- Email: [email protected]
Immobilization Techniques in Radiotherapy
C H A P T E R
6
Immobilization Techniques in Radiotherapy
6.1 Introduction
Proper immobilization techniques accomplish a variety of clinical goals including the following: Reduce patient motion and improve day-to-day reproducibility of setup. External beam treatments typically require several tens of minutes to complete, and during this time the operator is essentially blind to any motion of the patient or tissue with notable exceptions (e.g., orthogonal fluoroscopic imaging, 3D surface mapping, or combination magnetic resonance (MR) therapy devices). Although delivery techniques such as volumetric arc therapy (VMAT) can reduce treatment times, the addition of other technologies such as Image-guided radiation therapy (IGRT) increase the overall time of a treatment session. Movement during treatment is particularly important for stereotactic treatments, especially “frameless” stereotactic radiosurgery,1 since these are long treatments delivering high doses. Effective immobilization is crucial for minimizing motion during treatment. ■ Improve patient comfort. This is not only an end in itself but also is also a key factor in reducing motion. An uncomfortable patient will tend to move to try to find a more comfortable position; therefore, ensuring that the patient is as comfortable as possible will reduce motion. In addition to reducing motion, immobilization devices also help ensure that the patient anatomy is aligned reproducibly from day to day. Patient comfort also plays into reproducibility in that patients will settle into a comfortable position. Though this may be thought to be less important in the era of IGRT, the reality is that most patients do not receive IGRT on every day of treatment and some sites still provide challenges even with IGRT. An example is the head and neck. Because of neck flexion or extension it may only be possible to accurately align one anatomical region (see Chapter 18). ■ Accommodate the requirements of physical devices. Immobilization devices and patients must fit not only through the bore of the computed tomography (CT) simulator (or magnetic resonance imaging (MRI) or positron emission tomography (PET) unit) but must also fit within the envelope of devices used during treatment (e.g., a lung patient with a lateralized lesion whose arms collide with the IGRT imaging panel on treatment). ■ Avoidance of normal tissue. The thoughtful use of immobilization technologies can result in better avoidance of normal tissue, a classic example being the “belly board,” whereby a patient undergoing abdominal treatment is placed prone in order to pull the bowel out of the high dose region. ■
87
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P A R T I Building Blocks
A
C
B Figure 6.1 Head and neck immobilization with a thermoplastic mask. A, Carbon fiber table top with head rest, shoulder bars and hand grips. B, Thermoplastic mask for head immobilization. C, High temperature water bath used to soften thermoplastic material.
6.2 Immobilization Devices and Techniques
CRANIAL IMMOBILIZATION Head frames using pins represent what is arguably the most accurate (and most invasive) methods of cranial immobilization and also allow localization using the Brown-Roberts-Wells (BRW) stereotaxy system. Examples include the Leksell G frame (Elekta Inc.) or the Talon system (Best Nomos Inc.), the latter of which allows multiple fraction treatments with two self-tapping titanium screws anchored in bone. A relocatable head frame was introduced in the early 1990s, the Gill-Thomas-Cosman (GTC) frame, which employs a dental appliance on the maxillary bone to anchor the cranium.2 One version uses a vacuum to maintain the position of the dental appliance combined with a patient-controlled release mechanism, an example of which is the HeadFix system (Elekta Inc.) available since the late 1990s.3 For further information see AAPM TG-68 on intracranial stereotactic systems.4 A widely used noninvasive cranial immobilization option relies on a thermoplastic mask (Figure 6.1B). These perforated plastics are typically 1/16″ to 1/8″ thick and soften when heated in a water bath at approximately 150° to 165° F (Figure 6.1C). Some masks are designed to soften in a convection oven. Some newer formulations include Kevlar for rigidity or may have reinforced
CHAPTER 6 Immobilization Techniques in Radiotherapy
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areas to increase rigidity. Others have large holes around the eyes to increase patient comfort. Vendor recommendations should be followed to minimize mask shrinkage after formation. Numerous studies have examined the reproducibility of patient positioning with these devices5 and have noted small intrafraction motion.
