IMMOBILIZATION OF PATIENTS UNDERGOING RADIATION THERAPY FOR HEAD AND NECK MALIGNANCIES

By Gunilla C. Bentel, RN, RTT
Duke University Medical Center
Durham, North Carolina

INTRODUCTION
Spatial misalignment of a treatment field can have adverse effects on the outcome of the treatment. Missing part of the target may result in failure to cure and inadvertent irradiation of adjacent normal tissues may cause irreversible injury. Immobilization techniques directed toward precise repositioning of the patient is therefore needed. In the head and neck area, where field margins around the tumor often must be small in order to avoid adjacent radiosensitive organs, effective immobilization techniques are particularly important. A large number of head and neck immobilization techniques, consisting of bit block systems or different casting materials, have been described.1,4,5,6,9,11,13,14,19,20,21
The frequency and magnitude of treatment field misalignment in irradiation of head and neck malignancies have been studied by several authors.8,10,16,21 Marks and Haus14 found localization errors in 16% in a series of head and neck cancer patients. These errors were significantly reduced by using a bite block immobilization system. Bkyhardt et al. found field placement errors > 5 mm in 17% and > 10 mm in 9.6% of patients treated for head and neck malignancies.2 Field placement errors were defined as deviations from the simulation film, block position, patient position, or field rotation.
This report describes a head and neck immobilization system consisting of a customized head support and a facial mask secured to a plastic base plate which is fastened to the treatment couch. This system has routinely been used in our department for approximately 12 months. This new immobilization system replaced one which used a standard set of foam rubber head supports and strips of a casting material over the face.

IMMOBILIZATION DEVICE
The head and neck immobilization device consists of a customized support under the patients head, neck, and shoulders, and a perforated thermoplastic sheet draped over the patient’s face (Fig. 1).

Fig. 1 The patient’s head fits tightly between the facial mask and the custom support. (A) Base plate, (B) Rectangular elevation, (C) Head support, (D) Protrusions which help fasten the mask to the base plate, (E) Clamp securing the mask to the base plate.
Both the head support and the mask are secured to a base plate which fits over the treatment couch and is fastened to the couch to prevent motion. In the center of the base plate is an elevated rectangular area, approximately 7” wide and 10” long. This area has an approximately 1” high wall on three sides with an opening on the remaining side for the patient’s neck. A plastic bag, approximately 10” wide and 20” long, is placed inside the walled area and extending down under the shoulders. The desired head position is determined and if additional head elevation is needed, a piece of Styrofoam®* is placed inside the bag to maintain the head elevation while the support is being made. If the chest needs to be elevate, the patient’s torso is placed on a sheet of Styrofoam® of appropriate thickness. The customized head support is made by pouring approximately 100 cc of foam** into the plastic bag. The patient lies down with the head on the plastic bag and within a few minutes the foam expands and fills the empty space between the walls and the head, neck, and shoulders (Fig. 2).

* Registered by Dow Chemical
** Smithers Medical Products, Inc.

Fig. 2 Customized support follows the posterior contour of the patient’s head and neck (A) Customized head support, (B) Base plate, (C) Clamps securing the mask to the base plate.

The head support becomes firm within 5 – 7 minutes. A perforated thermoplastic sheet, which is softened in hot water, is then draped over the patient’s face fro the middle of the forehead to below the chin. The thermoplastic sheet is also placed over several protrusions on each side of the base plate. Clamps with matching openings are then fastened over these protrusions to secure the mask to the base plate. The thermoplastic sheet becomes rigid within 5 minutes. Efforts are made to make a tight fit over the bridge of the nose. This area, where there is a thin layer of soft tissue between the skin and the bone, is an excellent anchor point for the mask. For patients requiring a bite block to keep the mouth open or a nasogastric feeding tube, a small hole is made in the thermoplastic sheet prior to softening. The bite block or nasogastric tube is then threaded through the hole as the mask is placed over the patient’s face. In patients where lead markers need to be placed on a particular surface structure during the simulation procedure (lymph node, canthus, etc.) a small opening is cut in the thermoplastic sheet following hardening of the mask.
The custom support fits tightly within the walled area preventing any motion. Two pegs, approximately ¼” in diameter and protruding about ½” from the elevated head area, are indented into the foam support preventing it from being placed in the wrong direction. Since the patient’s head is tightly “sandwiched” between the head support and the mask, a difference in clothing can cause the mask to be too tight if the patient wears other clothes when he/she returns for treatment. All clothing above the waist is therefore removed prior to making the mask and during treatment. In this head immobilization system, the mask secures the head to the base plate and the base plate is fastened to the treatment couch, preventing any skewing or rotation of the head.

