (BPI) magnetic tape for transfer of diagnostic images and archival storage of data Optical disk capability for archival storage of data 1024 pixel X 1024 pixel X 24 bit frame buffer with color capability Image processing speed of at least 107 pixels per second Graphics processing speed of at least 106 vectors per second Color hardcopy device capable of faithfully reproducing CRT display Digitizer with f 0.5 mm accuracy Network capability to tie RTP computer to diagnostic machines (CT, MRI, simulator) and treatment machines Image correlation/enhancement capability Beam definition parameters should be defined and changed interactively via keyboard or graphic devices Handling of any clinically used modalities, e.g., any combination of stationary or moving photon and electron beams, and brachytherapy Dose calculational accuracy ? 3% or 2 3 mm Dose grid spacing 3 mm maximum in plane of input slices Hardcopy of a treatment plan should contain all information as recommended by the ICRU (43) Plan evaluation tools, e.g., dose-volume histogram, dose surface display, dose statistics, TCP, and NTCP Software to be hardware independent Standards should be followed when specifying treatment geometry Complete documentation including: (a) training for system operation; (b) user’s guide; (c) data file formats; (d) algorithm descriptions; (e) source listings; (f) test examples of system operation; (g) training manuals for hardware maintenance for simple local service

Techniques have also been developed for calculating doses within the patient from entrance and exit dose measurements. More recently, a method based on comparing measured and calculated exit dose distributions has been proposed (77).

IDEALIZED 3-D RADIATION TREATMENT PLANNING SYSTEM A “state-of-the-art” 3-D radiation treatment planning system will have advanced features pertaining to data acquisition, dose calculation and information display. Requirements are listed in Table 1. Specifically, a 3-D planning system should provide the following capabilities:

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1

1. synthesis of all relevant diagnostic information for evaluating the patient’s disease, 2. appreciation and efficient delineation of anatomy and target volumes, 3. design of treatment fields, 4. design of treatment aids (e.g., shielding blocks, compensators, etc.), 5. dose calculation and optimization of 3-D dose distributions , 6. display of 3-D dose distributions, 7. treatment plan evaluation, 8. field verification (e.g., computer generated simulation and port films). Hardware Ideally, hardware should be chosen so that it is not the limiting factor in the RTP system design. This is not always possible due to cost or limits in technology. The hardware described in Table 1 will allow one to develop an RTP system with the features described above. All of the technology is currently available, most at reasonable prices. It is assumed that prices for these technologies will continue to fall to the point that a system may be configured for a price comparable to today’s commercial systems. It is not clear whether a single large computer or a network of several smaller computers is best suited for 3-D planning. Much depends on the numbers of users, type of computing and the state of technological development. Analysis should be based on system flexibility, memory limitations and the effective speed at which calculations can be done. It may be faster to have several users work on many smaller computers than on a single larger, more powerful computer. Array processors improve calculation speeds at the expense of increased software development time. At least one commercial RTP system makes use of an array processor. Unfortunately, many formalisms and algorithms used in RTP dose calculation and display are difficult to vectorize, a process necessary to make optimum use of the power of the array processor. Many of the calculations done in RTP may be better suited for parallel processor technology which will be readily available in the next five years. Real-time 3-D display is proving invaluable for both beam placement and plan evaluation (58). Clinical experience now being gained coupled with further technological improvement and cost reduction should make relatively inexpensive systems having this capability widely available within the next five years. Sofbvare In contrast to a research system where the users are those expert individuals involved in the actual development of each specific component, the clinical system needs to be organized in a user-friendly fashion so that treatment planning technologists or physicians can readily master the use of the system. It likely will be inefficient to simply extend the present commercial 2-D system to 3-D since

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much of the data structure and algorithm requirements will be different and the volume of data that needs to be examined and manipulated is much greater. A more effective approach is to examine the goals of the system and organize the flow of the different jobs for easy operations and speed. In view of the rapid advances continuously being made in hardware technology, another important consideration is development of “portable” software that will run on succeeding generations of computers without great modification. To the application program, this means that the primary source code of the program cannot contain commands or constructs which are not a part of a standard environment (or operating system). Data files should be an established standard to allow easy transfer of data between institutions. Documentation Complete documentation is essential for proper use of an RTP system, yet it is almost universally inadequate. It should contain sufficient detail to allow its correct use, maintenance and improvement. Uniform data structures Data structures (machine data, patient data) should be in a universal format. This will facilitate transfer of data among the diagnostic machines, treatment machines, and RTP computers within a department as well as data transfer among different departments. A format for the exchange on magnetic tape of radiation treatment planning information has been developed for this and for related NC1 RTP contracts (31). It follows the recommendations of the AAPM for digital image transfer

(l), while adding other types of data such as structure contours and dose matrices. The tape contents are stored as files, with CT images written in binary form and all other data as ASCII character strings. As the treatment planning systems at the four sites use and produce information in varying forms, some thought and effort was required for adaptations to the exchange tape format. SUMMARY

AND CONCLUSION

A virtual revolution in computer capability has occurred in the last few years, based largely on rapidly decreasing costs and increasing reliability of digital memory and mass-storage capability. Several methods of three-dimensional dose computations in heterogeneous media capable of 3% accuracy are being developed, but significant work still remains especially for high energy x-rays where electron transport, and possibly pair production, need to be considered. Innovative efforts in 3-D RTP displays are progressing rapidly and while this project concentrated on photon treatment planning only, clearly, 3-D RTP systems must integrate planning and dose distribution display for photon, electron and brachytherapy. Beam’s eye view and physician eye view have been helpful in 3-D interactive treatment planning. Dose-volume histograms have proven to be an excellent method of summarizing a large body of information, particularly when comparing two or more plans. Dose surface displays with real-time interactivity also appear to be extremely useful for plan evaluation. As display techniques improve and as 3-D planning systems take more advantage of real-time interactivity, their impact on clinical practice is likely to be substantial, However, it must be emphasized that for many of these applications a tremendous software development effort is required.

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