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Prolonged "ultra" low-dose-rate irradiation: effects on Chinese hamster cell population growth, survival, and radiation sensitivity.

The apparent clinical efficacy of125 I implants of solid tumors and32 P as an intraperitoneal adjuvant in patients with early ovarian carcinoma has raised clinical interest in the radiobiology of continuous radiation at dose rates in the range of 10 rad/hr. Cell population growth, survival, and radiation survival curve characteristics were examined for two Chinese hamster lines, CHO and V-79. Nonconfluent monolayers of these cells were passed serially and maintained at 37°C in226 Ra fields with dose rates of 9.5 or 12.5 rad/hr. Cell population growth continued with an increased doubling time (T2) for long (4-12 weeks) intervals during which the surviving fraction (S.F.) initially decreased to the range 0.10-0.05 for prolonged periods (23-103 days) until an abrupt drop to less than 0.00001 occurred. During the prolonged stable period (re T2 and S.F.) no change in sensitivity to acute doses of X irradiation was observed in the asynchronous populations surviving the continuous radiations at 9.5 or 12.5 rad/hr.