RBE Values Of High-Energy Electron Beams at VariOus Depths in Cultured Mammalian Cells

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Mouse leukemic cells (L5178Y) were used to determine the RBE values of high energy electron beams at various depth doses in a water phantom at 37℃. The RBE values of 10, 20 and 30 MeV electrons relative to 180 kV X-rays were 0.77, 0.83 and 0.86 at 100% depth dose, respectively. The RBE values of 30 MeV electrons were 0.85 at 95%, near the surface, and 0.89 at 68% depth doses. At 18% depth dose, a 0.78 RBE value was detected. The problem of RBE of high energy electrons has been the subject of many investigators in radiobiology. It is of theoretical interest in attempting to correlate theories relating RBE with LET, and is indeed of increasing pratical interest because of the increasing use of electron accelerators in radiotherapy. RBE values have been reported according to variations in high energy electrons at various depths in tissue. Markus and Sticinsky reported increasing RBE with depth, using Drosophila embryos, HeLa cells and onion roots. Fritz-Niggli and Schinz found no such effect using Drosophila embryos, nor did Wambersie et al. using E. coli and a strain of diploid yeasts. Robinson's results, using cultured human cells for 35 MeV electrons, agreed with those of Wambersie et al. and Fritz-Niggli and Schinz. Schwarz et al. reported no variation in the RBE with depth at 60% to 100% isodose levels except for an increase in RBE value to 1. 48 at the 12% isodose level.
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