6 The Simulation Process in the
Determination and Definition of the Treatment Volume and Treatment Planning
Sasa Mutic, James A. Purdy, Jeff M. Michalski, and Carlos A. Perez
S. Mutic, MS; J. M. Michalski, MD; C. A. Perez, MD Department of Radiation Oncology, Washington University School of Medicine, St. Louis, MO 63110, USA
James A. Purdy, PhD
Department of Radiation Oncology, University of California Davis Medical Center, 4501 X Street, Sacramento, CA 95817, USA
6.1
Introduction
Radiation therapy is a continually evolving medi- cal specialty, especially considering the technology used for treatment planning, treatment, and deliv- ery verification. During the past two decades, the field has evolved from treatment planning based primarily on planar radiographs to planning based on volumetric study sets. Currently, the vast major- ity of radiotherapy treatment plans are based on volumetric study sets. This significant increase in use of volumetric imaging in radiotherapy is due to the overwhelming acceptance of conformal radia- tion therapy as a standard of care for many treat- ment sites. The four primary imaging modalities employed in modern radiation therapy treatment planning process include computed tomography (CT), magnetic resonance imaging (MR), positron emission tomography (PET), and ultrasound (US).
Planar X-ray radiography remains an important component of treatment planning and treatment verification process. Due to the current importance of treatment planning based on volumetric images, the primary emphasis of this chapter is CT simula- tion and incorporation of other imaging modali- ties in the CT simulation process. The use of con- ventional simulator in the age of CT simulation is addressed accordingly.
The radiation therapy simulator has been an integral component of the treatment-planning pro- cess for over 30 years. Conventional simulators are a combination of diagnostic X-ray machine and certain components of a radiation therapy linear accelerator. The conventional simulator (Fig. 6.1) consists of a diagnostic X-ray unit and fluoroscopic imaging system. The treatment table and the gantry are designed to mimic functions of a linear accel- erator. The gantry head is designed to accommo- date different beam modification devices (blocks, wedges, compensating filters), similar to a linear accelerator. The images are transmission radio- graphs with field collimator setting outlined by
CONTENTS
6.1 Introduction 107 6.2 Technology Overview 109 6.2.1 Conventional Simulator 110 6.2.1.1 Imaging Chain 110 6.2.1.2 Simulation Software 110 6.2.2 CT Simulator 111 6.2.2.1 Large-Bore CT 111 6.2.2.2 Multislice CT 112 6.2.2.3 CT-Simulator Tabletop 113 6.2.3.4 Patient-Marking Lasers 114 6.2.3 MR Simulator 114 6.2.4 PET/CT Simulator 116 6.2.4.1 Stand-Alone PET 116 6.2.4.2 Combined PET/CT 116 6.2.5 Virtual Simulation Software 117 6.3 Multimodality Imaging 118 6.3.1 Detection 118
6.3.2 Staging 118
6.3.3 Target Definition and Altered Dose Distributions 118
6.3.4 Evaluation of Response to Therapy and Follow-up 119
6.4 Patient Positioning and Immobilization 119 6.5 Simulation Process 124
6.5.1 Conventional Simulation 124
6.5.1.1 Patient Positioning and Immobilization 125 6.5.1.2 Verification of Patient Position Using Fluoroscopic Imaging 125
6.5.1.3 Determination of the Isocenter Location 125 6.5.1.4 Beam-Placement Design 126
6.5.1.5 Transfer of Simulation Information for Treatment Planning and Treatment 126 6.5.2 CT Simulation 126
6.5.2.1 Scan and Patient Positioning 126 6.5.2.2 Image Transfer and Registration 129 6.5.2.3 Target and Normal Structure Delineation 130 6.5.2.4 Treatment Techniques 131
6.6 Discussion and Conclusion 131 References 132
delineator wires. Using primarily bony landmarks, the physician outlines areas to receive therapeutic radiation doses. A shortcoming of a conventional simulation process is that very little anatomy, other than bony anatomy, is available for design of treat- ment portals. Shortly after the introduction of clini- cal CT scanners in early 1970s, it was realized that this imaging modality has much to offer in a radia- tion oncology setting. The CT images provide infor- mation not only about target volumes but about critical structures as well. Using CT images for radiation therapy treatment planning has enabled us to improve dose delivery to target volumes while reducing dose to critical organs. The CT images also provide density information for heterogeneity- based dose calculations.
A major weakness of CT imaging is a relatively limited soft tissue contrast. This limitation can be overcome by using CT images in conjunction with MR studies for treatment planning. The PET images can be used to add physiological information. Ultra- sound has also been useful for imaging in brachy- therapy. Multimodality-imaging-based treatment planning and target and normal structure delinea- tion offer an opportunity to better define the ana- tomic extent of target volumes and to define their biological properties.
Tatcher (1977) proposed treatment simulation with CT scanners. This short article described the feasibility of a CT simulator and indicated potential economical benefits. Goitein and Abrams (1983)
and Goitein et al. (1983) further described multidi- mensional treatment planning based on CT images.
They described a “beam’s-eye-view” (BEV) function which “provides the user with an accurate reproduc- tion of anatomic features from the viewpoint of a treatment source.” They also described how “pro- jection through the CT data from any desired origin provides an alignment film simulation which can be used to confirm accuracy of treatment, as well as help establish anatomic relationships relative to the margins of a treatment field.” In reality, this was a description of the major characteristics of a system that we know today as a CT simulator or virtual sim- ulator. An alignment film created from a divergent projection through the CT study data is commonly known as a digitally reconstructed radiograph (DRR). Additionally, use of DRRs in radiation ther- apy has been developed (Sherouse et al. 1990a).
Sherouse et al. (1987, 1990b) described a CT- image-based virtual simulation process which they referred to as a “software analog to conventional simulation.” They described software tools and addressed technical issues that affect the present CT-simulation process. They pointed out the need for fast computers and specialized software, but also the need for improved patient immobilization and setup reproducibility.
The radiation oncology community eagerly embraced the concept of virtual simulation and in early 1990s commercial packages became available.
These systems consisted of a diagnostic CT scanner, external laser positioning system, and a virtual sim- ulation software workstation. One of the early com- mercial CT simulation packages is shown in Fig. 6.2.
(AcQSim Oncodiagnostic Simulation/Localization System, Philips Medical Systems).
The CT simulators have matured to a point where they are one of the cornerstones of modern radiation oncology facilities. The present systems incorporate specially designed large-bore CT scanners, mul- tislice CT scanners, high-quality laser positioning systems, and sophisticated virtual simulation pack- ages. Many systems incorporate dose calculation capabilities and treatment-plan analysis and evalu- ation tools.
Additional virtual simulation software features and functions along with increased efficiency and flexibility have enabled CT simulators to replace conventional simulators in many facilities. This trend seems to be further fueled by the increased demand for imaging studies for 3D and IMRT treat- ment planning where conventional simulators are of limited value.
Fig. 6.1 Modern version of a conventional simulator. (Courtesy of Varian Medical Systems, Palo Alto, Calif.)
Figure 6.3 shows the place of CT simulation in the treatment-planning process. The implementation of simulation and treatment-planning process varies greatly between radiation oncology departments.
This diversity is in part driven by significant tech- nical differences between simulation and treatment- planning systems offered by different manufactur- ers. It would be impractical to address all possible variations in implementation of these processes.
