Immobilization Techniques in Radiotherapy

6.1

Introduction

Proper immobilization techniques accomplish a variety of clinical goals including the following:

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  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, since these are long treatments delivering high doses. Effective immobilization is crucial for minimizing motion during treatment.

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  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 ).

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  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).

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  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.

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. 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. For further information see AAPM TG-68 on intracranial stereotactic systems.

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 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 devices and have noted small intrafraction motion.