Evaluation of Tissue Inhomogeneity for Gamma-knife Radiosurgery Using Film Dosimetry / 대한방사선종양학회지

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.