The Response of Chinese Hamster Ovary Cells to Fast Neutron Radiotherapy Beams: III. Variation in Relative Biological Effectiveness with Position in the Cell Cycle

The response of synchronized Chinese hamster cells to fast neutrons was investigated. Two different neutron beams (mean energies 7 and 21 MeV) produced at the Texas A & M University Variable Energy Cyclotron were used in these studies. The results showed that the variation in sensitivity of cells to neutrons as a function of position in the cell cycle was substantially reduced compared to the variation observed with 60 Co γ rays. The degree of this reduction was similar for the two neutron beams studied. A detailed analysis of single-dose survival curves generated at radiation-sensitive and radiation-resistant positions in the cell cycle demonstrated that the relative biological effectiveness (RBE) of the sensitive moiety was much smaller than that of the resistant cells. For example, the RBE at the clinically relevant neutron dose of 75 rad was 4.7 for the mid- to late-S phase cells, 2.2 for the mitotic cells, and 1.7 for the G1/ S cells. Moreover, the ${\rm RBE}_{n/\gamma}$ of the sensitive moiety was dose independent while that of the resistant cells strongly depended on dose.