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J Korean Soc Ther Radiol Oncol > Volume 25(4); 2007 > Article
The Journal of the Korean Society for Therapeutic Radiology and Oncology 2007;25(4): 268-277.
Distortion of the Dose Profile in a Three-dimensional Moving Phantom to Simulate Tumor Motion during Image-guided Radiosurgery
Mi Sook Kim, Seonghwan Ha, Dong Han Lee, Young Hoon Ji, Seong Yul Yoo, Chul Koo Cho, Kwang Mo Yang, Hyung Jun Yoo, Young Seok Seo, Chan Il Park, Il Han Kim, Seong Jun Ye, Jae Hong Park, Kum Bae Kim
1Department of Radiation Oncology, Korea Institute of Radiological and Medical Sciences, Korea. mskim@kcch.re.kr
2CyberKnife Center, Korea Institute of Radiological and Medical Sciences, Korea.
3Department of Radiation Oncology, Seoul National University College of Medicine, Seoul, Korea.
Respiratory motion is a considerable inhibiting factor for precise treatment with stereotactic radiosurgery using the CyberKnife (CK). In this study, we developed a moving phantom to simulate three-dimensional breathing movement and investigated the distortion of dose profiles between the use of a moving phantom and a static phantom.
The phantom consisted of four pieces of polyethylene; two sheets of Gafchromic film were inserted for dosimetry. Treatment was planned to deliver 30 Gy to virtual tumors of 20, 30, 40, and 50 mm diameters using 104 beams and a single center mode. A specially designed robot produced three-dimensional motion in the right-left, anterior-posterior, and craniocaudal directions of 5, 10 and 20 mm, respectively. Using the optical density of the films as a function of dose, the dose profiles of both static and moving phantoms were measured.
The prescribed isodose to cover the virtual tumors on the static phantom were 80% for 20 mm, 84% for 30 mm, 83% for 40 mm and 80% for 50 mm tumors. However, to compensate for the respiratory motion, the minimum isodose levels to cover the moving target were 70% for the 30~50 mm diameter tumors and 60% for a 20 mm tumor. For the 20 mm tumor, the gaps between the isodose curves for the static and moving phantoms were 3.2, 3.3, 3.5 and 1.1 mm for the cranial, caudal, right, and left direction, respectively. In the case of the 30 mm tumor, the gaps were 3.9, 4.2, 2.8, 0 mm, respectively. In the case of the 40 mm tumor, the gaps were 4.0, 4.8, 1.1, and 0 mm, respectively. In the case of the 50 mm diameter tumor, the gaps were 3.9, 3.9, 0 and 0 mm, respectively.
For a tumor of a 20 mm diameter, the 80% isodose curve can be planned to cover the tumor; a 60% isodose curve will have to be chosen due to the tumor motion. The gap between these 80% and 60% curves is 5 mm. In tumors with diameters of 30, 40 and 50 mm, the whole tumor will be covered if an isodose curve of about 70% is selected, equivalent of placing a respiratory margin of below 5 mm. It was confirmed that during CK treatment for a moving tumor, the range of distortion produced by motion was less than the range of motion itself.
Key Words: Moving phantom, Isodose, Radiosurgery, CyberKnife
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