Monte Carlo Study of Electron Spectra and Dose from Backscattered Radiation in the Vicinity of Media Interfaces for Monoenergetic Photons of 50-1250 keV

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Abstract
In the present paper electron fluence spectra and backscatter dose factors for monoenergetic photons (50, 100, 250 and 1250 keV) are presented. The influence of the atomic number of the backscatter materials glass, bone, steel and titanium is studied. For all radiations increases in dose were found in the immediate vicinity of the interface in a region extending over a distance determined by the photon energy. In a thin layer extending several tens of micrometers from the interface, a large increase in dose was found for 50 and 100 keV photons. The largest backscatter dose factor (3.5) was found for 50 keV photons and a steel interface. This large effect was found to be due largely to backscattered photoelectrons. K-shell fluorescent photons from titanium and steel were also included but were found to have almost no effect on backscatter dose factors or electron spectra. With both K-shell fluorescence and sampling of the photoelectron angular distribution switched off, much smaller backscatter factors were obtained. Beyond the thin region near the interface where the dose is increased, significant dose depressions were found for photons of these energies in a region extending several centimeters from the interface. The effect was maximum for 50 keV photons, for which dose depressions of about 35% for steel and titanium and 15% for glass and bone were obtained. For 250 and 1250 keV photons a more modest dose enhancement was found close to the interface (40-50%) but extending over a larger region (e.g. 5 mm for 1250 keV photons). Small differences in radiation quality nearer to the interface were found as expressed by track-averaged and dose-averaged restricted linear energy transfer, LET100,T and LET100,D, with a maximum effect for the 100 keV photons. For this radiation quality a decrease of 50% in LET100,T was found close to the interface.

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