This chapter discusses the most common points and describes general differences between some popular approaches.
6.2
Technology Overview
One of the major recent changes in radiotherapy imaging is the approach of imaging equipment manufacturers towards radiation oncology and its unique imaging needs. Conventional simulators were always designed specifically for radiotherapy purposes and provided tools for accurate and effi-
cient simulation and treatment planning. The scan- ners used for volumetric imaging were, on the other hand, historically designed with diagnostic radiol- ogy needs in mind, with little or no concern for radiation therapy needs. A compounding factor to this problem is that scanner characteristics which are extremely important in radiotherapy are often not a significant concern or not needed in diagnostic radiology. The scanners used in radiotherapy should have flat table tops, larger openings to accommo- date immobilization devices and patients in con- ventional treatment positions, and software tools which can improve patient positioning and target delineation (Mutic et al. 2003). Typically, diagnostic scanners were modified to meet the radiotherapy needs; flat tabletops and external patient position- ing lasers were added to these scanners. This process worked well but had many limitations for simulation and treatment planning of certain tumor sites. The CT simulation process was also not as efficient as possible due to technological limitations. To rectify this, in the past few years, several CT scanners have been introduced with features designed specifically for radiotherapy. Even PET/CT scanners, whose pri-
Fig. 6.2 A CT-simulator room layout.
(Courtesy of Philips Medical Systems, Cleveland, Ohio)
Fig. 6.3 Place of CT simulation in ra- diotherapy treatment-planning process.
(From Mutic et al. 2003) CT Simulator
CT Scanner Virtual
Simulation
Treatment Planning System Dose Calculation
mary purpose is diagnostic scanning, are designed with radiotherapy needs in mind. Radiotherapy needs have begun to drive product development.
This change in manufacturer approach towards radiotherapy has resulted in a variety of imaging options available to radiation oncology departments and it has improved our ability to image patients for treatment-planning purposes. This development in volumetric scanning capabilities has inevitably led to improvements in conventional simulator design as well, as this technology has to be able to compete and keep up with other advances in treatment plan- ning and delivery techniques.
Although there have been numerous improve- ments in CT technology for radiotherapy and accom- panying virtual simulation and treatment-planning software, there remains lot of room for progress.
Dynamic CT acquisition for radiotherapy treat- ment planning needs significant development and is really just in its infancy. Virtual simulation software (contouring, isocenter and beam placement, and port definition capabilities) needs to reside directly on CT-scanner control consoles to improve simu- lation efficiency. The CT acquisition protocols for radiotherapy need further improvements to capital- ize on the fact that radiation dose from a CT scan is not a significant concern for radiotherapy patients, and that significant increases in CT acquisition techniques are acceptable in order to improve soft tissue contrast and image quality. Correspondingly, virtual simulation and treatment planning software needs to be able to accommodate increased image- acquisition capabilities of multislice CT scanners, as described later in this chapter. The vast majority of the modern treatment-planning systems are practi- cally limited to 200–300 images per patient study.
Multislice CT scanners can produce several hundred to several thousand images per scan which can be used to improve tumor definition, understanding of tumor and normal structure breathing motion, and verification of patient treatments accuracy.
6.2.1
Conventional Simulator
The conventional simulator (Fig. 6.1), consists of a fluoroscopic imaging chain (X-ray tube, filters, collimation, image intensifier, video camera, etc.;
Bushberg et al. 2002), generator, patient support assembly (treatment table), laser patient positioning / marking system, and simulation and connectivity software. The imaging chain and simulator software
have undergone several improvements during the past few years.
6.2.1.1 Imaging Chain
One of the major changes in the imaging-chain design for conventional simulator was the replace- ment of the image intensifier and video camera system with amorphous silicon detectors. The new imagers produce high spatial and contrast resolu- tion images which approach film quality (Fig. 6.4).
More importantly, these images are distortion-free, a feature that is important for accurate geometric representation of patient anatomy. The introduc- tion of high-quality digital imagers in conventional simulation further facilitates the concept of filmless radiation oncology departments.
Fig. 6.4 Digital image of a head from a modern conventional simulator equipped with an amorphous silicon imager. (Cour- tesy of Varian Medical Systems, Palo Alto, Calif.)
6.2.1.2
Simulation Software
The conventional simulation software has also undergone many improvements. Modern simula- tors have Digital Image Communications in Medi- cine (DICOM) standard import capabilities (NEMA 1998). Treatment field parameters can be imported directly from the treatment-planning computer.
The software can then automatically set the simu- lator parameters according to the treatment plan.
This facilitates efficient and accurate verification
of patient treatment setup on the conventional sim-
ulator. These simulators also have DICOM export capabilities which enable transfer of treatment setup parameters directly to a record and verify system or to a treatment-planning computer. The ability to import and capture digital images enables conven- tional simulators to have tools for automatic correla- tion of treatment planning and verification fields.
Vendors also offer solutions for some shortcom- ings of older conventional simulators. For example, older simulators were not equipped with tools to verify portal shapes created with multileaf collima- tors (MLCs). Newer simulators have features which can project MLC shapes directly on the patient’s skin or on the portal films (Fig. 6.5).
A possibility for future of conventional simula- tion imaging is cone-beam CT. Since newer simula- tors are equipped with digital imagers, it is possible that cone-beam CT technology, which is available on linear accelerators, can also be implemented on con- ventional simulators. This will significantly improve imaging capabilities and usefulness of these devices.
Figure 6.6 shows a cone beam CT image from a con- ventional simulator.
While it is often mentioned that conventional simulators can be completely replaced with CT sim- ulators, new features and usefulness of conventional simulators are slowing down this process. Conven- tional simulator continues to be an important com- ponent of radiotherapy process, even though its use for treatment planning of many tumor sites has been significantly reduced.
6.2.2 CT Simulator
The CT simulator consists of a CT scanner, laser patient positioning / marking system, virtual simula- tion / 3D treatment planning software, and different hardcopy output devices. The CT scanner is used to acquire volumetric CT scan of a patient which represents the virtual patient and the simulation software creates virtual functions of a conventional simulator. The three most significant changes in CT- simulation technology, in recent years, have been the introduction of a larger gantry bore opening (large- bore CT; Garcia-Ramirez et al. 2002), multislice image acquisition (multislice CT; Klingenbeck et al. 1999), and addition of CT-simulation software directly on the CT scanner control console. These innovations improve efficiency and accuracy of CT- simulation process. They also improve patient expe- rience by allowing patients to be positioned in more comfortable positions and reducing the simulation procedure time.
6.2.2.1 Large-Bore CT
Large-bore CT scanners were specifically designed with radiation therapy needs in mind. One of the requirements in treatment of several cancer sites (breast, lung, vulva, etc.) is for extremities to be
Fig. 6.5 Digital image of a chest from an amorphous silicon imager with multileaf collimator shape projected on the im- age. (Courtesy of Varian Medical Systems, Palo Alto, Calif.)
