Increased radiosensitivity of rat rhabdomyosarcoma cells induced by protracted irradiation

Cells of a solid rat rhabdomyosarcoma, which can be cultured in vitro and upon inoculation grow in vivo into new tumors, were irradiated at low dose rates in vitro or in vivo. Assays of fractions of clonogenic cells show that repair of sublethal damage takes place during protracted irradiation. Nevertheless an acute single dose administered at the end of a protracted irradiation of many hours is more effective with respect to induction of cell lethality than a single acute dose applied without pretreatment. This increased radiosensitivity at the end of a low dose rate treatment can be explained however by the observation based on DNA measurements, that cells were redistributed over the cell cycle phases with relatively more cells in the G2 + M phase