PHOTON-ABSORBED FRACTIONS FOR CYLINDRICAL GEOMETRY - A TLD PHOTON-ABSORBED FRACTION MODEL

The purpose of this study is to develop a mathematical model and calcu late photon-absorbed fractions in a homogeneous nonradioactive cylinde r placed inside off-center and outside a cylindrical homogeneous distr ibution of activity. In the second case, both the radioactive cylinder and the nonradioactive one are placed in a tissue-equivalent nonradio active medium. The values of the photon-absorbed fractions are investi gated for various geometrical configurations using water as the materi al filling the cylinders and the medium in between and an isotope comm only used in Nuclear Medicine, Tc-99m. The calculations for off-center cylinders allows for modeling inhomogeneous distributions of activity within a tumor by placing several ''cold'' cylinders of various sizes in a radioactive finite cylinder. This three-dimensional model calcul ates photon-absorbed fractions for inhomogeneous activity distribution s that can be used in quantitative nuclear medicine for self-absorptio n correction, thus introducing a more realistic correction than the on e-dimensional corrections. These calculations are also used to model t he response of a cylindrical TLD (thermoluminiscent dosimeter) placed inside a homogeneous radioactive cylinder and outside the homogeneous radioactive cylinder, in an absorbing nonradioactive surrounding mediu m. The purpose of these calculations is to evaluate the photon-absorbe d fraction in the TLD as an instrument of measuring the time-integrate d activity of a homogeneous radioactive source versus an inhomogeneous one. The dependence of the TLD-absorbed fraction on the position of t he TLD with respect to the radioactive cylinder is investigated. (C) 1 996 American Association of Physicists in Medicine.