Accurate daily repositioning of the patient in the treatment position and reduction of patient movement during treatment is essential to accurately deliver the prescribed dose and achieve the planned dose distribution. As we will see in this section, modern day immobilization and repositioning systems are designed to be able to be attached to the simulation and treatment couches, so that the immobilization device and the patient are registered to the treatment machine coordinate system. Once the immobilization device has been locked into a specified position, the patient is then aligned to the immobilization system. The end result is that a set of coordinates is obtained from the CT simulator that is used in the virtual simulation process and that can be correlated to the treatment room isocenter.
The anatomic sites most often needing immobilization in radiation therapy are the head and neck, breast,
Figure 6. Typical CT simulation suite showing the scanner, flat tabletop, orthogonal laser system, virtual simulation workstation and hardcopy output device. (Courtesy of Philips Medical Systems.)
Thorax-esophagus, shoulders and arms, pelvic areas (especially if obese), and limbs rotated to unusual positions. The precision achievable in the daily treatment positions of a patient depends on several factors other than the anatomic site under treatment, such as the patientís age, general health, and weight. In general, obese patients and small children are the most difficult to reposition.
Simple patient restraint and repositioning devices can be used in treating some anatomic sites. For example, the disposable foam plastic head holder shown in Fig. 7 provides stability for the head when the patient is in the supine position. If the patient is to be treated in the prone position, a face-down stabilizer can be used as shown inFig. 8. This device has a foam rubber lining and a disposable
Figure 7. Disposable foam plastic head holder provides stability to the head when the patient is in the supine position.
Figure 8. Face-down stabilizer. This formed plastic head holder has a foam rubber lining and disposable paper liner with an opening provided for the eyes, nose, and mouth. It allows comfort and stability as well as air access to the patient in the prone position.
Figure 9. Polyurethane body mold. The chemical mixture is poured into the foam mold under a latex sheet. The patient is positioned in the foam mold as the polyurethane mixture expands to body shape. These body molds are easy to make, save time in patient alignment, and increase patient comfort. (Courtesy Smithers Medical Products, Inc.)
Paper lining with an opening provided for the eyes, nose, and mouth of the patient. It allows comfort and stability as well as air access for the patient during treatment in the prone position.
There are now several commercially available body mold systems that are in widespread use as immobilization and repositioning aids. Fig. 9 illustrates one such system that utilizes a foam block cutout of the general anatomic area and polyurethane chemicals, which when mixed expand and solidify to conform to the patientís shape in a matter of minutes. Another widely used system (Fig. 10) consists of a vinyl bag filled with plastic minispheres. The bag is positioned around the patient to support the treatment position and then a vacuum is applied causing the minispheres to come together to form a firm solid support molded to the patientís shape.
Plaster casts are still used in some clinics, but have not gained widespread use in the United States, probably because they are too labor intensive and time consuming. Also, transparent form-fitting plastic shells (Fig. 11) that
Figure 10. Vacuum-form body immobilizer. The system consists of a plastic mattress filled with microspheres connected to a vacuum pump. Under vacuum, the mattress shapes itself to the body contours. (Courtesy of MEDTEC.)
Figure 11. Plastic shell. Transparent form-fitting plastic shells fabricated using a special vacuum device. (See Ref. 11.)
Are fabricated using a special vacuum device are also used in some countries (e. g., Great Britain and Canada), but again are very labor intensive and have not gained acceptance in this country. Both methods are described in detail in the book by Watkins (11). In the United States, thermal plastic masks are much more commonly used (Fig. 12). The plastic sheet or mesh is placed in warm water to make it very pliable, and when draped over the patient conforms to the patientís shape and hardens upon cooling.
Fig. 13 illustrates a device called a bite block, which is used as an aid in patient repositioning in the treatment of head and neck cancer. With this device, the patient is
Figure 12. Thermoplastic cast. When placed in a warm (170įF) water bath, thermoplastic material becomes very flexible and can easily be molded to the patientís surface curvature. Immobilization using this material is less labor intensive than the conventional plaster cast or plastic shells and is therefore more readily adaptable on a routine basis for the immobilization of patients during radiation therapy. (Courtesy of MEDTEC.)
Figure 13. Bite-block system. Placement of the patient in a comfortable supine position and use of bite-block immobilization minimize patient movement for head and neck treatments. Note that the C-arm design allows both lateral and anterior beam arrangements to be used. (Courtesy of Radiation Products Design, Inc.)
