The Conventional Simulator

The purpose of a radiation therapy simulator is to mimic the radiation treatment unit, but, instead of a high-energy therapy beam, it produces low-energy, diagnostic quality x-rays for imaging. Most of the preparation for the treatment is done at a therapy simulator, which saves valuable treatment machine time. A simulator has most of the functionality of a treatment unit; the relevant components of a modern therapy simulator are shown in Fig. 11-2. The simulator head contains a diagnostic x-ray tube mounted at one end of a rotating gantry. At the other end are a film cassette holder and a fluoroscopic unit or digital imaging device.

FIGURE 11-2 • Schematic diagram of the main components of a radiation therapy simulator.

Housed in the simulator head is the collimator that has movable delineator (cross) wires and blades (jaws). On the simulator image, the delineator wires are projected as straight lines that define the field size. The blade positions determine the image size. A field localization light in the head projects the crosshairs and the delineator wires on the patient’s skin, which help in positioning the patient. The optical distance indicator measures the depth of the isocenter. The collimator includes an accessory mount that allows the treatment accessories, such as the block tray or the electron cones, to be mounted on the simulator. A gridded graticule tray with radiopaque markers provides a scale for measurement on the film. Fig. 11-3 shows a lateral head-and-neck radiograph on which the delineator wires, the crosshairs, and the graticule markers are projected. Fig. 11-4 is an anterior-posterior simulation film for a patient with lung cancer.

FIGURE 11-3 • Left-lateral simulation radiograph for a head-and-neck patient. The delineator wires, cross hairs, and graticule marks are projected on the x-ray film.

FIGURE 11-4 • Anterior-posterior simulation film for a lung patient.

The simulator couch looks like a treatment couch, but is modified to produce optimal images. It has three degrees of freedom for translational motion and rotates in a horizontal plane about the isocenter and the pedestal. The fluoroscopic unit contains an image intensifier and a video camera. The fluoroscope can move longitudinally, laterally, and radially. A collision sensor on top of the image intensifier senses potential collision with the patient or the couch and stops motion, preventing patient injury or machine damage. On a video monitor, fluoroscopic images are viewed in real time. An electronic storage unit to save images is often provided. A radiographic film cassette holder on top of the fluoroscopic unit is used for taking hard copy simulation radiographs.

A hand pendant allows the operator to adjust the couch and simulator settings from inside the simulator room. The operator can adjust the gantry angle and move the couch while watching closely for possible collision. The simulator settings and the couch position can also be adjusted from the console outside the simulator room. This is particularly useful during fluoroscopic imaging, when the delineator wires and the isocenter position can be adjusted interactively. A lead glass window separates the console from the simulator to shield the operating personnel from radiation exposure.

Alignment Lasers

The wall-mounted alignment laser system provides beams of laser lights. With the help of a lens system, each beam is spread out in a plane. The beams pass through the isocenter of the simulator. On the patient’s skin, these beams project as fine lines along three major planes—transverse, sagittal, and coronal. The intersections of these light beams on the patient’s skin help in marking the triangulation points on the skin. Generally, at the end of the simulation, the triangulation points are tattooed. Similar laser systems in the CT room and in the therapy room are helpful in placing the patient in the treatment position. Although the lasers used in the localization system are low power, they can nevertheless cause retinal damage. It is advisable not to stare at the laser lights or their reflection from a shiny surface.

Computed Tomography Scanner and Spiral Computed Tomography Scanner

Since the first CT images obtained by Hounsfield,⁶ CT imaging has experienced rapid development. The major advancements have been in the area of image quality and scan time reduction. A fourth-generation CT scanner is shown schematically in Fig. 11-5. The x-ray tube and the collimator assembly rotate around the patient, and the intensities of the transmitted radiation reaching the detectors are recorded. Either a solid scintillation detector (bismuth germanate, cesium iodide, cadmium tungstate) array or a gas-filled (xenon or xenon-krypton) array is used for the detection. The transmission data are stored in the computer, and the image is reconstructed based on the attenuation information. The linear attenuation coefficients are converted into CT numbers, and the image is displayed on a video monitor. The CT numbers for the pixels are expressed in Hounsfield numbers and are related to linear attenuation coefficients:

FIGURE 11-5 • Schematic diagram of fourth-generation CT scanner. A circular array of x-ray detectors remain fixed, and a rotating x-ray source with fan-shaped beam scans the patient.

where H is the CT number, µ is the linear attenuation coefficient for the pixel under analysis, and µ_w is the linear attenuation coefficient of water. The CT numbers in a scan range from −1000 to approximately +1000, with the CT number for water set at zero. A CT number of −1000 corresponds to air, and a CT number of +1000 is for very dense bone.