Fig. 6.6 Cone-beam CT image of a head acquired on a conven- tional simulator. (Courtesy of Varian Medical Systems, Palo Alto, Calif.)
positioned away from the torso. When acquiring a CT scan with a patient in such treatment position, extremities often cannot fit through a conventional 70-cm-diameter scanner bore opening. In such situ- ations, patient positioning needs to be modified to acquire the scan. This can result in less than optimal treatment position (patient may be less comfortable and therefore the daily setup reproducibility may be compromised). Large immobilization devices (slant board, body molds) are also difficult to fit through a conventional diameter scanner. The first large-bore CT-simulator was introduced in 2000, and several additional models with enlarged bore opening have been introduced since then.
Large-bore scanners also have increased scan field of view (SFOV). The SFOV determines the larg- est dimension of an object that can be fully included in the CT image and it is typically 48–50 cm on most conventional 70-cm bore opening scanners.
For treatment-planning purposes it is necessary to have the full extent of the patient’s skin on the CT image. Lateral patient separation can often be larger than 48–50 cm and the skin is then not visible on CT images. Increased SFOV available on large bore scanners solves this problem. There are, however, differences in implementation of extended SFOV and validity of quantitative CT values (quantita- tive CT) at larger image sizes. The CT numbers for some scanners are accurate only for smaller SFOVs and the values towards the periphery of large SFOV images are not reliable. This can be a concern for heterogeneity based dose calculations as inaccurate CT numbers can lead to dose calculation errors. The impact of CT number accuracy for increased SFOV images on dose calculation accuracy should be eval- uated during scanner commissioning.
6.2.2.2 Multislice CT
In 1992 Elscint (Haifa, Israel) introduced a scanner which had a dual row of detectors and could acquire two images (slices) simultaneously. Since then, mul-
tislice CT has gained widespread acceptance, and scanners which can acquire 4, 8, 10, 16, 32, 40, 64, etc.
(typically with sub-second rotation times) are now available from all major vendors. The basic premise behind the multislice CT technology is that multiple rows of detectors are used to create several images for one rotation of the X-ray tube around the patient.
The detector design and arrangement varies among the vendors. Figure 6.7 shows an example of imple- mentation for a 16-slice scanner available from a major vendor.
Although the scanner is considered a 16-slice scanner, there are 24 rows of detectors or detector elements. The center 16 have 0.75-mm collimated width at the isocenter and the outer four on either side have 1.5-mm collimated width at the isocenter. The total length coverage at the isocenter is then 24 mm.
The thinnest nominal slice thickness that the scan- ner can produce is slightly larger than 0.75 mm, but for practical purposes it can be considered here as 0.75 mm. With proper collimation (16u0.75) on the X-ray tube side, signal from the center 16 detector elements can be used to acquire 16 0.75-mm-thick images at a time. If the collimation is increased to 16u1.5, so the X-ray beam includes the outer eight detectors, 16 1.5-mm-thick images can be acquired.
In this situation, signals from the adjoining pairs of 0.75-mm detectors are combined to create 1.5-mm- thick images. Similarly, larger slice thicknesses can be created by combining signal from multiple detec- tor elements. The primary advantage of multislice scanners is the ability to acquire image studies many times faster than single-slice scanners.
One of the obstacles for radiation therapy scan- ning with single-slice scanners is the limited tube heat loading capability. Often, fewer images are taken, slice thickness is increased, mAs is decreased, or scan pitch is increased to reduce the amount of heat produced during the scan and to allow for the entire scan to be acquired in a single acquisition.
Due to the longer length of imaged volume per tube rotation (multiple slices acquired simultaneously), the tube heat loading for a particular patient volume is lower for multislice than for single-slice
Fig. 6.7 A detector array for a 16-slice CT scanner
4 x 1.5 mm 16 x 0.75 mm 4 x 1.5 mm
they should allow registration (indexing) of patient immobilization devices to the tabletop. Figure 6.9 demonstrates this concept. The CT simulator table- top has the same width as the linear accelerator used for patient treatment and both allow regis- tration of patient immobilization system to the treatment couch. Ability to register the immobili- zation device and the patient to a treatment table is extremely important and improves immobilization, setup reproducibility, accuracy, and efficiency. The patient is always positioned in the same place on scanners, and multislice scanners are generally
not associated with tube head-loading concerns.
Faster acquisition times and decreased tube load- ing of multislice scanners (which will allow longer volumes to be scanned in a single acquisition) can provide an advantage over single-slice systems for treatment-planning purposes. Multislice technol- ogy can be especially beneficial for imaging of the thorax where breathing artifacts can be minimized with faster scanning. Multislice technology also facilitates dynamic CT scanning, often referred to as 4D or 5D CT (Low et al. 2003). This application of multislice CT in radiation therapy has yet to be fully explored.
Multislice scanners are also capable of acquir- ing thinner slices which can result in better quality DRRs and more accurate target delineation (better spatial resolution; Fig. 6.8). Studies with thinner slices result in an increased number of images to process. Target volumes and critical structures have to be delineated on an increased number of images and treatment-planning systems have to handle larger amounts of data. Currently, this can result in increased time and labor required for treatment planning. Software vendors are creating tools which will allow easier manipulation of larger study sets, but that will likely take several years to implement.
In the meantime, the numbers of CT images that are acquired for a treatment plan needs to be balanced between resolution requirements and the ability to process larger number of images.
6.2.2.3
CT-Simulator Tabletop
This section and discussion about simulator tabletops applies equally to all simulators used in radiation therapy (conventional, MRI, CT, and PET) and treatment machines. Tabletops used for patient support in radiation therapy during imag- ing or treatment should facilitate easy, efficient, reproducible, and accurate patient positioning.
It is not only important that a tabletop improve patient positioning on a single device (i.e., treat- ment machine), but the repositioning of a patient from one imaging or treatment device to another also has to be considered. A great improvement in this process would be if all tabletops involved in patient simulation and treatment had a common design. They do not necessarily have to be identical, but they should have same dimensions (primar- ily width), flex and sag under patient weight, and
Fig. 6.8 a A 0.8-mm- and b a 3-mm-slice-thickness CT digi- tally reconstructed radiograph. Image a contains much more detail than b
a
b
the treatment machine and patient daily setup can be facilitated using the treatment-couch positions.
Actually, if the patient is registered to the treatment couch, couch coordinates used for patient treat- ment can become a part of parameters that are set and tracked in the record and verify system.
The tolerance for the couch parameters can be set according to type of treatment that the patient is receiving. For example, for conformal radiotherapy treatments the coordinates should allow minimal deviations (comparable to margins used for target delineation) in daily couch positioning. The thera- pist can then first place the treatment couch to the coordinates set in the record and verify system and then evaluate patient positioning. If the patient is well immobilized, minimal adjustments should be needed in patient setup.
6.2.3.4
Patient-Marking Lasers
A laser system is necessary to provide reference marks on patient skin or on the immobilization device. Figure 6.2 shows a laser system for a CT simulator.
Wall lasers
Wall lasers are vertical and horizontal, mounted to the side of the gantry. These lasers can be fixed or movable.
Sagittal lasers
The sagittal laser is a ceiling- or wall-mounted single laser, preferably movable. Scanner couch can move up/down and in/out but cannot move left/right; therefore, the sagittal laser should move left/right to allow marking away from patient mid- line.
Scanner lasers
Internally mounted, vertical and horizontal lasers on either side of the gantry and an overhead sagit- tal laser.
