J Korean Soc Ther Radiol Oncol.  1998 Sep;16(3):325-335.

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

• Cho HL
• Sohn SC
• Suh HS

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.

Keyword

Gamma knife; Tissue heterogeneity; Film dosimetry

MeSH Terms

Axis, Cervical Vertebra
Constriction
Film Dosimetry*
Neuroma, Acoustic
Pituitary Neoplasms
Radiosurgery*
Skull