In a conventional CT scanner, an x-ray source mounted on a gantry rotates 360° to collect the data for one CT slice. The slice thicknesses that are available depend on the scanner. Generally, they are of the order of 1, 3, 5, and 10 mm. A smaller thickness results in better resolution.

To obtain the volumetric data on a conventional CT scanner, after each scan the patient couch is translated to the new scan position. Because of the patient’s breathing and involuntary movement of the internal organs, motion artifacts are present unless special precautions are taken. In a spiral or helical CT scanner, the patient is transported through the gantry in synchrony with continuous data acquisition over a multitude of circular (360°) scans. Mathematical schemes⁷ have been developed to reconstruct axial scans that are similar to the scans obtained with a conventional scanner.⁸ On a spiral scanner, a volume or a subvolume can be scanned in a short time, on the order of 30 seconds. Another important development is the use of multisection imaging. The single narrow fan beam of x-rays is broadened into a cone beam, and each detector is replaced with a row of detectors (4 to 64). These detector rows are aligned parallel to the axis of the patient. These multidetector scanners are capable of imaging up to 64 sections of body simultaneously.⁹ Fig. 11-6 shows a multislice large bore (85 cm) scanner, used for CT simulation. The data for the whole study can be acquired during a single breath hold, resulting in images without interscan motion. From the diagnostic point of view, this is significant for locating small structures such as small pulmonary nodules, with which respiratory motion can cause detection problems.¹⁰ From the radiation therapy perspective, the advantages of the spiral CT scanner compared with a standard scanner are the ability to collect CT data with the patient in the immobilized treatment position within a shorter time frame and to create coronal and sagittal reconstructions with finer details.

FIGURE 11-6 • A multislice CT scanner with 85-cm-bore gantry.

The requirements for a therapy CT scan are different from those for a diagnostic scan.^10,11 Some of the requirements are: (1) the patient should be scanned in the treatment position (the couch top should be flat, similar to a treatment couch; the physical CT aperture diameter and circle of reconstruction [field of view] should be large enough to accommodate the patient in the treatment position; if an immobilization device will be used during the treatment, the patient should be scanned immobilized, but the device should be made of materials that do not cause CT artifacts; and the anterior and lateral isocentric lines and other landmarks are marked on the skin by taping radiopaque catheters or lead beads); (2) CT scans should be of high quality so that the disease, tissue at risk, and surrounding normal tissues can be identified clearly; (3) particularly for three-dimensional (3D) treatment planning, a volume scan is required in order to delineate the treatment volume in three dimensions; in the region of treatment volume, a contiguous set with 5-mm spacing or smaller should be used to make sure that the small tumor extensions and normal structures are not missed (closely spaced scans also allow better image reconstruction); (4) anterior-posterior and lateral scout views should be obtained with the CT images indexed to these (these views are helpful in comparing CT data with conventional simulation radiographs); (5) the CT scanning should be completed in a short time period so that the patient remains adequately immobilized during the CT data collection; (6) for the treatment planning scans in the thoracic region, the patient should be scanned under shallow-breathing condition. Although the breathing introduces some motion artifacts in the images and the image quality may be degraded slightly, the scans represent the condition of the patient during treatment. However, there are interventional techniques, such as active breathing control, deep inspirational breath-hold and respiratory gating, can minimize the effect of respiratory motion, as discussed later.

Virtual Simulation

During conventional simulation the oncologist relies on the plain radiographs for designing the treatment ports. At times, even with the help of MRI, CT, and other diagnostic information, it is difficult to draw these ports accurately on the films. In the mid-1980s, Sherouse and colleagues^12–14 introduced the term virtual simulation to describe a computer-based simulation process. They developed a software package that allowed the user to carry out simulation at a computer workstation instead of at a real simulator. The advantage of this method compared with plain radiographs is that the target volumes and anatomic structures, outlined on the CT scans, are used to make the decision about the position of the isocenter, field sizes, beam directions, and shielding blocks. CT scans provide better visualization of the disease and normal tissues, resulting in better target coverage and normal tissue shielding. The virtual simulator system has the ability to display and execute all the functions of a conventional radiotherapy machine, including gantry, couch, and collimator control.