Lasers should be spatially stable over time and allow positional adjustment. Properly aligned sim- ulator lasers greatly improve accuracy of patient treatments. Misaligned simulator lasers can intro- duce systematic errors in patient treatment; there- fore, simulator laser alignment should be checked daily and the alignment tolerance should be within 2 mm (Mutic et al. 2003).
6.2.3
MR Simulator
The MR images for radiotherapy treatment plan- ning are usually acquired in diagnostic radiology, and very few radiation oncology departments have a dedicated MR scanner. Furthermore, the vast major- ity of MR studies in radiotherapy are currently lim- ited to brain imaging. Magnetic resonance imaging has a superior soft tissue contrast compared with CT imaging, and there are several benefits that MR can offer for target delineation based on this advantage.
There have been several reports describing use of MR scanners for imaging and treatment simulation in radiotherapy (Potter et al. 1992; Okamoto et al.
1997; Beavis et al. 1998; Schubert et al. 1999; Mah et al. 2002). Some of these reports have suggested that MR studies can be used alone for radiotherapy treatment planning. Indeed, if spatial distortions (the geometry of imaged objects is not always repro- duced correctly), which is the largest concern with MR imaging, can be removed or minimized, MR studies can be used as the primary imaging modal- ity for several treatment sites. Superior soft tissue contrast provided by MR can also be an advan-
Fig. 6.9 Similarity in design of simulator and treat- ment machine tabletops allows effi cient and accurate reproducibility of patient positioning. (Courtesy of MED-TEC, Inc., Orange City, Iowa)
tage for treatment planning of certain extracranial tumor sites such as prostate (Lee et al. 2003; Chen et al. 2004).
Conventional MR scanners are not well suited for extracranial imaging for treatment planning. The main difficulty is placement of patient in treatment position with immobilization device in the scanner.
Small diameter and long length of conventional MR scanner openings severely limits patient position- ing options for imaging. Open MR scanners, how- ever, do not have this problem and patients can be scanned in conventional treatment positions. At least one manufacturer offers an open MR scanner which has been modified to serve as a radiotherapy simulator (Fig. 6.10). The scanner table is equipped with a flat top and external patient alignment lasers.
The geometry of the scanner is similar to the CT simulator shown in Figure 6.2. Another manufac- turer offers a 70-cm-diameter gantry opening con- ventional MRI scanner. The depth of the scanner opening is 125 cm. The dimensions of this scanner are very similar to a conventional CT scanner and in fact the scanner could be mistaken for a CT scanner.
The ergonomics of this scanner are also well suited for radiotherapy simulation. One of the major prob- lems with MR imaging for radiotherapy treatment planning are geometric distortions in acquired images. The MR scanners are often equipped with correction algorithms which minimize geometri- cal distortions. These corrections do not affect the entire image and only the center portion of the
image (center 20–35 cm diameter) is adequately cor- rect (within 2 mm); therefore, the representation of patient’s skin and peripheral anatomy for larger body sections may be inaccurate. The effect of these inaccuracies must be evaluated if dose distributions and monitor units will be calculated directly on MR images.
Virtually all treatment-planning systems allow import of MR images and image registration with CT study. Some treatment-planning systems also allow design of treatment portals and display of isodose distributions on MR images directly. If the treatment-planning system can calculate doses directly on MR images, and if it was determined that geometric distortions are not significant, then there may be no need for CT images and MR study may be the only image set used for treatment planning.
There should be a way to create images from the MR study which are equivalent to simulation radio- graphs for comparison with port films from the treatment machine. Another potential problem with MR images is that they do not contain information which can be related to electron density of imaged tissues for heterogeneity based corrections. This is not a significant problem, as bulk density correc- tions can be applied in the majority of treatment- planning systems. Due to availability of CT images in modern radiation oncology departments, it may be easiest if a CT study set is always acquired to com- plement the MR data and facilitate easier and more accurate heterogeneity-based dose calculations.
Fig. 6.10 An MR simulator. (Courtesy of Philips Medical Systems, Cleveland, Ohio)
6.2.4
PET/CT Simulator
The PET images for radiotherapy planning can come from a stand-alone PET scanner or a combined PET/CT unit. Combined PET/CT scanners are being installed in radiation oncology departments and are used for PET scanning, but also these machines can be used for CT scanning only without PET acquisition. Due to this purpose, these scanners can be classified as CT simu- lators, although the PET/CT simulator term may be more appropriate. Combined PET/CT scanners offer several advantages for radiotherapy imaging and are generally preferred over stand-alone units.
6.2.4.1
Stand-Alone PET
One of the major limitations of stand-alone PET scanners is a relatively small gantry bore opening, typically 55–60 cm. These scanners were designed to optimize image quality and not necessarily to accom- modate radiotherapy patients in treatment positions with immobilization devices. This design feature of stand-alone PET scanners can severely limit the size of immobilization devices and patient position that is used for treatment. The immobilization devices used with a stand-alone PET scanner are limited in size and cannot be wider than approximately 50 cm.
This is a major limitation for scanning patients with lung cancer where the patients need to have their arms positioned above the head. Also, larger patients may not be able to be scanned in an immobilization device. Even with these limitations, stand-alone PET scanners can be successfully used for radiotherapy
imaging and very good registrations (within 3 mm) can be achieved for the majority of patients. A stand- alone PET scanner has been used for radiotherapy scanning for several years at the Mallinckrodt Insti- tute of Radiology in St. Louis (Missouri). Successful registrations have been achieved for head and neck, thorax, abdomen, and pelvis scans.
6.2.4.2
Combined PET/CT
The first combined PET/CT prototype was intro- duced in 1998 at the University of Pittsburgh (Beyer 2000). Since then, all major manufacturers have pro- duced several commercial models. The key descrip- tion of PET/CT scanners is that a PET and a CT scan- ner are “combined” in the same housing (Fig. 6.11), meaning that there are two gantries (PET and CT) combined in one housing sharing a common couch.
Image reconstruction and scanner operation is increasingly performed from one control console.
Combined PET/CT scanner design varies among different vendors with respect to PET detectors, image quality and resolution, speed, image field of view; number of slices for the CT part, scanner couch design, gantry bore opening, and other con- siderations. All of the commercially available scan- ners have a 70-cm gantry opening for the CT portion, although large-bore CT scanners will likely become part of PET/CT scanners in the future. The PET gantry opening ranges in diameter from 60 to 70 cm, meaning that some of the commercial scanners have a non-uniform gantry opening as the patient travels from the CT portion of the scanner to the PET side.
More importantly, the scanners with the smaller
Fig. 6.11 A combined PET/CT scanner
gantry opening on the PET side will pose the same difficulties for radiotherapy scanning as stand-alone PET scanners. Again, the size of patient immobiliza- tion devices and patient scan/treatment position will have to be adapted to the size of the gantry opening.
The combined PET/CT technology offers two major benefits for radiotherapy planning. Firstly, because the images are acquired on the same scanner, providing that the patient does not move between the two studies, the patient anatomy will have the same coordinates in both studies. These images have been registered using hardware registration rather than software registra- tion. The second benefit of the combined PET/CT units is that CT images are used to measure attenuation cor- rection factors (ACFs) for the PET emission data, obvi- ating the need for a time-consuming PET transmis- sion scan (Bailey 2003; Bailey et al. 2003). The use of CT images to generate PET ACFs reduces the scan time up to 40% and also provides essentially noise- less ACFs compared with those from standard PET transmission measurements (Townsend et al. 2004).
