Delivery

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Immobilization

To accurately deliver treatments to the intended target, the patient must be immobilized in a comfortable and reproducible position. Immobilization of the trunk and extremities is often accomplished with the aid of vacuum-molded bags of polystyrene beads or polyurethane foam molds which can be customized to each patient. Masks of thermal plastic materials, which become compliant when heated can be applied to the head and neck, and secured to the treatment couch or head holder to ensure reproducibility of neck flexion and head position.

Once immobilized with one of these aids, the table and patient position can be adjusted such that the surface marks previously drawn on the patient at the time of treatment planning ''line up'' with the laser projection of the isocenter in the treatment room. This technique assumes that the anatomical target is in a constant relationship to the surface reference marks, which can often lead to inaccuracies. Thus, additional techniques are required to improve the accuracy of isocenter targeting.

Verification and Localization

At the outset and through a course of radiotherapy, plain radiographs of all treatment fields are obtained on the treatment unit to verify the location of the isocenter in reference to bony landmarks and to compare against planning DRRs. These radiographs can now be generated digitally, and verified by the clinician prior to delivery of the treatment. Although this step improves the setup accuracy achieved with surface marks alone (14,15), it does not account for internal organ motion. Motion of organs and targets in relation to bony structures is an unavoidable effect of respiration, peristalsis, and weight loss during and between treatments (16). Several techniques have recently been developed to account for interfraction and intra-fraction organ motion.

CT portal images or ''cone beam CT'' is a new technology being evaluated as a mechanism for providing portal images with soft-tissue contrast (17,18). These images can be generated with therapeutic megavoltage (MV) X rays, or with diagnostic kilovoltage X rays from a conventional system mounted at 90 degrees to the

Figure 4 Kilovoltage cone-beam CT system for image-guided radiation therapy. This system (A) employs a conventional X-ray source (arrow) and state-of-the-art flat-panel detector technology to generate a series (300-600) projection radiographs over 360°. The projections are used to reconstruct a 3D representation of the patient's internal anatomy while in treatment position. This technology has been adapted to the Elekta Synergy RP system. Performance is demonstrated in a small animal with a single axial slice through the reconstructed dataset displayed at lung windows (B). Source: Courtesy of D. Jaffray, Princess Margaret Hospital, Toronto.

Figure 4 Kilovoltage cone-beam CT system for image-guided radiation therapy. This system (A) employs a conventional X-ray source (arrow) and state-of-the-art flat-panel detector technology to generate a series (300-600) projection radiographs over 360°. The projections are used to reconstruct a 3D representation of the patient's internal anatomy while in treatment position. This technology has been adapted to the Elekta Synergy RP system. Performance is demonstrated in a small animal with a single axial slice through the reconstructed dataset displayed at lung windows (B). Source: Courtesy of D. Jaffray, Princess Margaret Hospital, Toronto.

treatment unit on the gantry (Fig. 4) (19). MV CT requires only minor adjustments to the linear accelerators, but results in CT images of a much lower soft-tissue and spatial resolution within a clinically acceptable dose range (20). The benefit of CT imaging is a much more accurate depiction of soft-tissue target location on the treatment unit. Although promising, this imaging technology remains investigational.

Alternatively, transabdominal ultrasound can be utilized in target positioning. The prostate gland is an example of a target that moves significantly between treatments. Changes in bladder and rectal filling can alter the location of the prostate by as much as 2 to 4 mm on a daily basis with occasional displacements of up to 20 mm (21). Daily ultrasound-guided positioning has led to greater certainty of the location of the prostate gland. The location of the treatment field can therefore be adjusted on a daily basis to account for any organ motion. This positioning system is reproducible, noninvasive, and effective for daily localization of the prostate gland (22).

Another option to localize targets at the time of therapy includes the placement of radiopaque markers or "seeds" in the soft-tissue target prior to planning and treatment (23,24). These markers can be readily visualized on planning CT images, DRRs, and portal radiographs. The optimal location of the markers in relation to the isocenter can therefore be known, verified, and adjusted prior to treatment.

Gating Techniques

Movement of organs during a treatment, or intrafraction motion, poses a significant problem for diseases such as lung cancer, where respiration can result in a substantial displacement of tumors. Giraud et al. found a mean displacement of 3 to 4 cm with maximal inspiration and expiration in lung tumors, with a maximal displacement of 7 cm (25). Additional margins of healthy lung are often treated to ensure coverage of the tumor during respiration. This results in higher risks of treatment-related complications, such as pneumonitis.

The simplest method of correcting for respiratory variation is to teach patients "quiet breathing'' techniques or breath holding that results in minimal diaphragmatic movement (26-28). Unfortunately, many patients receiving therapy for lung cancer are unable to hold their breath for prolonged periods of time. Another method is respiratory gating. Various gating technologies exist, but typically, a respiratory sensor is placed on the patient, which relays information to the treatment machine. Sensors of respiration include sensors of abdominal wall tension, light-emitting diodes (29), temperature-sensitive thermocouple devices placed in the nostril (30), and infrared sensors (31). The treatment is delivered at defined intervals during the patient's respiratory cycle. The planning CT scan and localization films must also be obtained in the same phase of respiration. Respiratory gating can also been used in treating tumors below the diaphragm, such as liver tumors (32).

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