IMMOBILIZATION DEVICE FOR PATIENTS UNDERGOING PROSTATE IRRADIATION

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

INTRODUCTION
Bladder and bowel toxicity associated with prostate irradiation is well documented and methods to reduce the frequency and severity of these toxicities have been studied. Conformal therapy techniques, where treatment fields are designed to conform to the target while limiting the volume of bladder and rectum in the beam’s-eye-view, has shown to reduce the morbidity in prostate irradiation.3,4,10,13,14 Prostate irradiation using small target margins, however, should not be undertaken without effective patient immobilization. 1,12
In early 1992, a three-dimensional treatment planning system (Virtual Simulator*)11 was phased in to clinical use in our department. The tumor margins in patients with prostate carcinoma undergoing three-dimensional treatment planning were frequently smaller and we therefore began using ALPHA CRADLE®** brand Patient Repositioning Systems for these patients in an attempt to improve set up accuracy.

IMMOBILIZATION DEVICE
The extended prostate ALPHA CRADLE® form, PFM, pelvic form modifed, which extends from the mid-chest to below the feet, consists of a Styrofoam®# form which is filled with foaming agents that expand and form a mold around the patient. The base of the extended prostate ALPHA CRADLE® form is a Styrofoam® sheet approximately ¾” thick, 6’ long and 2’ wide. With the patient on the base, several pieces of Styrofoam® are added to make the mold comfortable and sturdy (Fig. 1). The patient’s knees are bent to relax the lower back and a triangular-shaped piece of Styrofoam® is placed under each knee to support the position. A piece of Styrofoam® is added on each side and below the feet as close to the patient as possible. With the patient removed, the excess width of the base plate is cut away and the entire Styrofoam® form is placed inside a polyvinyl bag. This bag must be considerably wider and longer than the base plate to allow space for the foaming agents to expand up along the patient’s sides and between the legs and feet. Approximately 660 to 975cc of foaming agents are prepared as directed by the manufacturer** and are evenly distributed inside the form. Air is allowed to circulate inside the bag until bubbles are noted in the foam (1-2 minutes). The air is then forced out, the bag is sealed, and the patient is returned to the form. The patient enters the form from the head of the couch and gradually moves into the form until the knees are in proper position on the knee supports. The patient must adjust the chest, hips, and legs until he feels straight and the staff make a visual check that he appears straight. All clothing under and on the sides of the patient is removed allowing the mold to form to the skin surface. The excess width of the bag of pulled in toward the space between the patient’s torso and the Styrofoam® sides as well as between the patient’s legs allowing the foam to expand and form a tight mold to the patient’s torso and each side of both legs (Fig. 2).

* Sherouse Systems, Inc.
** Registered by Smithers Medical Products, Inc.
# Registered by Dow Chemical

Fig. 1 Stryrofoam® barriers are taped to three sides to prevent foam from leaking. A triangular Styrofoam support for each knee is also taped in place before placing the entire form in the polyvinyl bag.

Occasionally, form may leak and form a lip over the couch, causing an irregular surface under the extended prostate ALPHA CRADLE® form. To avoid having an irregular surface under the finished mold, the ALPHA CRADLE® form is built on a hardboard that is wider than the base of the mold. The process of building this extended prostate ALPHA CRADLE® form requires approximately 45 minutes.

ALIGNMENT MARKS AND TREATMENT
Extended, prostate ALPHA CRADLE® forms, PFMs, are made for all prostate patients undergoing three-dimensional treatment planning in our department. These extended prostate ALPHA CRADLE® forms are therefore made in the computed tomography room immediately prior to obtaining the CT based data set. Reference marks are made on the patient and on the ALPHA CRADLE® form to establish the coordinates for the Virtual Simulation. Laser alignment systems are used in each room to help reposition the patient. Alignment marks are made both on the ALPHA CRADLE® form and the

Fig. 2 The Alpha Cradle® forms a tight mold for the legs and the pelvis. The foam is forced up on each side of the patient’s legs, hips and chest. Repositioning marks are made on the Alpha Cradle®.

