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  • A retrospective study comparing the reproducibility of the WingSTEP breast board to the conventional MT-350

    breast board in tangential breast irradiationbreast board in tangential breast irradiationJensen, Christer; Fallmyr, Ida; and Skottner, Nils

    Cancer Department, lesund hospital, NorwayChrister.Jensen@helse-sunnmore.no

    IntroductionAt our clinic, the conventional MT-350 breast board from Civco [1] has been the standard immobilization device in use for breastcancer patients since 2003. The patients are daily positioned with individual settings before treatment is given.

    The reproducibility of patient positioning using the conventional breast board is challenging due to the numerous individual settings these boards offer. The WingSTEP is a facile breast board from IT-V [2], where only the head cushion offers individual settings. The aim of this study was to assess the precision and stability of the two immobilization devices, and determine whether the WingSTEP has as good, or better, precision and stability as the conventional MT-350 breast board from Civco. A secondary aim was to investigate the adequate CTV to PTV margin for both breast boards. An incentive was also that the WingSTEP costs around 30% of a conventional breast board.

    Materials & Methods

    Introduction

    Materials & MethodsA total of 62 patients were retrospectively included in the study. Of these, 31 were positioned in the conventional MT-350 breast board from Civco and, 31 were positioned in the more facile breast board called WingSTEP from IT-V.

    Patients were scanned in the supine position using a 16 slices Philips MX 8000 IDT Computer Tomograph (CT), the scans were reconstructed with 2 mm slice thickness. Knee supports were used for additional immobilization.

    The clinical target volume (CTV) was the palpable breast volume. Before CT-scanning a copper wire was placed marking the circumference of the breast. This was further used as a help for defining the CTV. The national guidelines define the dorsal limit as the fascia over the thoracic muscles and the ventral limit as 5 mm under the surface of the skin [3]. There was no planning target volume (PTV) delineated, but this volume is taken into account according to CTV to field edge margins. The clinics standard procedure dictates 15 mm margins from the CTV to the field edges in the cranial and caudal directions, and 10 mm towards the lung. There should also be an adequate anterior field flash of more than 20 mm above the breast.

    Figure 1: Baseline shift during treatment session. Distance measured with a Dimetix FLS-C10 laser distance sensor pointing at the isocenter.

    Margins needed to account for interfraction translations were calculated for each breast board, and are shown in Table 2. There are no significant differences in the margins needed to account for interfraction motion for the two breast boards.

    One has to keep in mind that these margins are calculated from portal images early in the treatment session, and does not account towards the lung. There should also be an adequate anterior field flash of more than 20 mm above the breast.

    All patients were planned in supine position to a mean dose of 50 Gy in 25 fractions. Patients were treated using an isocentric technique with medial and lateral symmetric tangential fields. Patients were treated with 6 MV photons.

    The cancer department of the lesund hospital operates two Elekta linear accelerators; one with a Synergy and one with a Preciseplatform. The Synergy platform is equipped with a kilo voltage cone-beam computed tomography (CBCT) system, XVI [4]. All patients performed a cone beam scan on the Synergy treatment machine prior to the first treatment. Electronic portal images (EPIDs) were taken during the 3 first fractions and thereafter once a week, and a total of 1252 images where analyzed.

    Translations in the cranio-caudal (CC) and anterior-posterior (AP) direction between the beams eye view digitally reconstructedradiograph (DRR) and the EPID image were used for calculation of intra-fraction motion and margins. In addition rotation around the left-right-axis (LR) between the dose plan CT and the EPID image was recorded. All EPID images were matched by one radiation therapy technologist (RTT). Seven randomly chosen patients were independently checked by another RTT; differences were negligible.

    One has to keep in mind that these margins are calculated from portal images early in the treatment session, and does not account for baseline shifts due to patient relaxation throughout the treatment session as seen for some patients. For loco regional and IMRT treatment the patients daily treatment time is longer, and intrafraction movement and baseline shifts have the potential to increase dislocation during the session.

    The margins are calculated from matching bony structures in the chest wall, which is a good estimate for the CTV localization. The margins do not include rotational errors or shape deviations, and are considered as a lower limit.