CUSTOMIZED VERSUS STANDARD HEAD SUPPORT
Various types of commercially available head positioning supports are often used in radiation therapy of head and neck patients. Sometimes these are used in conjunction with a restraining device to help maintain the head position during the treatment. Commercially available head supports, which have standard heights and shapes, frequently do not fit the posterior contour of a patient’s head and neck (Fig. 3), making the patient uncomfortable and leaving room for motion even when restraining devices are used.

Fig. 3
The degree o flexion of a patient’s head requires different head supports since variations in the position also changes the posterior contour of the head and neck area. The size of a patient’s chest is an important variable in the needed size and shape of a head support. The midplane of a patient with small shoulders is elevated more above the couch top than in a patient with small shoulders. To maintain a neutral head position, a patient with a large chest therefore requires more head elevation than a patient with a small chest (Fig. 4).

Fig. 4

Prior to introducing this new head immobilization technique, we used a set of standard foam rubber supports in each treatment, CT, and simulation room. Each set consists of several head supports with different height and shape. In our experience, we found that a certain headrest in one set may be different from the matching one in another set. Furthermore, we found that frequent use of these supports over long periods of time tends to cause changes in the shape. We have therefore elected to make customized head supports for all head and neck patients.
The shapes of the first 20 customized head supports were evaluated to find just how much the supports vary in size and shape. Each head support was cut along the sagittal plane and the contour was traced onto a graph paper. The height of the head and of the neck above the couch top was then measured (Fig. 5). The distance between the highest and lowest point on the head support (minimum head and maximum neck elevation) was also measured. Two head supports were excluded from the study because the patients were small children and the measurements were therefore vastly different from that of the adults.

Fig. 5

The height under the head, which is somewhat determined by the size of the patient, varied from 3.2 cm to 6.2 cm. The height under the neck varied from 5.8 cm to 9.8 cm. The difference between the elevation of the head and of the neck (Point A and Point B in Fig. 5) varied from 0.3 cm in a very large patient to 6.6 cm. The average difference between the head and the neck elevation was approximately 3.5 cm. The distance between the highest and lowest point on the head support varied from 5.0 to 14.0 cm with an average of 9.75 cm. The variation of the head and neck patients in a rigid yet comfortable position, a custom support is necessary.

PORT FILM REVIEW
To assess the effectiveness of the head and neck immobilization device using a customized head support add a thermoplastic mask, a retrospective review of port films was made. Port films of each field are routinely taken on the first treatment. If no misalignment is noted, the port films are repeated on a weekly basis. If a misalignment is noted, a correction is made and the port film is repeated during the following treatment. If parallel opposed fields are used, only one port film is taken each week alternative between the fields.
In this review, port films taken of two opposed fields on the same day were recorded as one port film day since it represented only one set up. Only the physicians’ routine review of port films was used to determine the frequency of misalignments. Small discrepancies between alignment systems in the simulator room and the treatment room and varying amount of sag of the two couches may be the cause of small errors in patient alignment observed on the first set of port films.3,7,12,15,17,18 However, in evaluation of the effect on the head immobilization system described in this report, all port films were included in the review.
Port films were reviewed retrospectively of 28 patients treated while immobilized with the customized head support and a thermoplastic mask between 11/92 and 7/93. The findings were compared with that of 25 patients treated between 2/92 and 8/92 using an earlier immobilization method consisting of a standard head support and three strips of casting material across the patient’s face.
The port film review of the group of patients using the customized head support and thermoplastic mask consisted of 207 port film days while the earlier group consisted of 200 port film days.

RESULTS
The rate of field misalignments in the earlier group of patients was 24% (48/200 port film days) while in the more recent group, using a customized head support and a thermoplastic mask, the misalignment rate was 10.1% (21/207 port film days). This represents a significant improvement in set up reproducibility. For both groups, 90% of all misalignments were <5 mm and no adjustment > 10 mm was ever needed in either group.
In addition to a reduction in the rate of misalignments observed on port films, the technologists find it easier and faster to position the patient and to set up the treatment and the patients are more comfortable. The reduction in the number of repeat port films is saving both technologist and physician time as well as cost of film and storage facility.

DISCUSSION
This head immobilization system, where the patient’s head fits tightly between the head support and the mask without room for motion, is comfortable and offers a reproducible head position. At the present time, more than 100 patients have completed therapy using this immobilization technique. No adverse effect from treating through the perforated thermoplastic sheet using 4 MV photons or higher energy has been observed with doses over 7000 cGy.
Since the patient’s head is locked into the position, ease in unlatching the locks in case of an emergency has been considered, however, the need for a quick release has never occurred. The device has been used without difficulty in small children as well as in adults.