Effects of continuous irradiation at low dose-rates on a rat rhabdomyosarcoma

An experimental rat rhabdomyosarcoma was irradiated with 137Cs γ radiation applied at dose-rates of 75, 110 and 150 rads/hour. The results show that the effectiveness per unit dose of the γ radiation applied at low dose-rates is relatively smaller than that of single acute doses of 300 kV X rays. An additional dose of 800 rads of 300 kV X rays was given to tumours in living animals and in animals that were dead for ten minutes after a dose of 2,000 rads given in 20 hours. This experiment allowed an evaluation of the oxygenation status at the end of the protracted irradiation. The results show that, during this protracted irradiation, (1) sub-lethal damage is repaired, (2) reoxygenation occurs so rapidly that the fraction of hypoxic cells remains essentially constant, and (3) cells are accumulated into a more radiosensitive phase of the cell cycle.