Shorter scan times can benefit radiotherapy patients who are scanned in treatment position which often can be uncomfortable and difficult to tolerate for pro- longed periods of time. One of the concerns with ACFs generated from CT images is mismatch or misalign- ment between CT and PET images. The PET images are acquired during many cycles of free breathing and CT images are acquired as a snapshot in time at full inspiration, partial inspiration, or some form of shal- low breathing. The breathing motion will cause mis- match in anatomy between PET and CT images in the base of lung and through the diaphragm region. This mismatch can result in artifacts in these areas which may influence diagnosis and radiotherapy target defi- nition in this region. There are various gating meth- ods that can be used during image acquisition to min- imize the motion component and essentially acquire true, motionless, images of patient anatomy. Gated or 4D CT (with time being the fourth dimension) can be used to generate more reliable ACFs and also for radiotherapy treatment planning where gated delivery methods are being used.
Contrast-enhanced CT images can cause inaccu- rate ACFs due to artificially increased attenuation through anatomy which contains contrast material.
The most obvious way to avoid this problem is to acquire a routine CT with contrast and another non- contrast CT. There is also an option to use software tools to correct for these artifacts. For radiotherapy scanning, it is preferred to acquire two separate scans.
The attenuation correction CT can be a whole-body, low-dose scan with greater slice thickness if desired.
The second CT would be a treatment-planning scan with thin slices for better resolution and DRR qual- ity. This scan is acquired only through the volume of interest, thus limiting the number of images and memory requirements to manipulate these images in the treatment-planning computer. This second scan can then be contrast enhanced if desired.
6.2.5
Virtual Simulation Software
As with all software programs, user-friendly, fast, and well-functioning virtual (CT) simulation soft- ware with useful features and tools will be a deter- mining factor for success of a virtual simulation pro- gram. Commercially available programs far surpass in-house written software and are the most efficient approach to virtual simulation. Several features are very important when considering virtual simulation / 3D treatment-planning software:
1. Contouring and localization of structures: con- touring and localization of structures is often mentioned as one of the most time-consuming tasks in the treatment planning process. The virtual simulation software should allow fast user-friendly contouring process with help of semi-automatic or automatic contouring tools.
An array of editing tools (erase, rotate, translate, stretch, undo) should be available. An ability to add margins in three dimensions and to automati- cally draw treatment portals around target vol- umes should be available. An underlining empha- sis should be functionality and effi ciency.
2. Image processing and display: virtual simulation workstation must be capable of processing large volumetric sets of images and displaying them in different views as quickly as possible (near real- time image manipulation and display is desired).
The quality of reconstructed images is just as important as the quality of the original study set.
The reconstructed images (DRRs and multiplanar reconstruction) are used for target volume defi ni- tion and treatment verifi cation, and have a direct impact on accuracy of patient treatments.
3. Simulator geometry: a prerequisite of virtual
simulation software is the ability to mimic func-
tions of a conventional simulator and of a medi-
cal linear accelerator. The software has to be able
to show gantry, couch, collimator, jaw motion,
SSD changes, beam divergence, etc. The software
should facilitate design of treatment portals with
blocks and multileaf collimators.
6.3
Multimodality Imaging
Imaging is involved in all steps of patient manage- ment, disease detection, staging, treatment modal- ity selection (intramodality and intermodality), target volume definitions, treatment planning, and outcome estimation and patient follow-up.
An overall goal of imaging in radiotherapy is to accurately delineate and biologically characterize an individual tumor, select an appropriate course of therapy, and predict the response at the earliest possible time. The requirement to biologically char- acterize an individual tumor means that an imaging modality must be capable of imagining not only the gross anatomy but also recording information about physiology, metabolism, and the molecular makeup of a tumor; therefore, the image information used in radiotherapy can be classified as anatomical and/
or biological. The four primary imaging modalities used in radiation therapy are CT, MRI, ultrasound (US), and nuclear medicine imaging.
No single imaging modality provides all the neces- sary information for treatment planning and patient management for several cancer sites, but multiple imaging modalities can be used to complement each other and improve disease detection, staging, therapy selection, target design, outcome prognosis, and follow-up. Figure 6.12 shows the information content possibilities of the imaging modalities used in radiation therapy. The maximum benefits may be realized if anatomical and biological imaging modalities complement each other.
6.3.1 Detection
Imaging of disease with CT or MRI (non-functional) is based on anatomic or physiological changes that are a late manifestation of molecular changes that underlie the disease. By detecting changes in the
molecular and biochemical process, biological imaging (PET or functional MRI) can demonstrate disease before it becomes anatomically detectable.
Changes in tumor detection capabilities can lead to modification in radiation therapy target volumes and dose prescriptions.
6.3.2 Staging
Positron emission tomography has improved patient staging in several treatment sites (Dizendorf 2003).
Better knowledge of the true extent of the patient’s disease can significantly alter patient management.
For some patients, who would otherwise undergo curative radiotherapy, PET may demonstrate distal disease or alter the extent of local disease and indi- cate that a palliative course of therapy is more appro- priate. These patients would not only be spared the side effects of futile curative treatment, but the over- all health care costs could also be lowered due to PET findings.
In addition to more accurate staging, PET may also be able to provide information about individ- ual tumor biology (phenotype). This would allow further stratification of patients within the same clinical stage. So rather than basing therapy selec- tion for an individual patient on the stage alone, which is statistically appropriate for a large group of patients, biological properties of an individual tumor can then be used for therapy selection. The tumor phenotype information may affect inter- modality and intramodality patient management depending on suspected radiation or chemotherapy sensitivity of an individual tumor. If we know more about biological properties of an individual tumor, it may be possible to incorporate biological response models in the therapy selection process to maximize the therapeutic ratio.
6.3.3
Target Definition and Altered Dose Distributions
The true extent of the disease may extend beyond anatomically defined volumes, and biological imag- ing with PET has already been shown to be valuable for defining the extent of target volumes. Further- more, PET can be used to differentiate areas of bio- logical importance within the boundaries of target volumes. Ling et al. (2000) have described a concept
Fig. 6.12 Information content of current imaging modalities in radiotherapy
of biological target volumes (BTVs). In addition to recommendations for target volume definitions pro- posed by the International Commission on Radia- tion Units and Measurements (ICRU) reports 50 and 62 (ICRU 1993, 1999), portions of target volumes would be identified as having increased growth activity or radioresistance. Identification of these volumes would be performed with biological imag- ing and these volumes would be labeled as BTVs.
Biological target volumes would then have a special consideration during the treatment-planning pro- cess and would be subject to dose escalation.
For example, Chao et al. (2001) have shown how PET imaging based hypoxia measurement technique with Cu(II)-diacetyl-bis(N
4-methylthiosemicarba- zone) (Cu-ATSM) tracer can be used to identify a BTV for head and neck tumors. Experiments have shown that increased Cu-ATSM uptake can be used to identify hypoxic tissues, which are also associ- ated with increased radioresistance. The proposed treatment technique is based on the idea that Cu- ATSM can be used to identify the hypoxic BTV and IMRT delivery can be used to deliver escalated doses to overcome the radioresistance of the BTV.
