Dosimetry of Electron Sources Near Planar Tissue Interfaces

Abstract

The beta dose distributions in red bone marrow equivalent material due to imbedded continuous sources were measured experimentally with ultra thin LiF thermoluminescent dosimeters near planar interfaces of cortical bone (CB) and red bone marrow (RBM), and RBM and air. It has been also investigated numerically by Cyltran, the Monte Carlo code. In the Monte Carlo approach, the dose enhancement ratio for a planar CB-RBM interface increases with electron energy and reaches a plateau at 0.50 MeV while the dose reduction ratio for a planar vacuum-RBM interface decreases to a steady value from 1.00 MeV onwards. With a semi-infinite source of 32P, dose enhancement ratios at 0-9, 79-88, and 157-166 mg/cm2 separations from a planar CB-RBM interface were measured to be 1.07±0.01, 1.03+0.03 and 0.99+0.03 respectively. The dose reduction ratios at these separations from a planar AIR-RBM interface were found to be 0.82±0.01, 0.94+0.03 and 0.97+0.03 respectively. Both the dose enhancement ratios and dose reduction ratios agree with the results calculated by the Monte Carlo approach within one standard deviation except for the dose reduction ratio at 0-9 mg/cm2 from the AIR-RBM interface. The experimental result in this case is about three standard deviations less than the Monte Carlo results. Using the same Monte Carlo code, the dose enhancement ratio at 0-20 micron separation from a planar CB-RBM interface due to a point or plane source of 0.50 MeV electrons at the interface reaches saturation at approximately 0.22 times the CSDA range of 0.5 MeV electron in CB for both plane and point source configurations. The saturation dose enhancement ratio for both configurations is 1.06±0.01.