Devices and apparatus in this category are designed to aid in visualizing and determining the extent of the tumor in relation to the treatment geometry (target volume localization) and to obtain measurements of the patientís body contours and thicknesses. In the past, target volume localization was usually accomplished by physical examination and the use of a device called an X-ray simulator, which combines radiographic and fluoroscopic capability in a single machine that mimics the actual treatment unit geometries (Fig. 1). The simulation process itself may be supplemented with other diagnostic imaging studies including computed tomography (CT), magnetic resonance images (MRI), and, more recently, positron emission tomography (PET).
Devices used to aid the conventional simulation process include a radiopaque fiducial grid (Fig. 2) projected on the patientís anatomy, which allows one to determine dimensions of the treatment volume from the simulator plane films. Examples of other devices used in the 2D target volume localization process are magnification rings placed in the irradiated field and lead-tipped rods that can be inserted into body openings, such as the vagina for carcinoma of the cervix or into the rectum for carcinoma of the prostate. The lead tip can be visualized clearly on simulator films or treatment portal films and allows evaluation of treatment field margins.
Note that a new generation of conventional simulators (Fig. 3) has recently been developed in which the image intensifier system has been replaced with an amorphous silicon flat-panel detector. This device produces high resolution, distortion-free digital images, including cone-beam CT.
Figure 1. The basic components and motions of a radiation therapy simulator: A, gantry rotation; B, source-axis distance; C, collimator rotation; D, image intensifier (lateral); E, image intensifier (longitudinal); F, image intensifier (radial); G, patient table (vertical); H, patient table (longitudinal); I, patient table (lateral); J, patient table rotation about isocenter; K, patient table rotation about pedestal; L, film cassette; M, image intensifier. Motions not shown include field size delineation, radiation beam diaphragms, and source-tray distance. (See Ref. 6.)
Figure 2. X-ray simulator radiograph showing fiducial grid projected on patientís anatomy. The grid is used for localizing target volume and determining treatment field size.
Figure 3. New generation radiation therapy simulator, in which image intensifier system has been replaced with amorphous silicon flat-panel that produces high resolution, distortion-free images and facilitates a filmless department. (Courtesy of Varian Medical Systems.)
Once the treatment geometry has been determined and the patient is in treatment position, the patientís body thicknesses and contours are measured and recorded for purposes of computing a dose distribution and determining treatment machine settings. Manual methods using calipers, lead solder wire, plaster cast strips, flexible curves, or other devices, such as the contour plotter (see Fig. 4) are the most common methods of obtaining this type of data when using the 2D planning approach.
Fields to be treated are typically delineated in the 2D simulation process using either visible skin markings or marks on the skin visible only under an ultraviolet (UV) light. Some institutions prefer to mark only reference setup points using external tattoos. These skin markings are used to reposition the patient on the treatment machine using the treatment machineís field localization light and optical distance indicator and laser alignment lights mounted in the treatment room that project transverse, coronal, and sagittal light lines on the patientís skin surface (Fig. 5).
In the new 3D era, CT simulators have become the standard of practice; a typical CT simulator facility design is shown in Fig. 6. A volumetric set of CT images is used to define the target volume, critical organs at risk, and skin surface. The CT numbers can be correlated with the electron densities of the tissues imaged to account for heterogeneities when calculating the dose distribution. Numerous studies have documented the improvements in target volume localization and dose distributions achieved with anatomic data obtained from CT scans as compared with the conventional simulation process (7,8). The CT simulators have advanced software features for image manipulation and viewing such as beamís eye view (BEV) display, and virtual simulation tools for setting isocenter, and digital reconstructed radiographs (DRRs)
Figure 4. Contour plotter. The device is a simple, easy-to-use precision pantograph that links a drawing pen to a stylus arm and, upon contact with the body, communicates body contours to an overhead drawing board. The contour plotter is suspended on a vertical column and can easily be adjusted and locked securely. A continuous plot is drawn as the operator follows the physical contours of the patient. Marks can be made along the contour to indicate beam entry and laser light locations. (Courtesy of MEDTEC.)
(9,10). Such systems provide all the functionality of a conventional simulator, with the added benefit of increased treatment design options and the availability of software tools to facilitate the understanding and evaluation of treatment options. In addition, the simulation process is more efficient and less traumatic to the patient. Laser alignment lights and repositioning devices registered to the treatment couch are used to facilitate repositioning the patient in the treatment machine coordinate system once the virtual simulation process is complete.
Figure 5. Laser alignment system. Patient in treatment position on treatment couch. Close-up of laser lines imaged on patient skin under typical treatment room lighting conditions. (Courtesy Gammex RMI, Inc.)