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J Korean Soc Ther Radiol Oncol > Volume 23(4); 2005 > Article
The Journal of the Korean Society for Therapeutic Radiology and Oncology 2005;23(4): 230-235.
Contralateral Breast Dose Reduction Using a Virtual Wedge
Inhwan Yeo, Dae Yong Kim, Tae Hyun Kim, Kyung Hwan Shin, Eui Kyu Chie, Won Park, Do Hoon Lim, Seung Jae Huh, Yong Chan Ahn
1Department of Radiation Oncology, Cooper University Hospital, UMDNJ-Robert Wood Johnson Medical School, Camden, USA.
2Research Institute and Hospital, National Cancer Center, Goyang, Korea. radiopiakim@hanmail.net
3Department of Radiation Oncology, Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul, Korea.
To evaluate the contralateral breast dose using a virtual wedge compared with that using a physical wedge and an open beam in a Siemens linear accelerator.
The contralateral breast dose was measured using diodes placed on a humanoid phantom. Diodes were placed at 5.5 cm (position 1), 9.5 cm (position 2), and 14 cm (position 3) along the medial-lateral line from the medial edge of the treatment field. A 6-MV photon beam was used with tangential irradiation technique at 50 and 230 degrees of gantry angle. Asymmetrically collimated 17 x 10 cm field was used. For the first set of experiment, four treatment set-ups were used, which were an open medial beam with a 30-degree wedged lateral beam (physical and virtual wedges, respectively) and a 15-degree wedged medial beam with a 15-degree wedged lateral beam (physical and virtual wedges, respectively). The second set of experiment consists of setting with medial beam without wedge, a 15-degree wedge, and a 60-degree wedge (physical and virtual wedges, respectively). Identical monitor units were delivered. Each set of experiment was repeated for three times.
In the first set of experiment, the contralateral breast dose was the highest at the position 1 and decreased in order of the position 2 and 3. The contralateral breast dose was reduced with open beam on the medial side (2.70+/-1.46%) compared to medial beam with a wedge (both physical and virtual) (3.25+/-1.59%). The differences were larger with a physical wedge (0.99+/-0.18%) than a virtual wedge (0.10+/-0.01%) at all positions. The use of a virtual wedge reduced the contralateral breast dose by 0.12% to 1.20% of the prescribed dose compared to a physical wedge with same technique. In the second experiment, the contralateral breast dose decreased in order of the open beam, the virtual wedge, and the physical wedge at the position 1, and it decreased in order of a physical wedge, an open beam, and a virtual wedge at the position 2 and 3.
The virtual wedge equipped in a Siemens linear accelerator was found to be useful in reducing dose to the contralateral breast. Our additional finding was that the surface dose distribution from the Siemens accelerator was different from a Varian accelerator.
Key Words: Virtual wedge, Contralateral breast, Tangential irradiation
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