Response of HeLa cells to irradiation with pi- mesons.

HeLa cell suspensions were irradiated at various depths of Perspex in a beam of 70 MeV pi- mesons. The dose-rate in the Bragg peak varied between 40 rad hour-1 and 150 rad hour-1. The cells were assayed in vitro for loss of reproductive integrity. The results for irradiation in the peak indicated an RBE of about 2-1 when compared to the response obtained using 60Co gamma-rays at comparable dose-rates. At dose-rates of 100-150 rad hour-1, the RBE peak-to-plateau was about 1-4, while at lower dose-rates, the RBE peak-to-plateau was about 1.