A TECHNIQUE FOR EVALUATING ALPHA CRADLE DEVICES USED IN BREAST CANCER RADIOTHERAPY

Kent McCune, RTRT, CMD
Linda Parsons, RTT
Linda Schoenfeld, RTT, CMD
Anne Maddeford, BSRT, RTT

INTRODUCTION
The treatment of primary breast cancer with excision plus radiation is becoming an accepted alternative to mastectomy for localized breast cancer.1,7,13,14,18,19 The use of conservative surgery and radiotherapy is based on studies demonstrating local control and survival comparable to those achieved with mastectomy.18 Minimizing the risk of normal tissue damage while delivering a dose distribution that will result in a high level of local control is the goal of radiation therapy. A three or four field technique is commonly used for radiation therapy of breast cancer when the patient has positive axillary nodes. The breast, chest wall and ipsilateral internal mammary nodes are generally irradiated with two opposing tangential fields. The third field irradiates the supraclavicular and axillary nodes and is angled fifteen degrees from the vertical to avoid the esophagus and spinal cord. The fourth field is 180° opposed to the scf/axilla field and irradiates the axilla only. Treatment techniques for field matching in primary breast irradiation have been described in detail.5,6,16 Matching fields in such a complex treatment requires precise positioning and immobilization of the patient to prevent overlapping treatment fields. Lack of patient immobilization will greatly increase the probability of field overlap. Litcher et al have studied the dosimetry at the matchline between the supraclavicular and tangential fields.11 Roberson et al have studied the amount of lung which is irradiated by various methods of treatment of the internal mammary nodes.15
Doses to the contralateral breast and the influence of lung density corrections on treatment planning for primary breast cancer have been investigated by Fraass et al.9 Bentel has studied port films of patients receiving radiation therapy for carcinoma of the breast utilizing a modified ALPHA CRADLE® brand BF2 positioning and immobilization device.2 This study was based on the review of port films only. Careful review of the treatment portal films is the only way to check the adequacy of the radiation fields to the target volume.4,8 The review of port films does not help to solve the difficulties arising from patient movement during treatment, or the difficulty of accurately reproducing the patient position relative to the beam, from one treatment session to another.8 We feel the review of port films alone in the treatment site is not a reliable indicator of how precisely the ALPHA CRADLE® device is reproducing the patient position. Our goal was to quantitate how accurately the original patient position relative to the ALPHA CRADLE® device was being reproduced. To date we are not aware of any study quantitating the reproducibility of patient position relative to a cast foam immobilization device. The objectives of this present study are to: a) Quantitate the reproducibility of patient position relative to a modified HS2 ALPHA CRADLE® device, b) give us a means to evaluate different ALPHA CRADLE® devices, c) develop guidelines and recommendations on how to most effectively use the ALPHA CRADLE® devices, d) justify to the hospital the extra time and staff required to make the patient immobilization devices.

METHODS AND MATERIALS
History
For primary breast cancer radiation therapy, generally the patient position is supine with head turned slightly to the contralateral side of the effected breast, and ipsilateral arm positioned on an armboard. It became apparent through the review of treatment portal films that the reproducibility of the prescribed fields was not accurate. Reviewed port films revealed that often the arm position was incorrect and tangent fields would not be reproduced to include the entire chestwall. The radiation therapists were asked to make an assessment of the reproducibility of the breast set ups. The information provided by the therapists reveled that the breast set ups were cumbersome, time consuming and patients often had to be repositioned in between fields to reproduce the correct set up. In order to correct these problems and eliminate the need to re-simulate patients, a method of positioning and immobilization using the ALPHA CRADLE® device was chosen.
This institution had no prior experience using ALPHA CRADLE® devices in breast set ups; therefore a technique was developed to evaluate and quantitate how accurately a patient’s position could be reproduced. We utilized the ALPHA CRADLE® MOLD MAKER system, the standard HS2 ALPHA CRADLE®, and our custom modified HS2 ALPHA CRADLE® in this study. The MOLD MAKER system utilizes a bag and chemicals to generate a custom immobilization and repositioning device for the patient. The standard HS2 cradle in contrast, uses a pre-fabricated form that positions both arms up 45°.  We extensively modified a standard HS2 ALPHA CRADLE® due to the preliminary findings of this study. The modified HS2 ALPHA CRADLE® positions both arms up 60°. Positioning both arms up enables: a) a three point set up on the lower thorax, b) provides symmetry to breasts, c) facilitates an accurate CT set up, d) correct treatment position for those patients which return for contralateral breast irradiation. The ALPHA CRADLE® device’s base and sides have been lengthened six inches to provide support, and to correctly position the lower thorax for treatment. The superior end of the form also positions and immobilizes the head to reduce the chance of treating the spinal cord and esophagus on those patients with a supraclavicular field.

