Radiation Therapy

In this article, written by the third-place winner in the Student category of the 2007 rt image Writer’s Competition, the author talks about patient positioning and immobilization devices.


Patient positioning and immobilization are said to be the most crucial parts of treatment in radiation therapy. However, patient positioning is one of the weakest links in treatment planning. Immobilization devices allow for accurate targeting in radiation therapy treatment.

Without proper immobilization, the patient is at risk for improper treatment and unwanted side effects due to mistreatment of the clinical target volume. Immobilization devices such as molds, casts, headrests, and other devices, are constructed to reduce setup error and patient movement during treatment.

The evolution of immobilization devices and techniques has revolutionized today’s therapy methods and their effectiveness. For example, the use of rigid immobilization devices can help reduce treatment errors in isocenter placement by 20 percent.

Simple immobilization devices can be adjusted prior to treatment to fit the patient’s body; however, they will not restrain those patients who insist on moving during treatment. Simple immobilization devices can include halos, wingboards, breastboards, and bellyboards.

Complex immobilization devices are made specifically for one patient and can be made and used in the treatment of any area to position and secure the patient.

However, this isn’t commonly done, due to the time and expense in manufacturing each device. In general, simple devices may not be as effective as custom devices because they may allow the patient to easily move off of their treatment marks.

Despite any benefit that standard or customized immobilization devices may provide, accurate treatment is ultimately up to the therapist and the patient. To address this issue, importance must be placed on the therapists’ role in monitoring the patient for movements and shifts.

Additionally, patients must be informed that treatment success is proportional to the immobilization of their body.

Disregard for the therapists’ instructions can affect the outcome of their treatment. As radiation treatment procedures become more complicated, the need for better immobilization techniques and practices are key to future improvements in radiation therapy methods.

Something Missing

Immobilization techniques were very limited in early radiation therapy. Patients were commonly allowed to sit in a chair, stand or lie on a table without being restrained from movement in any way.

As a result, the clinical target volume was often missed, and outer margins of the treatment area received a portion of the dose.

Using a free setup, the proportion of fractions with setup errors greater than 5 mm ranges from 17 percent to 57 percent, and those greater than 10 mm are observed in up to 15 percent of fractions.
Misalignments on normal tissue often resulted in widespread metastases.

Later, patients were taped down while lying flat on a table. Use of this method led therapists to move away from treating patients in any other position besides lying flat unless it was necessary.

Although this method falls short of our standards today, taping patients to the table greatly improved the effectiveness of radiation therapy.

Unlike treatment today, past methods of immobilization did not use custom devices for each patient. Each device was reused and failed to conform to the exact needs of every patient for immobilization.

Advancements in technology and medical studies have given us the knowledge to develop a variety of immobilization devices that can be customized to fit each patient to improve overall treatment effectiveness. 

Head, Neck, and Brain Immobilization

In the past, patients treated for head, neck, and brain cancers were taped to the table across their forehead. A bite block was occasionally used in order to tilt the neck in a reproducible fashion.

This technique proved to be quite effective despite its primitive design.

Current methods for head, neck, and brain cancer irradiation include a variety of masks, molds, and frame systems. In some instances, shoulder straps are used to pull the shoulders out of the treatment field for the neck.

Initial construction and selection of proper immobilization for head and neck cancers is one of the most important parts of ensuring proper treatment of the patient.

Studies on treatment errors in immobilization for head and neck cancers show that errors during fabrication and selection of the incorrect headrest account for 75 percent of the setup errors greater than 4 mm.

The most common immobilization device used to treat head, neck, and brain cancer is an aquaplast. This device is made from a rigid plastic that becomes pliable when placed in warm water.

When it is wet, it is placed over the patient’s head and conforms to the contour of the treatment area. The aquaplast will dry very quickly. After the mask has cured, the therapist can use the mask as an immobilization device and may also make direct marks on the plastic for precise positioning.

This prevents the therapist from having to mark directly on the patients’ skin. Masks provide great reproducibility in daily treatments, are lightweight and easy to use.

Additionally, modifications can be made to the mask if a patient experiences swelling or weight loss during treatment with the mask.

However, there are a few drawbacks to masks, such as patient discomfort and high costs due to personal customization. Due to the restrictive qualities the mask provides, some patients report experiencing anxiety while wearing it.

However, the mask may be cut to increase comfort if the patient feels claustrophobic. Aquaplasts can range in price, from $20 to $40, depending on its size and type.

Immobilization becomes more complex for radiosurgery or gamma knife procedures, which give a high dose in a single fraction, as opposed to spreading the dose out over a five-to six-week period.

During these procedures, the patient is fitted with a halo system and then bolted to the treatment table. Bolts are then fitted to the system and screwed into the patient’s skull to minimize any movement during treatment.

The smallest movement could jeopardize a patient’s treatment success due to the minimal margin of error in these procedures.

Without the halo system, these types of procedures would not be possible to successfully complete. Although the halo allows for very accurate treatment, the procedure has proven to be invasive for many patients, is costly and uncomfortable.

Thorax and Breast Immobilization

One of the most common sites of field placement error is the chest area. Immobilization of the chest has not been considered as important as the head and neck area.

There are two common immobilization systems used when treating tumors in the chest cavity. The most common and low-cost system is the wing board, which allows the patients’ arms to be pulled up and away from the treated area for lateral fields.

Additionally, it raises the shoulders and superclavs out of the treatment area. Wing boards can be reused for numerous patients, thus keeping the cost down.

However, because this is not a custom immobilization device, there is a chance for more error during treatment. With this system, the patient is easily able to move their lower body. Any small movement by the patient could cause them to rotate or shift from their treatment marks.

