Angular dependence of dose sensitivity of nanoDot optically stimulated luminescent dosimeters in different radiation geometries.

ABSTRACT

PURPOSE: A type of in vivo dosimeter, an optically stimulated luminescent dosimeter, OSLD, may have dose sensitivity that depends on the angle of incidence of radiation. This work measures how angular dependence of a nanoDot changes with the geometry of the phantom in which irradiation occurs and with the intrinsic structure of the nanoDot. METHODS: The OSLDs used in this work were nanoDot dosimeters (Landauer, Inc., Glenwood, IL), which were read with a MicroStar reader (Landauer, Inc., Glenwood, IL). Dose to the OSLDs was delivered by 6 MV x-rays. NanoDots with various intrinsic sensitivities were irradiated in numerous phantoms that had geometric shapes of cylinders, rectangles, and a cube. RESULTS: No angular dependence was seen in cylindrical phantoms, cubic phantoms, or rectangular phantoms with a thickness to width ratio of 0.3 or 1.5. An angular dependence of 1% was observed in rectangular phantoms with a thickness to width of 0.433-0.633. A group of nanoDots had sensitive layers with mass density of 2.42-2.58 g/cm(3) and relative sensitivity of 0.92-1.09 and no difference in their angular dependence. Within experimental uncertainty, nanoDot measurements agree with a parallel-plate ion chamber at a depth of maximum dose. CONCLUSIONS: When irradiated in cylindrical, rectangular, and cubic phantoms, nanoDots show a maximum angular dependence of 1% or less at an incidence angle of 90°. For a sample of 78 new nanoDots, the range of their relative intrinsic sensitivity is 0.92-1.09. For a sample of ten nanoDots, on average, the mass in the sensitive layer is 73.1% Al2O3C and 26.9% polyester. The mass density of the sensitive layer of a nanoDot disc is between 2.42 and 2.58 g/cm(3). The angular dependence is not related to Al2O3C loading of the nanoDot disc. The nanoDot at the depth of maximum dose has no more angular dependence than a parallel-plate ion chamber.