Radiation Physics

PV QA 3 - Poster Viewing Q&A 3

TU_4_3153 - Usability Evaluation of a New Elastic Immobilization with Metal Markers for Image-Guided Radiation Therapy for Breast Cancer

Tuesday, October 23
1:00 PM - 2:30 PM
Location: Innovation Hub, Exhibit Hall 3

Usability Evaluation of a New Elastic Immobilization with Metal Markers for Image-Guided Radiation Therapy for Breast Cancer
T. Fujimoto1, M. Yoshimura2, M. Shioji1, Y. Ono2, T. Ono2, H. Asada3, Y. Ishihara2, Y. Miyamae1, S. Yano1, M. Nakata1, K. Ueda1, and T. Mizowaki2; 1Division of Clinical Radiology Service, Kyoto University Hospital, Kyoto, Japan, 2Department of Radiation Oncology and Image-applied Therapy, Kyoto University Graduate School of Medicine, Kyoto, Japan, 3Kyoto University Hospital Nursing Administration, Kyoto, Japan

Usability evaluation of a new elastic immobilization with metal markers for image-guided radiotherapy for breast cancer

Purpose/Objective(s): We developed an elastic immobilization based on a commercially available brassier for postoperative radiotherapy for breast cancer. The purpose of this study was to validate the positioning accuracy and the surface doses of image-guided radiotherapy (IGRT) for breast cancer using the immobilization.

Materials/Methods: The elastic immobilization had six small metal markers. We developed the human phantom with a tungsten target in the breast, which had an equivalent form and flexibility to soft tissues of the human body. First, the positioning accuracy of IGRT was evaluated using onboard imaging devices. The immobilization was fixed on the human phantom; the landmarks were drawn on the phantom to indicate positional relationship between the immobilization and body. The setup of the phantom was performed five times, and then several x-ray images were produced. The position verification was performed using the metal markers of the immobilization. The margin of a breast target was estimated using the expanded uncertainty calculated from the displacement between simulated and measured the tungsten target positions. The expanded uncertainty value was obtained by multiplying the combined standard uncertainty value by a coverage factor (k = 2; confidence interval, 95%). Next, the surface doses were measured using the thermoluminescent dosimeters with kV planar imaging at 80 kVp / 100 kVp, 5 mAs, and cone beam computed tomography (CBCT) at 120 kVp, 180 mAs. The measured doses with and without the immobilization were compared. Finally, they were compared with the calculated dose using the EGSnrc Monte Carlo code under the same conditions.

Results: The displacement of the tungsten target was 2.31 ± 0.53 mm, -1.21 ± 1.90 mm and -0.11 ± 0.67 mm in the lateral, longitudinal and vertical directions, respectively. The three-dimensional translation vector of variation was 3.28 ± 0.59 mm. The accuracy of IGRT with the elastic immobilization was 3.30 ± 0.53 mm (k = 2). The measured surface doses with and without the immobilization were 0.53 ± 0.28 mGy and 0.97 ± 0.19 mGy in kV planar image pair, 5.50 ± 1.40 mGy and 5.90 ± 1.45 mGy in CBCT, respectively. Similarly, the calculated dose was decreased by using immobilization in CBCT.

Conclusion: This study showed a new elastic immobilization with the metal markers could provide positioning accuracy of IGRT for breast cancer with minimum skin markers. Furthermore, the immobilization decreased the surface doses of IGRT.
Table. Uncertainty budget of tungsten target position in IGRT with elastic immobilization (mm).
Source of uncertainty

Value (±)

Probability distribution

Divided factor

Standard uncertainty

u1: x-ray correction

0.59

normal

1

0.26

u2: imager position

0.12

normal

1

0.01

uc: combined

normal

0.27

U: expanded (k = 2)

normal

0.53

Author Disclosure: T. Fujimoto: None. M. Yoshimura: None. M. Shioji: None. Y. Ono: None. H. Asada: None. Y. Miyamae: None. S. Yano: None.

Send Email for Takahiro Fujimoto


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