HEAD AND NECK IMMOBILIZATION Thermoplastic masks are also commonly used for immobilization of head and neck cancer patients (Figure 6.1). As with cranial masks, a head holder is used that sets the head extension (Figure 6.1A). Various standard reusable head holder devices are available: Timo foam (polyurethane foam with a plastic coating), Silverman (clear radiologically thin plastic shell), or PosiFix (low density polyethylene foam, Civco Medical Solutions, Inc.). Alternatively, a custom device can be made such as the MoldCare cushion (Alcare Co. Inc.), which is a formulation of polystyrene beads coated in a moisture-curing resin in a fabric bag. The custom devices provide more support in the neck area and may be more reproducible. For head and neck treatments it is important to control the position of the shoulders, especially if supraclavicular nodal regions are to be included in the treatment. This can be accomplished with shoulder bars (Figure 6.1A) or with a thermoplastic mask that extends down over the shoulders, though there appears to be no clear advantage of one over the other.6 Hand grips set at known positions also aid with arm and shoulder position (Figure 6.1A). Thermoplastic mask arrangements are available that support prone treatments as required, for example, in craniospinal irradiation. One problem with thermoplastic devices is that they hold a rigid shape formed at the time of simulation. If a patient loses weight or the tumor volume resolves, the thermoplastic device may become loose. An example is shown in Figure 6.2 for a head and neck patient; the mask initially fits snugly (Figure 6.2A) but becomes loose by day 20 of treatment (Figure 6.2B). Similarly, a patient on steroids may develop swelling, which causes a mask to fit too tightly. In either case, resimulation and planning may be required. The use of a mask makes it difficult to measure the true source-to-surface distance (SSD). A simple depth gauge can be used to monitor the distance between the mask and skin.
THORACIC, ABDOMINAL, AND PELVIC IMMOBILIZATION A major consideration in the thorax is the position of the patient’s arms. Arms down at the sides is the most comfortable position for most patients but can present a challenge for three-point setup and precludes the use of some beam angles. Treating through the arms introduces dosimetric uncertainty (due to daily differences in arm position) and can also cause skin toxicity in the folds. A commonly used device is the wingboard (Figure 6.3B). Patients rest their heads on the headrest in the center and grip the two posts on the top with their hands. The arm position is set reproducibly by controlling the position of the grips. Body-forming immobilization devices are in common use, either on their own or in combination with other devices like the wingboard shown in Figure 6.3. One type is based on expandable polyurethane foam, such as AlphaCradle (Smithers Medical Products Inc.) or RediFoam (CIVCO Inc.), which consists of a two-part solution poured into a plastic bag that hardens after a few minutes into a rigid foam. Care must be taken because some foaming agents are toxic to the skin. It has been reported that KY jelly mixed into the foam can clean it from skin. One feature of the foam cradle system is that areas can be cut out once the cradle is formed. A section that prevents reading of an SSD can be removed, for instance. A downside is that if the foam cradle is not acceptable, it must be discarded and the process started over from the beginning. An alternative is the vacuum cushion system, which consists of polystrene beads in a plastic bag that is made rigid by evacuating the bag down to a pressure of approximately 650 mbar and then sealing it. One such device is shown in Figure 6.4—the BlugBag (Medical Intelligence Inc.)— which shows the vacuum-sealed immobilization bag under the patient (blue) and the optional
90
P A R T I Building Blocks
A
B
Figure 6.2 Cone-beam CT scans of a head and neck patient on day 1 of treatment (A) and day 20 of treatment (B) showing the fit of the thermoplastic mask.
A
B
Figure 6.3 Carbon fiber table top with indexing bar (A) and indexed wingboard (B).
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Figure 6.4 Vacuum bag for pelvic immobilization.
cover sheet that also vacuum seals over the top of the patient. Similar products are available from other vendors (e.g., Vac-Lok, CIVCO Medical Solutions Inc.). Care must be taken in handling and storing these vacuum bags since a rupture of the seal will inflate the device, which may necessitate resimulation and replanning. Vendor recommendations should be followed regarding periodic reapplication of the vacuum to maintain the proper rigidity. A third immobilization option, particularly useful for the pelvis, is a thermoplastic mold that stretches over the patient (e.g., HipFix, CIVCO Medical Solutions, Inc.). These are similar to the thermoplastic head mask but are constructed of thicker plastic. For the pelvis and abdomen special consideration must be given to the legs and feet because their large lever arm can affect the position of the hips. Leg and knee positions can be set with a vacuum bag (Figure 6.4) or a separate indexable knee roll and foot holder, which are commercially available. In addition, a “saddle” can be formed in the vacuum cushion to assist with accurate superior/inferior positioning.
