MONTE-CARLO STUDY OF ELECTRON-SPECTRA AND DOSE FROM BACKSCATTERED RADIATION IN THE VICINITY OF MEDIA INTERFACES FOR MONOENERGETIC PHOTONS OF 50-1250 KEV

In the present paper electron fluence spectra and backscatter dose fac tors for monoenergetic photons (50, 100, 250 and 1250 keV) are present ed. The influence of the atomic number of the backscatter materials gl ass, bone, steel and titanium is studied. For all radiations increases in dose were found in the immediate vicinity of the interface in a re gion extending over a distance determined by the photon energy. In a t hin layer extending several tens of micrometers from the interface, a large increase in dose was found for 50 and 100 keV photons, The large st backscatter dose factor (3.5) was found for 50 keV photons and a st eel interface. This large effect was found to be due largely to backsc attered photoelectrons, K-shell fluorescent photons from titanium and steel were also included but were found to have almost no effect on ba ckscatter dose factors or electron spectra. With both K-shell fluoresc ence and sampling of the photoelectron angular distribution switched o ff, much smaller backscatter factors were obtained. Beyond the thin re gion near the interface where the dose is increased, significant dose depressions were found for photons of these energies in a region exten ding several centimeters from the interface. The effect was maximum fo r 50 keV photons, for which dose depressions of about 35% for steel an d 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 inte rface (40-50%) but extending over a larger region (e.g. 5 mm for 1250 keV photons). Small differences in radiation quality nearer to the int erface were found as expressed by track-averaged and dose-averaged res tricted linear energy transfer, ($$$) over bar LET(100,T) and ($$$) ov er bar LET(100,D), with a maximum effect for the 100 keV photons. For this radiation quality a decrease of 50% in ($$$) over bar LET(100,T) was found close to the interface. (C) 1995 by Radiation Research Socie ty