Placed in the treatment position and instructed to bite into a specially prepared dental impression material layered on a fork which is attached to a supporting device. When the material hardens, the impression of the teeth is recorded. The bite-block fork is connected to a support arm that is attached to the treatment couch and may be used either with or without scales for registration.
There are many other devices used to help in the treatment setup of patients that are site specific. For example, breast patients are usually positioned supine, with the arm on the involved side raised and out of the treatment area. Fig. 14 shows a device called a breast or tilt board that is used to optimize the position of the patientís chest wall (or thorax). The device is constructed with a hinge section that can be positioned and locked into place at various angles to the horizontal treatment table top. Modern breast boards now provide options for head support, arm positioning, and breast support. Sometimes, it is more convenient to use a separate arm board that can be attached directly to the treatment couch (Fig. 15). The perpendicular support provides a hand grasp that can be adjusted to the proper height and assists the patient in holding their arm in a comfortable position away from the treatment field.
Another useful device is the breast bridge (Fig. 16), which can be placed on the patientís chest and adjusted to the skin markings, for determining separation of the tangential fields. Precise angulation of the beam portals is determined using a digital readout level. In addition, a squeeze bridge (Fig. 17) with plastic plates can be used to provide buildup when a higher surface dose is desired, or one with a wire mesh frame can be used when no additional surface dose is warranted. A beam alignment device (Fig. 18) is used to match multiple fields used in tangential breast irradiation (12). The alignment component of the device is a curved piece of aluminum with a row of nylon
Figure 14. Tilt board or adjustable breast board. The top piece is fabricated with a hinged section that allows the sloping chest wall to be more appositional to a vertical beam. (Courtesy of MEDTEC.)
Figure 15. Patient arm support used for breast irradiation assists patients in holding their arm in a comfortable position, away from the treatment field. (Courtesy of Varian Medical Systems.)
Figure 16. A Breast Bridge is used with tangential radiation fields and consists of a pair of plastic plates that can be locked at the appropriate separation determined for the individual patient. After the treatment area has been marked, the bridge is placed on the patientís chest and adjusted to the skin markings, thus determining separation of the fields. Precise angulation of the portals is determined by the digital readout level. Once the portals are set, the bridge may be removed. (Courtesy of MEDTEC.)
Pins protruding from its surface. The pins have a thin inscribed line to which the light field is aligned.
In some instances, it may be advantageous to treat the cancer patient in an upright position. The treatment chair (Fig. 19) is a device that facilitates such treatments by allowing a patient to be accurately repositioned in a seated position each day. The chair provides means of stabilizing the patient by the use of hand grips, elbow holders, and a seatbelt. The back of the seat is constructed of carbon fiber and thus the radiation beam can penetrate with minimal effects, and the angle of the seat back is adjustable.
Another device used to optimize patient position is the shoulder retractor. It provides a means by which the patientís shoulders can be pulled down in a reproducible manner, as illustrated in Fig. 20. Such a device is often used in the treatment of head and neck cancer involving lateral fields.
A standard feature on most accelerator treatment couches is a Mylar window or tennis racket-type table insert. This device consists of a thin sheet of Mylar stretched over a tennis racket-type webbing material and mounted in a frame that fits into a treatment table that has removable sections. Newer table insert devices made of carbon fiber (Fig. 21) eliminate the need to ĎĎrestringíí such panels and minimize the ĎĎsagíí that can occur with nylon string panels. Such inserts provide excellent patient support
Figure 17. A squeeze bridge used for breast treatments with and without bolus. The wire mesh device is used where no additional surface dose is desired and the plastic frame device is used when increased surface dose is needed. (See Ref. 1.)
Figure 18. A beam alignment device used to implement field matching techniques. (a) Schematic of beam alignment device. (b) Example of use of device with three-field technique for breast treatment. Superior edges of tangential breast fields are coplanar and abutted to the vertical inferior edge of the anterior supraclavicular field. (See Ref. 12.)
Figure 19. Treatment chair. Provides positioning and fixation for breast, lung, and thorax patients who require vertical-upright positioning; adjusts to different locking positions and can accommodates a thermoplastic mask for head fixation. (Courtesy of MEDTEC.)