6.3.4
Evaluation of Response to Therapy and Follow-up
Currently, tumor control and effectiveness of radio- therapy is evaluated in the weeks and months fol- lowing the completion of treatments. The evaluation, similar to detection and diagnosis, relies largely on anatomical changes, which take time to manifest. If the planned approach of radiotherapy is not effec- tive and the patient has a persistent disease or new growth, it is too late to make any modifications, as the therapy has already been completed. Addition- ally, by the time it is determined that a local tumor control has not been achieved, it may be too late to initiate a second line of therapy. Biological imag- ing may be used to detect response to therapy on a molecular level and allow evaluation of therapy effectiveness sooner after completion of treatments (Young et al. 1999). Ideally, biological imaging may be used shortly after initiation of treatments to image tumor changes. This approach has had limited success thus far, but research in this area is active and it eventually may be possible to evaluate tumor response after initiation of therapy.
We are just beginning to exploit benefits of mul- timodality imaging in management of radiotherapy
patients. With time, use of several image types will be a commonplace for treatment planning of many cancer sites. This has already, to an extent, taken place for treatment planning of central ner- vous system tumors where CT images are comple- mented with MRI studies for a significant fraction of patients.
One concern with utilization of novel imaging data for treatment planning and management of radiation therapy patients is that the information contained in the images may be misinterpreted or may be incorrect resulting in inappropriate patient treatments. It is imperative for radiation oncologists to understand potential pitfalls and shortcomings of individual imaging modalities, and also to realize that the best results can be achieved if newer imag- ing techniques are used to supplement existing stag- ing and tumor delineation processes. This is espe- cially true if biological or functional information is used for target delineation where possibility of false positive or negative findings exists. The correlation of biological or functional signals with anatomic abnormalities detected by CT or MRI can provide an important validation in the target delineation and patient management process.
6.4
Patient Positioning and Immobilization The success of conformal radiation therapy pro- cess begins with proper setup and immobilization.
One of the primary rules for positioning patients for simulation and radiotherapy treatments is that the patient should be as comfortable as possible.
Patients who are uncomfortable typically have poor
treatment setup reproducibility. An uncomfortable
position that a patient was able to tolerate during
simulation may be impossible to successfully repro-
duce for treatment. Immobilization devices tremen-
dously improve reproducibility and rigidity of the
setup. Another important consideration in patient
positioning is that immobilization devices custom
made for individual patients can significantly
improve intra and inter fraction immobilization
and setup reproducibility. Standard immobiliza-
tion devices often do not provide an adequate fit
for all patients, they work for many, but not for all,
patients. This is well accepted and understood for
body molds, as described in Chapter 7, and also for
thermoplastic masks for immobilization of head and
neck region. One very important point that is often
overlooked is design of head supports (head cups) for treatment of head and neck region. There is a tendency to use a standard head cup and a custom thermoplastic mask. An analogy could be made that the head cup is a foundation for a house and ther- moplastic mask is framing for the walls. If the foun- dation is not appropriately constructed, the entire structure will be unstable; therefore, for conformal treatments of head and neck region improved setup reproducibility can be achieved with custom head supports made from body mold material as shown in Figure 6.13.
Similar approach and forethought can be applied to other treatment sites and respective immobiliza- tion devices. Figure 6.14 shows a body mold that is used at the Washington University School of Medi- cine for treatment of patients with breast cancer. This device was designed to facilitate CT-based treatment planning and to improve patient reproducibility from the simulator to the treatment machine. In the inside of the body mold are Styrofoam wedges which elevate the patient and provide adequate position- ing for the breast or chest wall. The handle improves patient comfort by providing a solid grip point for the arm, and the non-skid material prevents patients from sliding in the body mold. The ear mold, which is made from dental wax, improves the head posi- tion reproducibility. The device also registers to the treatment table so the couch coordinates are tracked during patient treatment in the record and verify system. Other authors have proposed an even more elaborate positioning device for the breast, such as the prone breast board which has an opening for the breast to hang freely beneath the patient.
The third important point about patient position- ing is that patients should be aligned straight on the simulator table, and patients should not be rotated or slanted. It is much easier to reproduce a straight patient position than a rotated one. Simulation and treatment policies should include positioning and immobilization specifications for individual treat- ment sites. Patient setup design should consider loca- tion of critical structures and target volumes, patient overall health and flexibility, possible implants and anatomic anomalies, and available immobiliza- tion devices. Immobilization devices should not produce artifacts on CT images. Table 6.1 shows a treatment-site-specific set of instructions CT simu- lation instructions. The instructions are designed to be used with a single-slice CT scanner and non- uniform CT slice thickness and spacing is used to minimize heat production on the X-ray tube while obtaining excellent image quality. Table 6.2 shows the CT scan parameters that accompany instruc- tions in Table 6.1.
Fig. 6.13 Head and neck immobilization device with custom headrest
Fig. 6.14 Breast treatment immobilization device. 1 Portion of the mold removed on the ipsilateral side; 2 arm grip; 3 ear mold made from dental wax; 4 non-skid surface
Table 6.1. An example of treatment site specifi c CT simulation instructions. Initial reference refers to the initial set of positioning marks placed on the patient. In most instances, these marks will be the same as the treatment isocenter. The CT scan protocol parameters are provided in Table 6.2. SSD source-to-skin distance, SAD source–axis distance. SCV supraclavicular, supraclavicle, SIM simulation, CAX central axis, BIT bottom of ischial tuberosity, IC iliac crest, SI sacroiliac, AC alpha cradle SitePatient positionImmobilizationSetupProtocolSlice (mm)Index (mm)Scan limitsContrastSpecial instructions Head and neck IMRTSupine, head straightThermoplastic mask; custom head mold; IMRT registration device, no arm stretcher, no pad; initial reference midline and midplane per doctor’s instructions Wire initial referenceHead 5u555Top of orbits/top of skullNoneTriangle sponge under knees; no dentures; arms on abdomen; wire areas of interest; tongue blade or mouth piece on doctor‘s request
