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The Journal of the Korean Society for Therapeutic Radiology and Oncology 1998;16(3): 325-335. |
Evaluation of Tissue Inhomogeneity for Gamma-knife Radiosurgery Using Film Dosimetry |
Heung Lae Cho, Seung Chang Sohn, Hyun Suk Suh |
1Department of Radiation Oncology, Inje University, College of Medicine, Pusan Paik Hospital, Pusan, Korea. 2Department of Radiation Oncology, Seoul Paik Hospotal, Pusan, Korea. |
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ABSTRACT |
PURPOSE: Since the mid cranial fossa is composed of various thickness of bone, the tissue inhomogeneity caused by bone would produce dose attenuation in cobalt-60 gamma knife irradiation. The correction factor for bone attenuation of cobalt-60 which is used for gamma knife source is -3.5%.
More importantly, nearly all the radiosurgery treatment planning systems assume a treatment volume of unit density; any perturbation due to tissue inhomogeneity is neglected.
This study was performed to confirm the bone attenuation in mid cranial fossa using gamma knife.
MATERIALS AND METHODS: Computed tomography was performed after Leksell stereotactic frame had been fixed to the Alderson Rando phantom (human phantom) skull area. Kodak X-omat V film was inserted into two sites of pituitary adenoma point and acoustic neurinoma point, and irradiated by gamma knife with 14mm and 18mm collimator. An automatic scanning densitometer with a 1mm aperture is used to measure the dose profile along the x and y axis.
RESULTS: Isodose curve constriction in mid cranial fossa is observed with various ranges. Pituitary tumor point is greater than acoustic neurinoma point (0.2-3.0 mm vs.
0.1-1.3 mm) and generally 14 mm collimator is greater than 18mm collimator (0.4-3.0 mm vs. 0.2-2.2 mm). Even though the isodose constriction is found, constriction of 50% isodose curve which is used for treatment reference line does not exceed 1 mm. This range is too small to influence the treatment planning and treatment results.
CONCLUSION: Radiosurgery planning system of gamma knife does not show significant error to be corrected without consideration of bone attenuation. |
Key Words:
Gamma knife, Tissue heterogeneity, Film dosimetry |
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