Measurement of back scattered radiation at tissue lead interface in high energy electron beams

In electron beam treatment introduction of a metal sheet under the tissue to shield vital organs increases the dose to the tissue at the junction due to the back scattering of electrons from the metal. Investigations were made to observe this effect for electron energies from 9 to 20 MeV. It was found that the increase in the relative ionisation is from 26 to 50 percent at the tissue-lead junction. The range of the back scattered electrons for each electron energy was determined by introducing polystyrene (density = 1.02 gm/cc) and wax sheets between the ionisation chamber and metal. A 1.0 gm per sq. cm thick wax sheet (density = 0.85 gm/cc) reduces the relative ionisation to 101, 102.6, 106.6 and 106% for 9, 12, 16, and 20 MeV electron beams respectively. (author). 4 refs., 4 figs., 1 tab