Head 3u333Top of shoulders/ top of orbits Head 5u5553 cm below clavicles/top of shoulders Brain IMRTSupine, head straightInitial reference per doctorInitial reference: midline and midbrain or per doctor‘s instructions
Head 3u333Base of skull/top of skullNoneHave MR images ready for simulation, if necessary Head 5u555Chin/base of skull Head and neck conventional
Supine, head straightThermoplastic mask; clear head rest; arm stretcher, (clear mouth piece or dental wax for dentures per doctor‘s instructions), no pad Initial reference just above shoulders at 3 cm depth, wire initial reference Head 5u555Mid-orbits/top of skull100 ml Optiray 300Head position per doctor‘s instructions: neutral/clear „C”; extended clear „D“; hyper-extended clear „D“; chin tucked/clear „F“ reversed; tongue blade on doctor‘s request
Head 3u333Top of shoulders/ mid-orbits Onc med thorax 5u5553 cm below clavicle/top of shoulders OrbitsSupine, head straightThermoplastic mask; clear head rest; bolus may be used. Bolus made from thick thermoplastic material
Initial reference: vertical–lateral canthus; long- middle orbit; sagittal midline (bilateral); center of orbit (ipsilateral) Head 5u5553 cm above orbits/ top of skullNonePer doctor‘s instructions wire lateral edge of canthus; -cut mask over orbits out, tape with silk tape; have patient look straight up during scan; head neutral, Clr. „F“, if this tucks head clear „C“; start scan at bottom Head 3x3333 cm below/3 cm above orbits Head 5u555Bottom of chin/ 3 cm below orbits
Table 6.1. (Continued) SitePatient positionImmobilizationSetupProtocolSlice (mm)Index (mm)Scan limitsContrastSpecial instructions ParotidSupine, slightly oblique, „chicken wing“
Thermoplastic mask, clear head rest, triangle sponge to support oblique position Initial reference: at angle of mandible or in center of MD fi eld, affected side parallel to fl oor
Head 5u555Top of orbits/top of skullNoneRemove dentures; pad can be used; wire areas of interest Head 3u333Top of thorax/top of orbits Onc med thorax 5u5553 cm below clavicle/top of thorax BrainSupine, head straight, arms on abdomen
Thermoplastic mask, clear head rest, pad on table, triangular sponge under knees Initial reference: midline and midbrain or per doctor‘s instructions Head 3u333Base of skull/top of skull100 ml Optiray 300Neutral/Clear „F“/chin tucked or neck fl exed/clear „F“ reversed; mask reinforced; contrast per doctor‘s instructions; have MR images ready for simulation, if necessary
Head 5u555Bottom of chin/ base of skull Breast/SCV/ SSD/SADSupine. Bent elbow to clear scanner opening, if necessary
Body mold registered to table with arm holder and dental wax ear mold Doctor to mark upper, lower, medial, and lateral borders; place thick wires on marks
Onc med/large thorax 5u5551–2 cm above upper borderNoneSee breast simulation procedures for determining isocenter Onc med/large thorax 5u533Through SCV/ breast fi eld Onc med/large thorax 5u5551–2 cm below lower border Lung 3D/ CT-SIMSupine; chin extended; arms above head, folded, may rest on 5- or 7-cm sponge
3D: body mold registered to table; CT-SIM: none, unless necessary per doctor‘s instructions; no pad; triangle sponge under knees Initial reference at carina and midplane or per doctor‘s instructions on simulation request form Onc med/large thorax 5u555Chin-to-lung apex125 ml Optiray 320 and/or two tablespoons of Esophocat CAX drawn on patient‘s anterior and lateral surfaces. Per doctor‘s instructions the CAX can be placed mid-depth and midplane at highest level of thorax; contrast given just before the scan; start scanning after half contrast in
Onc med/large thorax 3u333Through target and CAX, most of lung Onc med/large thorax 5u555Through rest of the lung Onc med/large thorax 8u888Top of kidneys or per MD request EsophagusSame as lungSame as lungSame as lungSame as lungSame as lung
Same as lung
Same as lungNone or EsophocatSame as lung
AbdomenSupine; head on 5- or 7-cm sponge, as comfortable Body mold. Arms above head, hands on 7 sponge.
Initial reference at T12/L1 interspace, midplane and midline, or per doctor‘s instructions Onc med/large body 5u555Above diaphragm to above initial reference
450 ml; Readi-Cat 2, 1 h prior to scan
Readi-Cat 2 given to visualize small bowel on the scan Onc med/large body 3u333Through initial reference Onc med/large body 5u555To IC or through pelvis per doctor‘s instructions Standard pelvisSupine; arms on upper abdomen or chest
Feet banded with rubber band; none otherwise, unless requested by doctor; head on 5- to 7-cm sponge Initial reference midline and midplane of area requested by doctor
Onc med/large body 5u555Top of L3-above upper port edge450 ml; Readi-Cat 2Standard pelvis: 16.5u20, inferior border/bottom of ischial tuberosity; 16.5u16.5, inferior border/bottom of obturator foramenOnc med/large body 3u333Above/below port edge Onc med/large body 5u555Below port edge/ peritoneum Pelvis and peri-aorticSupine; arms above headSame as aboveSame as aboveOnc med/large body 5u555Diaphragm/L3/L4 interspace450 ml; Readi-Cat 2Upper port edge: T12–L1; lower port edge: bottom of obturator foramen; place center between upper and lower borderOnc med/large body 3u333L3/L4 interspace/ ischial tuberosity Onc med/large body 5u555Ischial tuberosity peritoneum 3D\IMRT prostateSupine; arms on upper chestBody mold, feet banded with rubber band, AC close on lateral thighs, lines on body mold, registration; head on 5- to 7-cm sponge (or as comfortable)
Initial reference midline and at level of the prostate Onc med/large body 5u555Physician will specify upper margin; if not, go to L3/L4
10–15 cc Conray 30Rectal markers; option 1: fl exible Foley; option 2: rectal marker with metal balls; urethrogram by doctor; after body mold is fi nished, have patient get up and then reposition the patientOnc med/large body 5u555Mid-SI joints/top of iliac crest /JMM Onc med/large body 3u333BIT/mid-SI joints Onc med/large body 5u555Below urethral stricture/BIT Brachy prostateSupine; arms folded on chest
None; no pad; head on 7-cm sponge; triangle sponge under knees Do not need initial referenceOnc med/large prostate implant33Bottom of SI joints/ bottom of ischial tuberosity NonePatient to drink a glass of water 30 min prior to scan
6.5
Simulation Process
Like the other areas of radiotherapy treatment plan- ning and treatment, simulation requires a team approach involving physicians, physicists, dosime- trists, therapists, nurses, etc. The team needs to understand individual components of the process and their specific technical requirements. A well- informed and knowledgeable personnel is needed to fully exploit benefits of modern treatment simula- tion equipment. Furthermore, treatment-site-spe- cific written procedures can significantly improve efficiency, consistency, and accuracy of simulations.
Table 6.1 shows an example of such procedures.
Written procedures are also helpful for training of new staff and performing simulations for less fre- quent treatment procedures. A well-designed and simple simulation process greatly increases treat- ment planning efficiency and improves patient setup reproducibility between the simulator and treatment machine. This section describes conven- tional and CT simulation processes. The CT simu-
lation is given larger emphasis as this has become the primary source of treatment planning informa- tion for large number of radiation oncology depart- ments.