INDEXING OF IMMOBILIZATION DEVICES Figure 6.3A and B illustrate an important principle common to almost all modern immobilization devices: indexing. With indexing, the immobilization devices are clamped to labeled positional notches in the table (e.g., “H2” in Figure 6.3A). This ensures that the device and patient will be at the same position on the table for each day of treatment. Indexing enables an important safety feature—namely, an interlock for the position of the treatment table. Since the table is expected to be in the same place each day, the treatment delivery system can read its position and determine whether it is at its intended location to within a predetermined tolerance. This can help prevent inadvertent treatment of the wrong location. Immobilization equipment must sometimes be used in combination with other devices. An example is an abdominal compression plate that might fit on a bridge over the patient or an array of stereo-camera markers that may also be fixed on a bridge over the patient. Such devices must be accommodated within the constraints of the immobilization devices.
BREAST IMMOBILIZATION AND SETUP Immobilization for breast treatments presents some unique challenges. If a supine setup is used, elevation on an adjustable incline board is often helpful to increase comfort and to provide a more reproducible fall of the breast. It is important to reduce rotations and to provide a reproducible position for the arm, particularly when matching supraclavicular or axilla fields to the breast tangent fields. Commercial devices are available for positioning the ipsilateral arm. Supine setups
92
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on an incline board are also subject to problems with clearance with the treatment devices or CT simulator bore. Alternatively, prone setups can be used, which have the advantage of pulling the breast tissue away from the lung and heart and may have a particular advantage for larger women. Special care must be taken, however, to reduce rotation for these setups. It is also important to position the breast at the proper place relative to the opening in the prone board and to avoid having the breast touch the table top as this may cause excess skin dose. Some prone breast devices are attached as a table extension to eliminate this effect. Bolus presents a special challenge for breast treatments because the bolus should conform to the skin as much as possible, which may be difficult with typical tissue equivalent bolus material. Alternatives include a brass metal mesh,7 a beam spoiler, or even a thick coating of Vaseline.
IMMOBILIZATION FOR EXTREMITIES Extremities represent a special challenge for immobilization and setup. If a single leg or arm is to be treated, it is necessary to avoid the contralateral limb with the creative use of immobilization devices, elevating and holding one of the legs in place with a vacuum cushion, for example. For treatments of the hand or foot, the hand or foot can sometimes be clamped down with a thermoplastic head mask. If bolus is required for the foot or hand it can sometimes be treated in a water bath.
6.3 Dosimetric Effects of Table Tops and Immobilization Devices
Immobilization devices and table tops disturb radiation beams that pass through them, a topic that is considered in detail in AAPM TG-1768 with reference to roughly 20 studies that have been published on the topic. Two issues are considered: 1) increased skin dose due to the blousing effect of devices and 2) attenuation of the beam by devices.
INCREASED SKIN DOSE A standard reference for dose-related skin toxicities is Archambeau et al.9 (the topic is not considered in the QUANTEC study, which is more focused on late effects). For a patient receiving standard 2 Gy fraction treatment, skin erythema starts at approximately 20 Gy, dry desquamation at 45 Gy, and moist desquamation above that. Most damage heals within 4 weeks of the completion of therapy, and late skin necrosis is a rare phenomenon in modern practice. The two skin layers relevant to acute and late effects are the epidermal layer (approximately 0.1 mm deep), which is home to proliferating cells, and the dermal layer (1 to 3 mm deep), which supplies the vasculature.9 AAPM TG-1768 notes that thermoplastic devices can increase surface does by 30% to 70%, depending on the final thickness of plastic and the beam parameters. Vacuum bags and foam cradles have also been observed to increase skin dose by roughly the same amount. It is important to note that solid carbon fiber table tops also have a skin bolusing effect. These tables (e.g., Figure 6.3A) are now in wide use, especially for IGRT, and can substantially increase skin dose. For low energy posterior beams this increase can be a factor of two or more, to the point that most skin sparing may be lost.
BEAM ATTENUATION Carbon fiber tables also attenuate the transmitted beam with dose reductions of 3% to 6% depending on the energy, angle, and field sizes. The table top support rails present on some units
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may severely attenuate the beam, and treatment through these should be avoided. AAPM TG-1768 recommends that attenuation and bolusing effects of table tops be explicitly included in the treatment plan. Some treatment planning system vendors provide an automatic means of accomplishing this, while others require users to develop their own method using CT scans of these devices.