ALPHA CRADLE® FORM PREPARATION
The ALPHA CRADLE® devices are prepared as directed by Smithers Medical Products, Inc., i.e. the protective sheeting secured with tape, chemicals shaken mixed, shaken and poured, the patient placed in the form. Before the foaming agents begin to foam, the patient is maneuvered under the simulator gantry and positioned for visual anatomical straightness. Then using fluoroscopy the patient is aligned so that the spinous processes are coincident with the longitudinal axis of the projected central axis of the X-ray beam before the mold hardens. Reference marks are placed on both sides of the form and the patient to indicate the transverse laser location. The patient is now indexed to the form in the longitudinal plane. Next the patient sits up and out of the form so that the side of it can be cut away to visualize the entrance of the lateral tangent field. Then the patient lies back into the form and the patient and form are adjusted so that all reference marks lie in the same transverse plane and are coincident with the transverse lasers of the simulator. Simulation is then completed, localizing the tangent fields along with the nodal areas to the physician’s prescription.
After the fields have been approved, patient positioning marks are added. Our institution has chosen to locate a mark on the anterior midline in a transverse plane that encompasses the inferior border of the tangent field. Marks are also placed on the patient’s right and left, in the same plane at a specified depth. The anterior mark will follow in a sagittal line with the set up point for the tangent fields. A third point is placed at the superior edge of the tangent fields. These positioning marks are used to reproduce the longitudinal axis of the treatment set up. Additionally, the form alignment marks are positioned to lie in the same transverse plane as the anterior set up point of the tangent fields.
With the patient and modified HS2 form marked, the patient sits straight up. Four millimeter lead sphere markers are positioned and embedded in the ALPHA CRADLE® device coincident with the projected central axis of the X-ray beam. One marker at the central axis, another approximately 8-10 centimeters superior, inferior, right and left of the central axis. The markers are then covered with masking tape and the central axis lines drawn on the bottom of the form. The patient is then repositioned in to the ALPHA CRADLE® device and all positioning marks are checked and realigned if necessary.
With the gantry positioned to direct the X-ray beam vertically down through the set up point for the tangent fields, one additional film is exposed. This film will record the anatomical position of the patient in relationship to the ALPHA CRADLE® device and central axis of the X-ray beam. On the first day of treatment, a port film is recorded with the X-ray beam directed vertically down through the set up point for the tangent fields. This film is recorded with a Port Film Marker* in place. The port film marker registers the location of the transverse and longitudinal axis of the X-ray beam on film, and also provides a means to determine the source film distance of the film. An additional film is taken midway through the treatment course.

