'Radial' scanning of trabecular bone: consideration of the probability distributions of path lengths through cavities and trabeculae.

Previous methods of dosimetry for radionuclides deposited in trabecular bone have been based on probability distributions of random path lengths through the trabeculae and marrow cavities. These distributions have been measured by a technique in which thin bone sections are effectively scanned along sets of parallel grid lines. A new technique is described in this paper, in which all scan lines pass through a selected point in the section scanned, so that they effectively radiate from the point (or focus) in the manner in which particles emerge isotropically from a radioactive atom. By scanning from a number of foci a 'radial-random' distribution can be obtained for a whole bone, and this distribution compared with that obtained from 'parallel' scanning. Differences between the two types of distribution have been observed, especially where the bone exhibits structural anisotropy, the effect on the lengths being up to 13% longer for the radial scanning mode. The effect on the calculated dose factors is not great; based on radial scanning, the factors for a lumbar vertebra are from 5 to 12% less than those for parallel scanning, depending on particle energy. The differences are very small for bones with more uniform trabecular structures, so that for the skeleton as a whole the differences will be smaller than those quoted.