Vac-loks can also be used as an immobilization device for the chest cavity. These devices are custom, beanbag type pillows that are placed around the patients’ upper body. The air is then vacuumed out of the bag for a custom fit and sealed in order to retain its shape.

Vac-loks are reusable, so long as the patient does not have any significant weight gain or loss.

This customization factor of the Vac-lok system improves consistency in patient setups. Use of a Vac-lok, in addition to applied weight to the patient’s abdomen, can suppress lung movement during respiration up to 50 percent.

Vac-loks can also be used to improve patient setups for the breast field. This type of treatment can involve the simultaneous use of the Vac-lok system with either a wing board or the prone breast board, which will be discussed later.

Both treatment methods allow for the boards to be customized to better fit the patients’ body, resulting in less movement.

A study in the British Journal of Radiology tested the use of Vac-loks versus the standard breast boards. Differences were found in the superoinferior direction and rotational devices. The use of the Vac-lok system was associated with a reduction in treatment errors in patient positioning.

The breast board is similar to the wing board; however, it is used specifically for the treatment of breast cancer. Breast boards have several adjustable features to allow for the manipulation of patients’ arms, wrists, head, and shoulders.

Today, breast boards are generally constructed of 100 percent carbon, allowing the device to be lightweight and durable. Additionally, carbon fiber boards have lower attenuation levels, which permit maximum beam penetration.

The breast can also be immobilized using a Thermoplastic Breast Support System, similar in construction to the aquaplast. A plastic mesh mold is taken of the breast to aid in placement when lying supine on a breast board. The breast board is most effective for patients with small-to-medium sized breasts.

When a patient lies on the board, larger breasts tend to push up high into the superior region of the chest. This does not allow therapists to accurately plan and treat the tumor volume consistently, which can result in “marked dose inhomogeneity” or “an inferior cosmetic outcome.”

The prone breast board is used to treat patients in the prone position and enables more effective treatment for patients with larger breasts. During treatment, the breasts hang in between the upper and lower sections of the board, thus allowing for complete and consistent coverage of the breast.

A wedge system in between the upper and lower sections of the prone breast board compresses the contra lateral breast out of the treatment area to avoid irradiation.

Pelvis Immobilization

Like the chest, immobilization of the pelvis region customarily hasn’t been considered as important as the head and neck. However, studies have shown that the use of rigid immobilization devices eliminate the 10 percent of daily positioning errors.

The use of rigid immobilization improves the accuracy of treatment delivery for the prone position, especially in the anteroposterior direction. The number of major deviations greater than 10 mm, which would result in mistreatment, was reduced from 31 percent to 11 percent.

Similar immobilization devices, such as Vac-loks, belly boards, and the HipFix system, are commonly used during treatments of the pelvis region. Vac-loks are used when the patient is treated supine. The Vac-lok secures the patient’s legs and feet to prevent the pelvis from moving out of alignment.

Using the Vac-lok in conjuction with hip straightening devices can reduce treatment deviation averages by 40 percent and portal verification film shifts by 50 percent. Belly boards are used when the patient is treated prone.

Belly boards allow the small bowel to drop below the lateral field to avoid severe side effects of irradiation.

A study conducted in the British Journal of Radiology showed that the average volume of small bowel within the planning target volume, in a series treated with standard box technique, was 100 cm.

The average volume decreased to 23 cm when the belly board was used. Use of the belly board significantly decreases the average volume of small bowel that would otherwise be within the lateral field during standard box technique treatment.

The HipFix system is a cast molding made from plastic that becomes pliable when placed in warm water. The mold is then placed on the patient, fastened to the table, and conforms to the contour on the patients’ pelvis.

Like the aquaplast, the cast is then used throughout the patient’s treatment to reproduce consistent setups. Standard deviations in isocenter placement have shown to be as little as 1.9 mm to 2.6 mm in a recent study on pelvic immobilization using HipFix for prostate irradiation.

An effective system of patient immobilization reduces the normal setup variability and its effectiveness helps to determine the margin width required around the clinical target volume.

Many factors must be considered when designing and selecting an immobilization device. It is important that one is easy to use, lightweight, durable, and comfortable for the patient. In addition, they must attenuate as little of the beam as possible.

The ease of use of a particular device is important in order to make the most efficient use of the therapist and patient’s time.

Immobilization devices can be large in size. For instance, a belly board can be 5- to 6-feet long. Mobility of such a large device can be difficult, therefore it is important that it be lightweight. Patient comfort is an essential factor during treatment.

Complicated treatment plans and intricate immobilization devices are not as effective if a patient is not comfortable and is not still during treatment. Devices must also be durable enough to be reused and withstand a treatment regimen.

Treatments may be delayed and time is wasted when immobilization devices break and must be replaced.

Today, carbon fiber is commonly used in the construction of immobilization devices. Carbon fiber allows the device to be lightweight yet durable and allows for less beam attenuation than acrylic devices.

Looking Ahead

The use of immobilization techniques discussed in these studies show great promise to the improvement and progression of radiation therapy techniques. The need for accurate patient positioning through the use of immobilization is evident.

Despite this, attention has been focused on the development of new radical treatment methods through the use of machinery, which promises to deliver higher doses to tumors and improve cure rates.

Little attention has been directed toward immobilization improvements in comparison to the development of new treatment machines. Studies on immobilization techniques show that stricter immobilization produces better treatments with less deviation from the control.

Individuals must be informed about the importance of immobilization devices, in order to understand that more focus must be placed in continuing to improve the technology behind them.

Improved immobilization techniques and devices can contribute to smaller treatment volumes, higher controlled doses, lower patient side-effects – and ultimately, a higher cure rate.     


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