6.4 Emerging Questions
Clinical requirements for patient immobilization continue to evolve. One important question in the near future will be the compatibility of the devices with MRI. This is important for two reasons. First, to facilitate the image fusion of diagnostic MRI for sites outside the cranium, it is highly desirable to scan the patient in a pose that mimics treatment. In this sense, large bore MRI units are valuable, such as the MAGNETOM Espree (Siemens Inc.), a 1.5 T MRI unit with a bore that is 70 cm diameter wide and 120 cm long. Whenever possible the patient should be scanned on a flat surface with the immobilization devices used for treatment. Second, the emerging use of MRI-guided radiation therapy (MRgRT) necessitates the use of MR-compatible immobilization devices. Many modern devices are designed to be MR compatible, a notable exception being the carbon fiber table top, which can exhibit large eddy currents that can distort images or even harm the patient.
FOR THE PHYSICIAN Immobilization is one of the first technical steps taken in providing radiation treatment. The subsequent quality of treatment depends crucially on this step. Good immobilization acts not only to reduce motion during treatment, but also to provide reproducible patient setup from day to day. This is important even in the era of image guidance, because some alignment issues cannot be corrected by imaging alone (e.g., variability in flexion of the neck, which means only one region can be aligned with the intended position). Immobilization can also be used to increase normal tissue sparing, the classic example being the prone “belly board” that moves bowel away from the high dose region. A variety of immobilization technologies exist, including thermoplastic masks, bite blocks, wing boards to assist in arm positioning, and inclined boards for breast cancer patients. Though most modern devices, including table support assemblies, are made to be radiologically thin, care must be taken to avoid excess skin toxicity since some immobilization devices and couch tops can increase skin dose by a factor of two or more. The recently published AAPM Task Group 176 report8 offers a wealth of information on this topic. Most modern immobilization devices provide indexing, which offers a means of locking the device into the treatment support table at exactly the same place each day. Because the device is in the same place each day relative to the table, this means that the table X, Y, and Z coordinates should also be the same each day. These table coordinates can be checked by the treatment delivery computer. Indexing therefore provides an important double-check safety mechanism to prevent the treatment of the patient in the wrong location.
References 1. Bova FJ, Buatti JM, Friedman WA, et al. The University of Florida frameless high-precision stereotactic radiotherapy system. Int J Radiat Oncol Biol Phys 1997;38:875–82. 2. Gill SS, Thomas DG, Warrington AP, et al. Relocatable frame for stereotactic external beam radiotherapy. Int J Radiat Oncol Biol Phys 1991;20:599–603. 3. Olch AJ, Lavey RS. Reproducibility and treatment planning advantages of a carbon fiber relocatable head fixation system. Radiother Oncol 2002;65:165–8.
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4. Lightstone AW, Benedict SH, Bova FJ, et al. Intracranial stereotactic positioning systems: Report of the American Association of Physicists in Medicine Radiation Therapy Committee Task Group no. 68. Med Phys 2005;32:2380–98. 5. Tryggestad E, Christian M, Ford E, et al. Inter- and intrafraction patient positioning uncertainties for intracranial radiotherapy: A study of four frameless, thermoplastic mask-based immobilization strategies using daily cone-beam CT. Int J Radiat Oncol Biol Phys 2011;80:281–90. 6. Sharp L, Lewin F, Johansson H, et al. Randomized trial on two types of thermoplastic masks for patient immobilization during radiation therapy for head-and-neck cancer. Int J Radiat Oncol Biol Phys 2005;61:250–6. 7. Healy E, Anderson S, Cui J, et al. Skin dose effects of postmastectomy chest wall radiation therapy using brass mesh as an alternative to tissue equivalent bolus. Pract Radiat Oncol 2013;3:e45–53. 8. Olch AJea. Dosimetric effects of immobilization Devices: Report of AAPM Task Group 176. Med Phys 2014;41(6), 061501-1 to 061501-30. 9. Archambeau JO, Pezner R, Wasserman T. Pathophysiology of irradiated skin and breast. Int J Radiat Oncol Biol Phys 1995;31:1171–85.