    90%/95% 99%/99%

    AP [mm] CC [mm] AP [mm] CC [mm]

    MT-350 9.23 5.21 12.93 7.28

    WingSTEP 9.67 5.06 13.54 7.07were negligible.

    Since the imaged fields are oblique, and making the assumption that most of the motion is in the AP-direction, less in the LR-direction, a correction to the measured displacement was calculated. Dividing the measured displacement from the EPIDs by thesinus of the gantry angle deviation with respect to the vertical will yield the correct value.

    Patient setup deviations can have a random component related to patient motion, and a systematic component due to the equipment or protocol.

    The setup variability of the bony anatomy, in most cases the thoracic wall, was calculated. The setup variability of the thoracic wall is a good estimate of the CTV-variability in most cases, but breast tissue is non-rigid and can deform and move with respect to the thoracic wall. We calculated the group mean (m), systematic error (), and random error () using the framework of vanHerk et al. [5].

    Table 2: The margins needed to account for interfraction translations in the AP- and CC-directions. All values in mm

    We can also calculate a PTV margin taking into account the intrafraction displacement in the AP-direction found from the internal data from 10 patients. Linearly adding the mean intrafraction displacement of 1.28 mm yields the margin in the AP-direction of 10.95 mm for the WingSTEP breast board to ensure a minimum dose to the CTV of 95% for 90% of the patients.

    All patients had a CBCT-scan before the first treatment fraction to verify immobilization and field setup. Rotations around the LR-axis in the CBCT-scan, the axial plane, were divided into 0.5 intervals as shown in Figure 2. There are no significant differences between the two breast boards, but there is a small trend that the MT-350 breast board, possibly due to the inclination, has somewhat bigger rotations.

    )sin( egantryanglAPAP EPIDntdisplaceme =

    Herk et al. [5].

    The van Herk et al. margin recipe cover the CTV for 90% of the patients with the 95% isodose for a 2D-situation:

    Margin = 2.15 + 0.7,

    where is the total standard deviation (SD) of the patient means; it reflects the preparation (systematic) uncertainty. is the SD of the execution (random) errors.

    In addition margins were calculated with the requirement of a minimum dose to the CTV of 99% for 99% of the patients. This requirement yields the margin recipe

    Margin = 3.04 + 0.95

    Conclusions

    Figure 2: CBCT-scan rotations around the LR-axis. Each interval is +/- 0.25 degrees.

    There is no group mean difference in the AP direction between the two breast boards.

    ResultsThe WingSTEP breast board offers minimal individual settings, but all patients included were able to perform the immobilization and treatment without problems.

    The results of the EPID setup verification are shown in Table 1. There are close to no systematic group mean in the AP-directionfor the two breast boards. This also yields for the CC-direction for the WingSTEP board, but there is a small systematic group mean towards the caudal-direction for the MT-350 board. This is probably due to a standard 5 inclination for this breast board. The reason for introducing this inclination years ago was mainly patient comfort. Another reason was also that it was thought that the breast would fall more naturally.

    MT-350 31 patients There is no group mean difference in the AP direction between the two breast boards.

    The MT-350 breast board has a small CC group mean displacement possibly due to the board inclination and patients slipping down caudally.

    There are no significant CTV-PTV margin differences between the two breast boards.

    Rotations around the LR-axis from CBCT-scan performed at first treatment session show no statistical differences.

    There are major baseline shifts for some patients possibly due to the patient being tense during setup, but relaxes during the treatment session. Intrafraction displacement is not as significant as interfraction displacement.

    The radiation therapy technologists prefer the WingSTEP breast board due to its lower weight and easier set-up.

    MT-350 31 patientsAP [mm] CC [mm] Rot []

    m 0.42 -1.50 0.82

    2.31 1.69 1.02

    2.23 2.25 0.67

    WingSTEP 31 patientsAP [mm] CC [mm] Rot []

    m 0.29 0.19 0.15

    2.44 1.67 0.82

    References[1] Civco Medical Solutions http://www.civco.com

    [2] IT-V Medizintechnik GMBH http://www.it-v.net

    [3] Norwegian Breast Cancer Group NBCG http://www.nbcg.no

    [4] Jaffray, D. A., J. H. Siewerdsen, J. W. Wong, and A. A. Martinez. Flat-panel cone-beam computed tomography for image-guided radiation therapy. Int. J. Radiation On