SPECIFIC ABSORBED FRACTIONS FOR PHOTON POINT SOURCES WITHIN A SCATTERING MEDIUM

Dose distributions from point gamma -ray sources within bound and unbound tissue-like absorbers were obtained by means of Monte Carlo calculations. Results are presented for source energies ranging from 30 to 2750 keV. These results are compared with approximate dose distributions utilizing an 'effective' absorption coefficient and with moment method calculations of the dose distribution within an infinite water phantom. Boundary effects were evaluated by comparing the dose in spheres of radii 10, 20, 30 and 40 cm with the dose in an unbounded absorber. In some cases, the introduction of a non-reflecting boundary reduces the dose at the boundary by nearly a factor of two. However, boundary effects are shown to be unimportant at distances greater than one mean free path from a non-reflecting interface.