The CT data are obtained by scanning the patient in the immobilized treatment position. The CT scans are transferred to the planning system, and the skin, target, dose-limiting organs, and bony landmarks are contoured on the CT scans. The planner can view a 3D display of the patient model that consists of the skin, target, and other anatomic structures abstracted from the CT data. The contoured structures utilize different surface rendering and variable degree of transparencies to enhance the visualization. Fig. 11-7 shows a typical virtual simulator screen. Like a physical simulator, the virtual simulator allows the planner to adjust treatment parameters such as the gantry angle, field size, collimator angle, and so on. The beam’s-eye view (BEV) provides images that are similar to fluoroscopic images. The planner can select the treatment beam direction by watching projections of the target and surrounding structures in the BEV. The fields are then shaped by drawing the beam apertures using a mouse or other pointing device. At the end of the virtual simulation, digitally reconstructed radiographs (DRRs) are produced for each treatment field. A DRR¹⁵ is similar to a radiograph, but is designed by the computer based on the attenuation information derived from CT numbers in the original data set. The image quality of a DRR depends on the CT slice spacing—a smaller spacing results in a better DRR. The divergence of anatomic structures projected on a DRR is similar to that in the projection on a radiograph. The magnification of a DRR can be adjusted to match the simulation x-ray film. Along with the beam aperture and collimator, the target and other structures of interest are projected onto it. A list of beam, table parameters, and the templates for the beam modifiers are generated. Before the treatment, to verify the treatment ports, the localization port films can be compared directly with DRRs. Or, if the DRR quality is not good, the simulation radiographs in the treatment position can be taken and compared with the corresponding DRR. In many planning systems, the beam’s energy can be altered so that the DRR will look like a high-energy radiograph, making comparison to the port film simpler.

FIGURE 11-7 • Typical virtual screen. The beam’s-eye view and simulator controls are displayed on the screen.

Computed Tomographic Simulator

The CT simulator is a whole-body CT scanner that is designed for radiation therapy simulation.^16,17 It consists of three basic components: (1) a CT scanner, (2) an interactive workstation capable of virtual simulation, and (3) a mechanism for marking the beam portals on the patient’s skin. The patient is scanned in the treatment position and the CT images are transferred directly to virtual simulation workstation. The target volume and the relevant critical structures are outlined on the CT scans, the beam geometries are optimized, and DRRs are produced for the block definition and treatment verification.

The lasers for patient alignment and marking are similar to those used in a conventional simulator, except that the sagittal laser can be moved laterally with respect to the longitudinal axis of the couch. The wall-mounted lasers are mounted at a longitudinal distance from the gantry to facilitate marking the patient reference point for setup.

The advantages of a CT simulator over a conventional simulator include: (1) the CT simulator allows for the visualization of the treatment volumes for the portal design; on a conventional simulation radiograph it is difficult to identify the target volume; (2) on DRRs the target volume and critical structures can be superimposed, and the treatment ports can be designed based on this; if needed, margins can be added around the target before outlining the shielding block; (3) the DRRs for the boost phase of the treatment can be produced from the same CT data set, without the need for the patient to be reimaged; (4) conventional simulators have physical limitations; for example, radiographs for the vertex field can not be obtained, whereas a virtual simulation software can produce these DRRs; (5) generally, a virtual simulation software package includes software to enhance the DRRs by adjusting the window and contrast levels. For example, the AcQsim (Philips Medical Systems, Cleveland, OH) package includes Digital Composite Radiograph (DCR), which groups CT numbers into ranges corresponding to bone, fat, muscle, and so on. The CT numbers in each category are modified by a weighting factor and redisplayed to provide greater enhancement of the specified tissue range¹⁸; Fig. 11-8A, B, and C show the regular DRR, DCRs for the bone and skin, respectively, for a head-and-neck patient; (6) the DRRs corresponding to each treatment field are generated; the magnification of the DRRs can be adjusted; cross hairs and grid points can be projected onto the DRR for direct comparison with the portal images.

FIGURE 11-8 • A, Digitally reconstructed radiograph. B, Digital composite radiograph for bone. C, Digital composite radiograph for the skin for a head-and-neck patient.

