ABSTRACT
These systems, variously called whole body immobilization systems, stereotactic body frames, or patient immobilization cradles, perform several functions: • Constrain the patient’s body, thereby minimizing the chance of inadvertent patient motion during
treatment • Maximize patient comfort
Contents 8.1 Objectives 173 8.2 Stereotaxy and image guidance 174 8.3 Patient positioning 175 8.4 Patient comfort 175 8.5 Patient immobilization 177 8.6 Abdominal compression 177 8.7 Intrafractional imaging 180 8.8 Head-and-neck systems 180 8.9 Commercial systems 181 8.10 Summary 182 References 182
• Provide a means to ensure that the patient position and pose on the expanded foam or vacuum cushions used to support the patient are the same as those set at the time of the simulation planning CT scan
• Through the use of external scales or a stereotactic localizer frame, provide a means of positioning the body frame and patient in the treatment room in preparation for setup using image guidance
8.2 STEREOTAXY AND IMAGE GUIDANCE The use of a body frame and its use in the setup of patients undergoing hypofractioned RT was first described by Ingmar Lax and Henric Blomgren at Karolinska Hospital, Stockholm (Lax et al. 1994). Their approach was strictly stereotactic; the body frame had both an internal and an external coordinate system (Figure 8.1). The internal system consisted of a set of wires, some of which were set obliquely into the base and sides of the body frame that were visible in the CT. To use the stereotactic approach to target positioning, a reference point, possibly a soft tissue feature seen in the CT, is chosen. Its position with respect to the positions of the wires seen in the CT slice containing the feature is measured, thus the threedimensional (3-D) coordinates of the feature in the internal system are determined. The coordinates of this reference point are then expressed in the external coordinate system. This consists of a set of scales affixed to the body frame or an accessory positioning arch. Setup of this soft tissue reference point to the machine isocenter is achieved in part by positioning the body frame on the couch of the treatment machine such that
the isocenter room lasers strike the external scales at the correct coordinates. What remains, however, is to ensure that the target is positioned correctly with respect to the radiation beams. This was approached by attempting to reproduce the position of the patient in the body frame. To do this, isocenter and alignment tattoos were placed on the trunk (preferably the sternum) and both tibial tuberosities. A laser mounted on the body frame was used to ensure optimum patient alignment. In the initial implementations, the patient was set up in the body frame and the body frame positioned in the treatment room using the room lasers. This approach of setting up to coordinates and treating was the best that could be done in the early 1990s as in-room image guidance was not available. Several investigators have investigated the setup accuracy of bony targets using this approach (Fuss et al. 2004; Lohr et al. 1999; Negoro et al. 2001; Wulf et al. 2000). The average 3-D displacements of the bony target from the simulation position ranged from 3.6 to 4.9 mm. Note that soft tissue targets will have poorer reproducibility. The setup errors seen in individual treatments, of prime concern to hypofractionated and single-fraction SBRT, may be much greater.