With a minimum of surface buildup effect for opposing beam portals, minimum reduction of beam intensity, and good visual access to the treatment surface. In addition, most medical linacs come with clamps that can be attached to
Figure 20. Shoulder retractor. Device used to pull the patientís shoulders down in a reproducible manner for treatment of the head and neck with lateral radiation fields. (Courtesy MED-TEC, Inc.)
Figure 21. Treatment couch insert. Carbon fiber table and spine inserts for simulator and treatment couches. Rigid carbon fiber minimizes the ďsagĒ that can occur with nylon string panels. (Courtesy of MEDTEC.)
The treatment couch and can be used with several different accessories, such as hand grips.
The development of 3D conformal radiation therapy (3DCRT) and more recently intensity-modulated radiation therapy (IMRT) has greatly enhanced the radiation oncologistís ability to plan and deliver very high doses that conform closely to the target volume, and falls off sharply, thus avoiding high dose to the nearby organs at risk (13,14). Both 3DCRT and IMRT invite the use of tighter margin to achieve higher dose escalation, and thus have spurred the development of new accessories and processes to better account for setup variation and organ motion that can occur during one (intra-) fraction, and between (inter-) fractions. Efforts thus far have focused on accounting for internal movement of the prostate gland and internal motion cause by respiratory function.
For example, for prostate cancer, the use of daily ultrasound imaging (Fig. 22), or daily electronic portal imaging of implanted radiopaque markers, has now become standard practice in many clinics (15,16).
Devices-methodologies used to address the problem of breathing motion in radiation treatment include: (1) gating and/or tracking and (2) breathhold devices-strategies. In gating and tracking, the state of the treatment machine is adjusted in response to a signal that is representative of a patientís breathing motion. With breathholding, the lung volume of the patient is directly immobilized prior to beamon, and released after the beam is off. The basic components of a gating or tracking system consist of a respiration sensor whose signal is processed and evaluated by a computer for suitability to trigger, or gate, the radiation. An example of a respiratory gating system is the Real-time Position Management (RPM) system (Fig. 23) commercially available from Varian Medical Systems (Palo Alto, CA) (17,18). An example of a breath control device is the Elekta Inc. (Norcross, GA) Active Breathing Coordinator (ABC) (Fig. 24) (19). The ABC apparatus is used to suspend breathing at any predetermined position along the normal breathing cycle, or at active inspiration, and consists of a digital spirometer to measure the respiratory trace, which is in turn connected to a balloon valve. Another example is the ExacTrac system (BrainLAB AG) that combines X-ray imaging and infrared tracking that permits correlation of internal 3D tumor motion with the patientís breathing cycle (Fig. 25) (20). Automatic fusion of digitally reconstructed radio-graphic (DRR) images computed from the treatment planning CT data to the live X-rays allows any set-up error or target shift and rotation to be identified and any discrepancy compensated for via robotic table movement.
Figure 22. BAT (B-mode Acquisition and Targeting) SXi system. Targeting device that provides fast ultrasound localization of a treatment target on a daily basis. (Courtesy North American Scientific)
Figure 23. Real-time Position Management system. Device used to aUow respiratory gating treatment delivery. (Courtesy of Varian Medical Systems)
For stereotactic radiosurgery or fractionated stereotactic radiotherapy, a suite of accessories is now available. Most important is a head stereotactic localizer such as the Gill-Thomas-Cosman (GTC) relocatable head ring (Fig. 26), which enables precise fixation and localization and repositioning of targets in the cranium (21). Another
Figure 24. Active Breathing CoordinatorTMsystem. Device allows the radiation oncologist to pause a patientís breathing at a precisely indicated tidal volume and coordinate delivery with this pause. (Courtesy of Elekta AB)
Important device when using the newly emerging radiotherapy treatment, stereotactic body radiation therapy (SBRT), in which a high dose is delivered in either a single fraction or just a few fractions, is the frame-based body stereotactic immobilization system (22). There are several now commercially available; an example is the Elekta Stereotactic Body Frame shown in Fig. 27. This device provides a reference stereotactic coordinate system that is external to the patientís body, so that the coordinates of a target volume can be reproducibly localized during simulation and treatment. This frame has built-in reference indicators for CT or MR determination of target volume coordinates. In addition, a diaphragm control attached to the frame can be used to minimize respiratory movements. Horizontal positioning of the frame, on the CT simulator or treatment couch, is achieved using an adjustable base on the frame.