6
Immobilization Techniques in Radiotherapy
6.1 Introduction
Proper immobilization techniques accomplish a variety of clinical goals including the following: Reduce patient motion and improve day-to-day reproducibility of setup. External beam treatments typically require several tens of minutes to complete, and during this time the operator is essentially blind to any motion of the patient or tissue with notable exceptions (e.g., orthogonal fluoroscopic imaging, 3D surface mapping, or combination magnetic resonance (MR) therapy devices). Although delivery techniques such as volumetric arc therapy (VMAT) can reduce treatment times, the addition of other technologies such as Image-guided radiation therapy (IGRT) increase the overall time of a treatment session. Movement during treatment is particularly important for stereotactic treatments, especially “frameless” stereotactic radiosurgery,1 since these are long treatments delivering high doses. Effective immobilization is crucial for minimizing motion during treatment. ■ Improve patient comfort. This is not only an end in itself but also is also a key factor in reducing motion. An uncomfortable patient will tend to move to try to find a more comfortable position; therefore, ensuring that the patient is as comfortable as possible will reduce motion. In addition to reducing motion, immobilization devices also help ensure that the patient anatomy is aligned reproducibly from day to day. Patient comfort also plays into reproducibility in that patients will settle into a comfortable position. Though this may be thought to be less important in the era of IGRT, the reality is that most patients do not receive IGRT on every day of treatment and some sites still provide challenges even with IGRT. An example is the head and neck. Because of neck flexion or extension it may only be possible to accurately align one anatomical region (see Chapter 18). ■ Accommodate the requirements of physical devices. Immobilization devices and patients must fit not only through the bore of the computed tomography (CT) simulator (or magnetic resonance imaging (MRI) or positron emission tomography (PET) unit) but must also fit within the envelope of devices used during treatment (e.g., a lung patient with a lateralized lesion whose arms collide with the IGRT imaging panel on treatment). ■ Avoidance of normal tissue. The thoughtful use of immobilization technologies can result in better avoidance of normal tissue, a classic example being the “belly board,” whereby a patient undergoing abdominal treatment is placed prone in order to pull the bowel out of the high dose region. ■
87
88
P A R T I Building Blocks
A
C
B Figure 6.1 Head and neck immobilization with a thermoplastic mask. A, Carbon fiber table top with head rest, shoulder bars and hand grips. B, Thermoplastic mask for head immobilization. C, High temperature water bath used to soften thermoplastic material.
6.2 Immobilization Devices and Techniques
CRANIAL IMMOBILIZATION Head frames using pins represent what is arguably the most accurate (and most invasive) methods of cranial immobilization and also allow localization using the Brown-Roberts-Wells (BRW) stereotaxy system. Examples include the Leksell G frame (Elekta Inc.) or the Talon system (Best Nomos Inc.), the latter of which allows multiple fraction treatments with two self-tapping titanium screws anchored in bone. A relocatable head frame was introduced in the early 1990s, the Gill-Thomas-Cosman (GTC) frame, which employs a dental appliance on the maxillary bone to anchor the cranium.2 One version uses a vacuum to maintain the position of the dental appliance combined with a patient-controlled release mechanism, an example of which is the HeadFix system (Elekta Inc.) available since the late 1990s.3 For further information see AAPM TG-68 on intracranial stereotactic systems.4 A widely used noninvasive cranial immobilization option relies on a thermoplastic mask (Figure 6.1B). These perforated plastics are typically 1/16″ to 1/8″ thick and soften when heated in a water bath at approximately 150° to 165° F (Figure 6.1C). Some masks are designed to soften in a convection oven. Some newer formulations include Kevlar for rigidity or may have reinforced
CHAPTER 6 Immobilization Techniques in Radiotherapy
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areas to increase rigidity. Others have large holes around the eyes to increase patient comfort. Vendor recommendations should be followed to minimize mask shrinkage after formation. Numerous studies have examined the reproducibility of patient positioning with these devices5 and have noted small intrafraction motion.