However, there is a limitation of the CT simulator compared to the conventional simulator. Because of the finite bore size of the CT gantry, typically 65 to 70 cm, it is impossible to accommodate all patients and all patient positions. For example, extremely large patients, some patients in conventional position for breast cancer treatment, and patients requiring special immobilization devices may not be able to enter the 65- to 70-cm opening without compromising the position. A larger-opening 85-cm-bore, scanner has the potential to eliminate these problems. The image quality and the doses for the larger bore have been found comparable to the 70-cm-bore scanner.¹⁹ Fig. 11-6 shows an AcQsim CT simulator with 85-cm-bore gantry.

Interventional Strategies

With the use of 3D conformal radiation therapy (3DCRT) and intensity-modulated radiation therapy (IMRT), dose distributions have become highly conformal. Great care must be taken to make sure that the dose is delivered as planned; otherwise, the risk of marginal failure increases. There are two main causes of treatment inaccuracy: (1) uncertainty in defining the clinical target volume (CTV) and (2) variation in treatment geometry during treatment. If other imaging modality data sets such as MRI or PET are available, the image registration or fusion²⁰ can be used to increase the accuracy of the CTV.

Variations in treatment geometry for sites such a thorax and abdomen are significant because of breathing. Larger margins are required in these regions to define the PTV. Currently, several techniques are being used to reduce the uncertainty due to respiratory motion. These include active breathing control (ABC), DIBH, and respiratory gating (RG). These techniques are used during simulation, CT, and treatment delivery. The three interventional techniques are discussed briefly here, and RG is described in more detail elsewhere.

Active Breathing Control

In this method, the patient’s nose is clamped and he or she breathes through an ABC apparatus via a mouthpiece, as shown in Fig. 11-9. The signal from the flow monitor is processed and a trace on the computer monitor displays the lung volume. During a particular sequence of the breathing cycle, at a predetermined flow direction and lung volume, a valve is activated to stop the airflow temporarily, resulting in the suspension of the breathing motion.^21–23 The duration of the breath-hold is such that the patient can easily tolerate it. The amount of breath-hold a patient can tolerate depends on the disease site. For example, patients with lung disease can easily tolerate a breath-hold of 15 seconds near the end of normal inspiration, whereas for patients with Hodgkin’s disease and liver cancer, the breath hold ranges from 35 to 50 seconds when ABC is applied during inspiration.²³ ABC can substantially reduce the treatment margins without the need to modify the therapy accelerator.

FIGURE 11-9 • A patient breathing into the active breathing control (ABC) apparatus. A nose clip is used to make sure that the breathing is through the ABC unit.

(From Wong JW, Yan D, et al: Interventional strategies to optimize the delivery of radiation therapy. In Bragg DG, Rubin P, Hricak H, eds: Oncologic imaging, ed 2, Philadelphia, 2002, Saunders, p 116.)

Deep Inspiration Breath Hold Technique

In this method the patient is coached on the technique of reproducible deep inspiration through the various phases of treatment planning and delivery.^4,24,25 The DIBH uses a modified version of slow vital capacity maneuver²⁶ to bring the patient to approximately 100% vital capacity, followed by a breath hold, which maintains the patient at that level for a prescribed period, during which the patient is simulated, CT-scanned, treated, and port filmed. The patient breaths through a mouthpiece connected to a spirometer control unit. A nose-clip is used to make sure that the patient breaths through the mouth. The spirometer measures the airflow and it is interfaced with a computer. The airflow is integrated to yield the lung volume as a function of time and is displayed on a computer monitor. The DIBH maneuver begins with quiet tidal breathing, followed by a slow deep inspiration, slow deep expiration, then a slow deep inspiration to the maximum limit and breath-hold. A training session is held to familiarize the patient with spirometer and to measure patient parameters, such as the tidal volume, vital capacity, and comfortable breath-hold duration. Some patients are unable to follow the directions and are unsuitable for DIBH. The technique has two advantages. First, the breath-hold minimizes tumor motion due to breathing. Second, it expands the patient lung to its maximum volume, pushing healthy lung tissue out of the primary radiation field and consequently reducing the fraction of the lung in the treatment beam. Fig. 11-10 compares the free breathing and DIBH treatment plans for a patient. The images are coronal planes through the isocenter for the same patient. Hanley et al.⁴ have found that DIBH can reduce the volume of the lung receiving more than 25 Gy by 30% compared to free-breathing plans. It is found to be highly reproducible. Patients could perform 10 to 13 breath holds in one session, with a comfortable breath-hold duration of 12 to 16 seconds.