patient’s skin, however, since skin tends to move with respect to deeper tissues, the marks on the rigid ALPHA CRADLE® form are considered more reliable, in the case of a discrepancy. Motion of skin marks with respect to deeper tissues is more pronounced in areas where there is a thick layer of subcutaneous soft tissues. Tightening or relaxing a muscle also causes the skin to move with respect to deeper tissues, and in some situations, the skin is pulled in one of the other direction as the patient lies down. Another problem with skin marks is that they tend to fade and are therefore reinforced several times during a course of treatment. It is possible that each time the marks are reinforced, they are placed a little to one side, and over a period of time they could migrate, giving rise to a field shift. The problems with migrating marks are avoided when the marks are made on the ALPHA CRADLE® form. When the patient is repositioned correctly in the ALPHA CRADLE® form, the marks on the ALPHA CRADLE® form are fixed with respect to the patient, and since the marks on the ALPHA CRADLE® form do not fade or wear off, eliminating re-marking, they do not migrate.
The sagittal alignment line is marked on the skin of the pelvis and on the mold from between the patient’s thighs to below the feet (Fig. 3). A lateral (horizontal) alignment line is marked on each side of the ALPHA CRADLE® form. One transverse line is marked on the pelvic skin an extending onto the ALPHA CRADLE® form. A second transverse line is marked on the skin just below the knees where the skin moves very little compared with that in the pelvic area. The second line, which also is marked on the ALPHA CRADLE® form, is used to assure that the patient is repositioned correctly in the ALPHA CRADLE® form in the cepahlad/caudal direction.

Fig. 3 Set up marks are made on the Alpha Cradle® form and on the patient. The transverse line across the calf of the legs (arrow) are used to assure that the patient is positioned correctly in the cephalad-caudal direction.

The majority of patients are treated using a 4-field box technique. The position of the isocenter with respect to the coordinate system is marked on the ALPHA CRADLE® form and on the patient, now in a different color to distinguish between the original marks and the actual treatment marks. All four fields are treated daily 5 days a week.

PORT FILM REVIEW
A port film is obtained of all fields on the first treatment day to verify both the shape and the position of the field. Following approval of these films, weekly port films are taken of one lateral field and one anterior or posterior field alternating between these fields on a weekly basis so that over a two-week period, all fields have had a port film. If a misalignment is noted on any film, a correction is requested by the monitoring physician and the port film is repeated to verify the correction. In the retrospective review of port films to assess the effectiveness of the ALPHA CRADLE® form described in this report, the port films taken following a requested field shift were excluded. It was felt that such films would not truly represent the ability to reproduce the set up over a period of several weeks since requested port films were taken any one day following the routine weekly film. It was also felt that the technologists would try extra hard to set up the fields correctly since they had been asked to repeat the port film. It is well recognized by many authors, that small differences between laser alignment systems, couches, etc. in CT, simulation, and treatment room may be present and give rise to misalignments observed on the first set of port films.2,5-9 This problem was recognized in this port film review and the initial port films were therefore not included. The retrospective review of port films therefore included only the weekly routine port films.
To assess the effectiveness of the prostate ALPHA CRADLE® form described in this report, the routine weekly port films of 44 patients treated with the ALPHA CRADLE® form were reviewed for misalignments observed by the monitoring physician during the treatment course. The findings were compared with that of 30 patients undergoing prostate irradiation without any immobilization device and without having a 3-D treatment plan. All patients in both groups were treated during the same period of time and whether they had an ALPHA CRADLE® form and 3-D planning was the physician’s choice. Neither the technologists treating the patients nor the physicians evaluating the port films during the course of the treatment were aware that the port films would be reviewed later.
The port film review of the immobilized patients included 213 routine isocenter set-ups while in the non-immobilized group there were 130 routine isocenter set-ups. When ort films of more than one field were taken on the same day, it was counted as only one isocenter set up since it represented the same set up.

RESULTS
In the non-immobilized group of patients there was a misalignment rate of 23.1% versus 17.4% in the immobilized group. The most significant improvement in reproducibility when the immobilization device was used, was noted in the anterior – posterior direction where the misalignment rate was 12.6% without immobilization and 5.1 with the extended pelvic ALPHA CRADLE® form. In the cephalad-caudal direction the misalignment rate was approximately 5% in both groups. In the right-left direction the misalignment rate was reduced from 18.0% without immobilization to 12.2% with immobilization.
For both groups, approximately 75% of the misalignments were <5mm and 25% were <10mm. Only one requested isocenter shift was >10mm and this was a non-immobilized patient.

DISCUSSION
The reason for the low number of errors in the cephalad-caudal direction is thought to be due to the fact that both anterior and lateral marks could be used when setting up the isocenter. In the right-left direction, the misalignment rate would likely have been lower with the ALPHA CRADLE® form if the technologists had exclusively used the sagittal line on the ALPHA CRADLE® form to set up the isocenter as instructed. The technologists have been setting up treatment fields to skin marks for many years and many of these patients had been treated before all of the technologists recognized that using the marks on the ALPHA CRADLE® form lead to better reproducibility.
Reproducibility of treatment field placement is fundamental to successful radiation therapy, but in treatment of prostate malignancies, reducing inadvertent irradiation of bladder and rectum may reduce acute and chronic injuries to these organs. The ability to reproduce the set up of prostate treatment fields accurately is critical, particularly when dose escalation is contemplated.