HEAD AND NECK IMMOBILIZATION Thermoplastic masks are also commonly used for immobilization of head and neck cancer patients (Figure 6.1). As with cranial masks, a head holder is used that sets the head extension (Figure 6.1A). Various standard reusable head holder devices are available: Timo foam (polyurethane foam with a plastic coating), Silverman (clear radiologically thin plastic shell), or PosiFix (low density polyethylene foam, Civco Medical Solutions, Inc.). Alternatively, a custom device can be made such as the MoldCare cushion (Alcare Co. Inc.), which is a formulation of polystyrene beads coated in a moisture-curing resin in a fabric bag. The custom devices provide more support in the neck area and may be more reproducible. For head and neck treatments it is important to control the position of the shoulders, especially if supraclavicular nodal regions are to be included in the treatment. This can be accomplished with shoulder bars (Figure 6.1A) or with a thermoplastic mask that extends down over the shoulders, though there appears to be no clear advantage of one over the other.6 Hand grips set at known positions also aid with arm and shoulder position (Figure 6.1A). Thermoplastic mask arrangements are available that support prone treatments as required, for example, in craniospinal irradiation. One problem with thermoplastic devices is that they hold a rigid shape formed at the time of simulation. If a patient loses weight or the tumor volume resolves, the thermoplastic device may become loose. An example is shown in Figure 6.2 for a head and neck patient; the mask initially fits snugly (Figure 6.2A) but becomes loose by day 20 of treatment (Figure 6.2B). Similarly, a patient on steroids may develop swelling, which causes a mask to fit too tightly. In either case, resimulation and planning may be required. The use of a mask makes it difficult to measure the true source-to-surface distance (SSD). A simple depth gauge can be used to monitor the distance between the mask and skin.
THORACIC, ABDOMINAL, AND PELVIC IMMOBILIZATION A major consideration in the thorax is the position of the patient’s arms. Arms down at the sides is the most comfortable position for most patients but can present a challenge for three-point setup and precludes the use of some beam angles. Treating through the arms introduces dosimetric uncertainty (due to daily differences in arm position) and can also cause skin toxicity in the folds. A commonly used device is the wingboard (Figure 6.3B). Patients rest their heads on the headrest in the center and grip the two posts on the top with their hands. The arm position is set reproducibly by controlling the position of the grips. Body-forming immobilization devices are in common use, either on their own or in combination with other devices like the wingboard shown in Figure 6.3. One type is based on expandable polyurethane foam, such as AlphaCradle (Smithers Medical Products Inc.) or RediFoam (CIVCO Inc.), which consists of a two-part solution poured into a plastic bag that hardens after a few minutes into a rigid foam. Care must be taken because some foaming agents are toxic to the skin. It has been reported that KY jelly mixed into the foam can clean it from skin. One feature of the foam cradle system is that areas can be cut out once the cradle is formed. A section that prevents reading of an SSD can be removed, for instance. A downside is that if the foam cradle is not acceptable, it must be discarded and the process started over from the beginning. An alternative is the vacuum cushion system, which consists of polystrene beads in a plastic bag that is made rigid by evacuating the bag down to a pressure of approximately 650 mbar and then sealing it. One such device is shown in Figure 6.4—the BlugBag (Medical Intelligence Inc.)— which shows the vacuum-sealed immobilization bag under the patient (blue) and the optional
90
P A R T I Building Blocks
A
B
Figure 6.2 Cone-beam CT scans of a head and neck patient on day 1 of treatment (A) and day 20 of treatment (B) showing the fit of the thermoplastic mask.
A
B
Figure 6.3 Carbon fiber table top with indexing bar (A) and indexed wingboard (B).
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Figure 6.4 Vacuum bag for pelvic immobilization.
cover sheet that also vacuum seals over the top of the patient. Similar products are available from other vendors (e.g., Vac-Lok, CIVCO Medical Solutions Inc.). Care must be taken in handling and storing these vacuum bags since a rupture of the seal will inflate the device, which may necessitate resimulation and replanning. Vendor recommendations should be followed regarding periodic reapplication of the vacuum to maintain the proper rigidity. A third immobilization option, particularly useful for the pelvis, is a thermoplastic mold that stretches over the patient (e.g., HipFix, CIVCO Medical Solutions, Inc.). These are similar to the thermoplastic head mask but are constructed of thicker plastic. For the pelvis and abdomen special consideration must be given to the legs and feet because their large lever arm can affect the position of the hips. Leg and knee positions can be set with a vacuum bag (Figure 6.4) or a separate indexable knee roll and foot holder, which are commercially available. In addition, a “saddle” can be formed in the vacuum cushion to assist with accurate superior/inferior positioning.
