RBE of Pulsed Radiations by Lethality in Mice: Cultured Mammalian Cells and Bacteria

The biological effectiveness of 25-MV pulsed x-rays was compared with that of Co-60 γ-rays and 180 kVp x-rays in studies using ICR-JCL female mice, mouse L5178Y leukemic cells, and E. coli ${\rm B}_{{\rm H}}$ and H/r-30 bacteria. Relative biological effectiveness (RBE) of pulsed x-rays with the mortality of mice as an end point was 1.11 that of Co-60 γ-rays; the value for 180 kVp x-rays was 1.28. Patterns of daily mortality and body weight fluctuations of mice exposed to pulsed x-rays were similar to those of Co-60 γ-rays. The RBE values obtained for cultured cells were the same as those listed above for mortality of mice. Changes in RBE by differences in cell phase between exponential and stationary growth were not observed for given radiations, although D0 values and extrapolation numbers did vary by cell phase. However, the sensitivity of E. coli did not depend significantly on the type of radiation delivered.