FIGURE 11-10 • Comparison of (A) free-breathing and (B) DIBH treatment plans. Images are coronal planes through the isocenter for the same patient. Lines representing the 10%, 50%, and 90% isodose levels are in red, green, and blue, respectively.

(From Rosenzweig KE, et al: The deep inspiration breath hold technique in treatment of inoperable non–small cell lung cancer, Int J Radiat Oncol Biol Phys 48:81, 2000.)

Respiratory Gating

There are commercially available systems (for example, Real-time Position Management [RPM] Respiratory Gating; Varian Medical Systems, Palo Alto, CA) that permit breathing-synchronized fluoroscopy on a treatment simulator, acquisition of breathing-synchronized CT imaging, and gated treatment on a linear accelerator.^5,27 The system consists of a charge-coupled-device (CCD) video camera attached to an infrared illuminator (Fig. 11-11) that tracks a patient’s respiratory motion by detecting reflected infrared light from two reflective markers on a lightweight block (Fig. 11-12). The block is placed on the patient’s chest or abdomen. The upper marker tracks the respiratory motion; the lower marker, separated by 3 cm from the upper marker, calibrates the system. The camera signal is processed by software. At the start of any session, whether simulation, CT, or treatment, the operator places the system into tracking mode for a few breathing cycles. This allows the system to determine the minimum and maximum vertical position of the upper marker. These values establish the scale of marker motion for subsequent display and for setting a gating threshold, as described later. At simulation, either amplitude or phase gating can be selected. For amplitude gating, the user adjusts the threshold levels that appear as two horizontal lines.

FIGURE 11-11 • Charge-coupled-device video camera attached to infrared illuminator for tracking the respiratory motion.

FIGURE 11-12 • Lightweight plastic block with reflective markers.

Operation During Simulation

The system allows synchronization of fluoroscopy with the breathing. The user selects the gating threshold for the breathing waveform. Only those fluoroscopy frames occurring during the gate intervals are played back. The operator then examines the anatomic motion in the playback to evaluate and optimize the choice of gating thresholds.

Operation on a CT Scanner

Scan acquisition can be synchronized with the normal breathing motion. The CT is operated in axial mode. The images are acquired only when the respiratory motion enters the selected gate. The breathing-synchronized images are used as the planning scans for the gated treatments. The breathing trace recorded during the planning scan is saved as a reference trace and is recalled before each gated treatment.

Operation During Treatment

During treatment, similar thresholds are selected. The beam is turned on only during the window defined by the threshold. Verbal breathing instructions have been found useful to improve the reproducibility and regularity in breathing.

Simulation Process

Once the radiation oncologist has decided to treat the patient with external beam radiation therapy, the objective is to treat the target volume, diseased tissue plus a margin, to a tumoricidal dose while minimizing the dose to the surrounding normal tissues. If critical tissues are located nearby, the aim is to keep the dose to these organs to a level that is within the acceptable limit of complication. To determine the target volume, the radiation oncologist relies on the physical examination, radiographs, and other diagnostic tests, such as CT, MRI, PET, and ultrasonography. Most patients treated with external beam radiation therapy undergo a treatment simulation before radiotherapy.

The planning for the simulation starts well before the patient is brought in for the actual procedure. Generally, the physician has a reasonably good idea about the treatment volumes, based on the physical examination and other diagnostic tests. Ideally, a team consisting of the radiation oncologist, the treatment planner, and the simulation therapist chalks out a tentative treatment approach beforehand. During the simulation, a strategy to treat the patient in a reproducible manner on a daily basis is developed. This includes decisions about the treatment position, the use of immobilization devices, the number of fields, gantry angles, collimator angles, field sizes, and field shapes. Appropriate points are tattooed on the patient’s skin to ensure that the patient can be brought back to the treatment position rapidly each day for treatment. If the target volume is located near critical structures (for example, the nasopharynx target volume wraps around the spinal cord) or the target volume extends between the retinas or is close to the optical chiasm, then intricate treatment planning is required to limit the dose to the critical organs. In these cases, customized plans are developed, and an outline of the patient’s external contour is needed. The outline is obtained either by a CT scan or by using one of the methods discussed later. To obtain the treatment planning CT scan, the patient is placed in the treatment position on the CT couch; special attachments are available to ensure that the couch top is similar to that of a treatment couch. The clinician outlines the target volumes and the critical structures on these scans, and the treatment plans are designed based on these data.