INDEXING OF IMMOBILIZATION DEVICES Figure 6.3A and B illustrate an important principle common to almost all modern immobilization devices: indexing. With indexing, the immobilization devices are clamped to labeled positional notches in the table (e.g., “H2” in Figure 6.3A). This ensures that the device and patient will be at the same position on the table for each day of treatment. Indexing enables an important safety feature—namely, an interlock for the position of the treatment table. Since the table is expected to be in the same place each day, the treatment delivery system can read its position and determine whether it is at its intended location to within a predetermined tolerance. This can help prevent inadvertent treatment of the wrong location. Immobilization equipment must sometimes be used in combination with other devices. An example is an abdominal compression plate that might fit on a bridge over the patient or an array of stereo-camera markers that may also be fixed on a bridge over the patient. Such devices must be accommodated within the constraints of the immobilization devices.
BREAST IMMOBILIZATION AND SETUP Immobilization for breast treatments presents some unique challenges. If a supine setup is used, elevation on an adjustable incline board is often helpful to increase comfort and to provide a more reproducible fall of the breast. It is important to reduce rotations and to provide a reproducible position for the arm, particularly when matching supraclavicular or axilla fields to the breast tangent fields. Commercial devices are available for positioning the ipsilateral arm. Supine setups
92
P A R T I Building Blocks
on an incline board are also subject to problems with clearance with the treatment devices or CT simulator bore. Alternatively, prone setups can be used, which have the advantage of pulling the breast tissue away from the lung and heart and may have a particular advantage for larger women. Special care must be taken, however, to reduce rotation for these setups. It is also important to position the breast at the proper place relative to the opening in the prone board and to avoid having the breast touch the table top as this may cause excess skin dose. Some prone breast devices are attached as a table extension to eliminate this effect. Bolus presents a special challenge for breast treatments because the bolus should conform to the skin as much as possible, which may be difficult with typical tissue equivalent bolus material. Alternatives include a brass metal mesh,7 a beam spoiler, or even a thick coating of Vaseline.
IMMOBILIZATION FOR EXTREMITIES Extremities represent a special challenge for immobilization and setup. If a single leg or arm is to be treated, it is necessary to avoid the contralateral limb with the creative use of immobilization devices, elevating and holding one of the legs in place with a vacuum cushion, for example. For treatments of the hand or foot, the hand or foot can sometimes be clamped down with a thermoplastic head mask. If bolus is required for the foot or hand it can sometimes be treated in a water bath.
6.3 Dosimetric Effects of Table Tops and Immobilization Devices
Immobilization devices and table tops disturb radiation beams that pass through them, a topic that is considered in detail in AAPM TG-1768 with reference to roughly 20 studies that have been published on the topic. Two issues are considered: 1) increased skin dose due to the blousing effect of devices and 2) attenuation of the beam by devices.
INCREASED SKIN DOSE A standard reference for dose-related skin toxicities is Archambeau et al.9 (the topic is not considered in the QUANTEC study, which is more focused on late effects). For a patient receiving standard 2 Gy fraction treatment, skin erythema starts at approximately 20 Gy, dry desquamation at 45 Gy, and moist desquamation above that. Most damage heals within 4 weeks of the completion of therapy, and late skin necrosis is a rare phenomenon in modern practice. The two skin layers relevant to acute and late effects are the epidermal layer (approximately 0.1 mm deep), which is home to proliferating cells, and the dermal layer (1 to 3 mm deep), which supplies the vasculature.9 AAPM TG-1768 notes that thermoplastic devices can increase surface does by 30% to 70%, depending on the final thickness of plastic and the beam parameters. Vacuum bags and foam cradles have also been observed to increase skin dose by roughly the same amount. It is important to note that solid carbon fiber table tops also have a skin bolusing effect. These tables (e.g., Figure 6.3A) are now in wide use, especially for IGRT, and can substantially increase skin dose. For low energy posterior beams this increase can be a factor of two or more, to the point that most skin sparing may be lost.
BEAM ATTENUATION Carbon fiber tables also attenuate the transmitted beam with dose reductions of 3% to 6% depending on the energy, angle, and field sizes. The table top support rails present on some units
CHAPTER 6 Immobilization Techniques in Radiotherapy
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may severely attenuate the beam, and treatment through these should be avoided. AAPM TG-1768 recommends that attenuation and bolusing effects of table tops be explicitly included in the treatment plan. Some treatment planning system vendors provide an automatic means of accomplishing this, while others require users to develop their own method using CT scans of these devices.
