EFFECT OF TISSUE INHOMOGENEITY ON DOSE DISTRIBUTION OF CONTINUOUS ACTIVITY OF LOW-ENERGY ELECTRONS IN BONE-MARROW CAVITIES WITH DIFFERENT TOPOLOGIES

Monte Carlo calculations have previously been performed by Eckerman to evaluate the absorbed fractions of continuous sources of monoenergetic electrons in marrow cavities of human bone. The difference in scattering power of electrons in cortical bone (CB) and the red marrow (RM) was neglected. In the present work the Integrated Tiger Series and Electron-Gamma-Shower Monte Carlo codes were used to investigate the effect of topology of the bone and bone marrow interface on backscatter dose increase to the marrow. Planar, cylindrical, and spherical geometries were included. For the planar geometry, a maximum dose increase of 9 +/- 1 (S.E. of the mean) % was obtained in the region within 12 mg/cm2 from the interface due to a semi-infinite source of electrons with energy greater than 0.5 MeV. An increase of 7 +/- 1% was observed experimentally in the same region due to a semi-infinite source of P-32. This was in good agreement with Monte Carlo calculation. Averaged over the region of RM embedding electron sources between two planar CB/RM interfaces 1000-mu-m apart, a dose enhancement of 10 +/- 2% was predicted for electron energies from 1 to 1.75 MeV. For the cylindrical interface with 500-mu-m radius of curvature, the maximum dose increase averaged over the whole cylinder due to an isotropic distribution of monoenergetic electrons inside the cylinder was 12 +/- 1%. This occurred at 0.75 MeV. For the spherical interface with 500-mu-m radius of curvature, the maximum dose increase in the region within 20-mu-m (2.1 mg/cm2) from the interface due to an isotropic distribution of monoenergetic electrons inside the sphere was as high as 21 +/- 1%. This occurred at about 0.5 MeV. The dose increase, averaged over the whole sphere, was 12 +/- 0.6%.