* Registered by Northwest Medical Physics

RESULTS
Preliminary results using the MOLD MAKER system and the unmodified HS2 ALPHA CRADLES® indicated that several problems were occurring with regard to patient set up reproducibility: a) the form was often skewed, b) the central axis of the X-ray beam was not coincident with the central axis indicated by the lead markers in the form, c) the patient’s position relative to the ALPHA CRADLE® device was not being reproduced in the inferior portion of the field. These problems were corrected by: a) placing thirty-six by two inch strips of Velcro® Loop on each side of the simulator and treatment tables and six inch by two inch wide Velcro® Hook on each corner of the ALPHA CRADLE® device’s base to secure the form to the table, b) instructing the therapist to align the central axis of the X-ray beam to the marks in the ALPHA CRADLE® device before the patient is positioned into the form, c) utilizing the modified HS2 ALPHA CRADLE® to correctly position the lower thorax for treatment. This eliminates the skewing of the form, the noncongruence of the central axis, and allows for patient adjustments without disturbing the form’s alignment to the X-ray beam.
The analysis of eight cases utilizing the modified HS2 ALPHA CRADLE® proceeded as follows: For each reference simulation film a number of spinous processes were not adequately visualized, rib facets were used as the reference bony landmarks. These landmarks were selected for their visibility and for their stability. The same landmarks were then identified on the treatment machine portal films. Measurements of the distance between the reference landmarks and longitudinal axis were done on both the simulation and treatment portal films. Alignment was judged on the position of the spinous process bony landmarks on the treatment machine portal film relative to the reference simulation film. Table 1 gives a summary of those measurements. Column 1 is the number of reference landmarks used in each case. Column 2 is the mean distance to the landmarks as measure on a line perpendicular to the reference longitudinal axis. Column 3, the mean set up error, is a measure of the difference between the reference landmark
location on the portal film relative to the simulation film.

DISCUSSION
Radiation therapy seeks maximization of the probability of local tumor control and minimization of the risk of significant complications. As the ability to accurately define the target volume improves, tighter margins are placed on the target volume to help decrease morbidity associated with treatment. The margins selected between the treatment volume and the target volume must reflect the uncertainty in the definition of the tumor volume, the possibility of patient motion, the proximity of radio-sensitive structures and beam penumbra. Accurate field placement is essential if maximum tumor response is to be achieved with minimal morbidity.8,17 The importance of immobilization for the accurate delivery of radiation to a target volume is well documented in the literature.10,12,17 To reproduce the patient position and to minimize patient movement during treatment the immobilization device must be supportive and comfortable. The therapists are instructed to align the longitudinal and transverse planes with the laser positioning lights to help ensure the reproducibility of the position. The ALPHA CRADLE® device will aid in positioning and immobilizing the patient, but it is still up to the radiation therapists to reproduce the correct patient position on a daily basis. Feedback from the therapists as well as personal observation of daily set ups confirms that minimal adjustment of the patient position is required once the patient lies back into the modified HS2 form. The therapist also states that there is no patient movement observed during treatment. As a result, patient treatments are being delivered with a greater degree of accuracy, the ease of set ups has improved, and the set up times on the machine have been reduced.
A diagnostic quality reference simulation film imaging the vertebral column and spinous process is critical to the success of this technique. The major drawback with this procedure is the poor definition and visualization of the reference anatomical landmarks on the treatment portal film. An imaging enhancement system would be very beneficial to this technique. To maximize imaging of the anatomical references on the treatment portal we use 3M XL* simulator film in lead screened cassettes. In addition, the therapists are instructed to position the collimator settings to that of the tangent fields for the first exposure, and full field for the second exposure. To fully realize the effectiveness of an ALPHA CRADLE® device, we recommend the following guidelines be adhered to: a) utilization of Velcro® or some other device to secure the ALPHA CRADLE® device to the simulation and treatment table, b) mark the projected simulator central axis cross hair location in the ALPHA CRADLE® device, c) align the ALPHA CRADLE® device to the treatment machine before patient lies back into the form, d) not manipulate form to align patient to treatment machine, position patient within the form coincident with the X-ray beam axes. Using these guidelines and the 4mm lead marker technique, we were able to evaluate the effectiveness of the ALPHA CRADLE® device and quantitate how accurately a patient’s position could be reproduced. On the average we were able to reproduce the original patient position to within 0.16 cm. Localizing the original reference longitudinal axis with 0.16 cm does not guarantee that the treatment fields will be reproduced within that same magnitude. To determine the adequacy of the radiation fields to the target volumes requires careful review of the treatment portal films. We feel this technique when used in conjunction with the careful analysis of the treatment portal films is very useful in determining the accuracy of patient repositioning, the effectiveness of the ALPHA CRADLE® device, and precise field placement.

*Registered by 3M Corporation

We recommend that other institutions use this technique and guidelines to evaluate their form’s performance due to the variation in the construction and use of ALPHA CRADLE® devices. This is an excellent quality assurance procedure that can be easily implemented as deemed necessary to ensure the effective use of an ALPHA CRADLE® device.