Recovery of low-dose hyper-radiosensitivity following a small priming dose depends on priming dose-rate

T-47D human breast cancer cells were irradiated with 60Co γ-radiation and radiation response was measured by loss of ability of single-cells to form colonies. The influence of the dose-rate on the ability of priming irradiation to abolish Low-Dose Hyper-Radiosensitivity (LDHRS) was investigated. In agreement with previous reports, the LDHRS was abolished by an acute priming dose of 0.3 Gy given 6 h prior to the challenge dose. When the interval between the priming- and challenge-doses was increased to 24 h, LDHRS was restored. However, when the priming dose was delivered at a low dose-rate (0.3 Gy/h), the abolition of LDHRS that again appeared 6 h following the priming dose, now persisted for intervals of up to 14 weeks between doses even after 28 passages during the 14 weeks.