6.4 Emerging Questions
Clinical requirements for patient immobilization continue to evolve. One important question in the near future will be the compatibility of the devices with MRI. This is important for two reasons. First, to facilitate the image fusion of diagnostic MRI for sites outside the cranium, it is highly desirable to scan the patient in a pose that mimics treatment. In this sense, large bore MRI units are valuable, such as the MAGNETOM Espree (Siemens Inc.), a 1.5 T MRI unit with a bore that is 70 cm diameter wide and 120 cm long. Whenever possible the patient should be scanned on a flat surface with the immobilization devices used for treatment. Second, the emerging use of MRI-guided radiation therapy (MRgRT) necessitates the use of MR-compatible immobilization devices. Many modern devices are designed to be MR compatible, a notable exception being the carbon fiber table top, which can exhibit large eddy currents that can distort images or even harm the patient.
FOR THE PHYSICIAN Immobilization is one of the first technical steps taken in providing radiation treatment. The subsequent quality of treatment depends crucially on this step. Good immobilization acts not only to reduce motion during treatment, but also to provide reproducible patient setup from day to day. This is important even in the era of image guidance, because some alignment issues cannot be corrected by imaging alone (e.g., variability in flexion of the neck, which means only one region can be aligned with the intended position). Immobilization can also be used to increase normal tissue sparing, the classic example being the prone “belly board” that moves bowel away from the high dose region. A variety of immobilization technologies exist, including thermoplastic masks, bite blocks, wing boards to assist in arm positioning, and inclined boards for breast cancer patients. Though most modern devices, including table support assemblies, are made to be radiologically thin, care must be taken to avoid excess skin toxicity since some immobilization devices and couch tops can increase skin dose by a factor of two or more. The recently published AAPM Task Group 176 report8 offers a wealth of information on this topic. Most modern immobilization devices provide indexing, which offers a means of locking the device into the treatment support table at exactly the same place each day. Because the device is in the same place each day relative to the table, this means that the table X, Y, and Z coordinates should also be the same each day. These table coordinates can be checked by the treatment delivery computer. Indexing therefore provides an important double-check safety mechanism to prevent the treatment of the patient in the wrong location.
References 1. Bova FJ, Buatti JM, Friedman WA, et al. The University of Florida frameless high-precision stereotactic radiotherapy system. Int J Radiat Oncol Biol Phys 1997;38:875–82. 2. Gill SS, Thomas DG, Warrington AP, et al. Relocatable frame for stereotactic external beam radiotherapy. Int J Radiat Oncol Biol Phys 1991;20:599–603. 3. Olch AJ, Lavey RS. Reproducibility and treatment planning advantages of a carbon fiber relocatable head fixation system. Radiother Oncol 2002;65:165–8.
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4. Lightstone AW, Benedict SH, Bova FJ, et al. Intracranial stereotactic positioning systems: Report of the American Association of Physicists in Medicine Radiation Therapy Committee Task Group no. 68. Med Phys 2005;32:2380–98. 5. Tryggestad E, Christian M, Ford E, et al. Inter- and intrafraction patient positioning uncertainties for intracranial radiotherapy: A study of four frameless, thermoplastic mask-based immobilization strategies using daily cone-beam CT. Int J Radiat Oncol Biol Phys 2011;80:281–90. 6. Sharp L, Lewin F, Johansson H, et al. Randomized trial on two types of thermoplastic masks for patient immobilization during radiation therapy for head-and-neck cancer. Int J Radiat Oncol Biol Phys 2005;61:250–6. 7. Healy E, Anderson S, Cui J, et al. Skin dose effects of postmastectomy chest wall radiation therapy using brass mesh as an alternative to tissue equivalent bolus. Pract Radiat Oncol 2013;3:e45–53. 8. Olch AJea. Dosimetric effects of immobilization Devices: Report of AAPM Task Group 176. Med Phys 2014;41(6), 061501-1 to 061501-30. 9. Archambeau JO, Pezner R, Wasserman T. Pathophysiology of irradiated skin and breast. Int J Radiat Oncol Biol Phys 1995;31:1171–85.