6.5.1
Conventional Simulation
As described previously, the dependence on con- ventional simulators has decreased over the past several years as conformal radiation therapy has become the standard of care for several treatment sites, and CT simulators have evolved to overcome most of their initial limitations (gantry opening size, long scan times, connectivity, etc.). In this new era, many radiation oncology departments have determined that they can replace conventional simulators with CT simulators, and there are many departments that no longer have a conventional simulator. Other institutions have reduced the number of conventional simulators and/or number of conventional simulations. For example, during
Table 6.2. An example set of CT scan parameters for a single-slice scanner. FOV fi eld of view
Protocol kV mA Time
(s)
Display FOV (cm)
Pitch Thickness (cm)
Spacing (cm)
Pilot length
Head 5u5 130 300 1 48 1.3 5 5 450
Head 3u3 130 300 1 48 1.3 3 3 450
Onc child brain 130 280 1 35 1.3 3 3 450
Onc neck 130 280 1 48 1.3 3 3 450
Onc medium thorax 8u8 130 230 1 48 1.5 8 8 650
Onc medium thorax 5u5 130 230 1 48 1.5 5 5 650
Onc medium thorax 3u3 130 230 1 48 1.7 3 3 650
Onc large thorax 8u8 130 250 1 55 1.5 8 8 650
Onc large thorax 5u5 130 250 1 55 1.5 5 5 650
Onc large thorax 3u3 130 250 1 55 1.7 3 3 650
Child thorax 5u5 130 150 1 35 1.5 5 5 450
Child thorax 3u3 130 150 1 35 1.7 3 3 450
Onc medium body 5u5 130 250 1 48 1.5 5 5 650
Onc medium body 3u3 130 250 1 48 2 3 3 650
Onc large body 5u5 130 300 1 55 1.5 5 5 650
Onc large body 3u3 130 300 1 55 2 3 3 650
Onc child body 5u5 130 180 1 35 1.5 5 5 512
Onc child body 3u3 130 180 1 35 2 3 3 512
Onc medium prostate implant 130 250 1 30 1.5 3 5 650
Onc large prostate implant 130 300 1 30 1.5 3 3 650
Onc extremity 130 200 1 48 2 5 5 650
the 1990s, the Department of Radiation Oncology at the Washington University School of Medicine operated with three conventional simulators and one CT simulator. In 2000, the department replaced two of the conventional simulators for another CT simulator for a total of one conventional simulator and two CT simulators, while treating the same number of patients. Even though the conventional simulator use has been reduced, certain proce- dures can be performed much easier and more efficiently on a conventional simulator than with a CT simulation. For example, palliative treatments for bone metastasis, whole pelvis irradiation for gynecological tumors, pelvis irradiation for rectal cancer, certain extremity treatments, brachyther- apy treatment planning, and some other treatment procedures are very simple to simulate using con- ventional technology. One of the advantages of conventional simulators is that there are virtually no limitation on available patient positions and on size and shape of immobilization devices that are used for simulation. If a patient cannot lie down, the sitting position in a special treatment chair can be accommodated with a conventional simu- lator where a CT scanner would not be an option.
For some treatments of hands and arms it may be desirable for the patient to stand next to the treatment table; again, this type of simulation can only be performed with conventional simulator.
With better imaging capabilities, cone-beam CT, and better connectivity with the treatment-plan- ning system and treatment machine, conventional simulators are actually becoming more valuable than in the past.
Conventional simulation process consists of the following:
x Patient positioning and immobilization
x Verifi cation of patient positioning using fl uoro- scopic imaging
x Determination of isocenter location x Beam placement design
x Marking of patient and immobilization devices based on isocenter coordinates
x Acquisition of X-ray fi lms
x Outlining of treatment portals on the X-ray fi lms x Transferring or acquiring of patient setup data for
the record and verify system
x Transferring of simulation data to dosimetry for treatment planning and monitor unit calcula- tion
x Preparation of documentation for treatment x Performance of necessary verifi cations and treat-
ment-plan checks
6.5.1.1
Patient Positioning and Immobilization
The goals for patient positioning and simulation described earlier should be followed for conven- tional simulation. The flexibility in size of immo- bilization devices greatly simplifies patient posi- tioning for treatment. If the patient’s conventional simulation will be followed with a CT scan, then the limitations of the CT scanner should be considered in patient positioning and immobilization.
6.5.1.2
Verification of Patient Position Using Fluoroscopic Imaging
Prior to construction of the immobilization device, the patient should be aligned to lay straight on the treatment table. This means that the patient’s head, vertebra, and possibly extremities should be parallel with the longitudinal axis of the treatment table. The patient should also lay flat on the table with no rota- tion. It is much easier to reproduce a straight patient position on the treatment table than if the patient is rotated. The verification of patient position is per- formed under fluoroscopic guidance. After it has been verified that the patient is straight, an initial set of skin marks should be placed on the patient so this position can be reproduced throughout the simulation. Patient alignment should be monitored throughout the simulation procedure.
6.5.1.3
Determination of the Isocenter Location
Treatment isocenter is typically placed based on physician instructions. For the majority of standard treatments, the isocenter placement should be pre- determined and outlined in treatment and simula- tion policies. It is desirable to place the isocenter on a stable location on the patient where the skin or patient anatomy does not move significantly. If the treatment isocenter must be placed in a posi- tion where the overlaying external anatomy does move, then treatment setup point should be used.
Treatment setup point is a set of marks which are
placed on a stable position on patient’s anatomy (like
sternum). For treatment, the patient is first aligned
to the setup point and then shifted to the isocen-
ter location using the shifts which were determined
during the simulation. Other considerations for
placement of isocenter include limitations/capabili- ties of treatment machines and desired dose distri- butions. These considerations are beyond the scope of this chapter.
6.5.1.4
Beam-Placement Design
Beams should be placed according to the treatment policies. Fluoroscopic capabilities of the conven- tional simulator are used for this purpose. Other chapters in this book outline common treatment techniques. Outlining of treatment portals based on simulation X-ray films is also better addressed in other chapters in this book.
6.5.1.5
Transfer of Simulation Information for Treatment Planning and Treatment
The final step in simulation process is transfer of patient setup data to dosimetry for calculation of monitor units and possibly for some simple treatment planning. The setup information is also transferred to the linear accelerator. Depending of the connec- tivity of the conventionally simulator and its ability to acquire patient setup information electronically, some or all of the patient setup data can be exported from the simulator electronically. Integrity of cap- tured and exported electronic data should be verified through periodic quality assurance process.
6.5.2
CT Simulation
The CT-simulation process consists of the follow- ing steps:
x Patient positioning and immobilization x Patient marking
x CT scanning
x Transfer to virtual simulation workstation x Localization of initial coordinate system x Localization of targets and placement of isocen-
ter
x Marking of patient and immobilization devices based on isocenter coordinates
x Contouring of critical structures and target vol- umes
x Beam placement design, design of treatment por- tals
x Transfer of data to treatment-planning system for dose calculation
x Prepare documentation for treatment
x Perform necessary verifi cations and treatment plan checks
Again, this process and its implementation vary from institution to institution. The system design is dependent on available resources (equipment and personnel), patient workload, physical layout and location of different components, and proximity of team members. Communication channels need to be well established to avoid errors and unnecessary re-simulations. A simulation request form can be used to communicate simulation specifics between the physician and other team members (Fig. 6.15).
The following is a general description of major steps in the CT simulation process.
6.5.2.1
Scan and Patient Positioning
The CT simulation scan is similar to conventional diagnostic scans; however, there are several differ- ences. Patient positioning and immobilization are very important. Scan parameters and long scan vol- umes with large number of slices often push scan- ners to their technical performance limits. The CT- simulator staff must be aware of scanner imaging performance capabilities and limitations and also geometrical accuracy limitations. Imaging capabili- ties should be exploited to achieve high image qual- ity and geometrical limitations need to be consid- ered when positioning and marking patients.
Patient Positioning and Immobilization
General patient positioning and immobilization considerations are as described earlier in this chap- ter. Larger bore scanners typically can afford more comfortable patient positions and larger immobili- zation devices and offer a definitive advantage over conventional CT scanners. Pilot (scout) images are a very efficient tool for evaluation of patient position- ing prior to the actual CT scan. After patient initial immobilization, a preliminary pilot scan should be imaged to assure that the patient positioning is straight. Immobilization devices should not pro- duce artifacts on CT images.
Scan Protocol
