Changing sensitivity of neonatal rats to tumorigenic effects of N-nitroso-N-ethylurea and X-radiation, given singly or combined.

After neonatal injection of rats with 10 mg N-nitroso-N-ethylurea [(ENU) CAS: 759-73-9]/kg body weight, whole-body X-irradiation with 1.25 Gy X-radiation caused a reduction in induced neurogenic tumors which was dependent on the interval between the two treatments. The reduction was greatest when radiation was given 1 day after ENU and progressively decreased with irradiation at 5 and 30 days. Changes in radiosensitivity in the neonatal period were also seen for ovarian tumors and squamous cell carcinomas of the mouth. Ovarian tumors were induced by 1.25 Gy X-rays alone, with a higher incidence occurring after irradiation at 5 days than at 30 days of age. Squamous cell carcinomas of the mouth have a high spontaneous incidence in this strain of rat, but 1.25 Gy X-radiation caused a significant reduction which was greater after irradiation at 5 than at 30 days of age. Pituitary tumors occurred in untreated and treated rats with a higher incidence in females. Only in males was there any suggestion of an effect of the various treatments on pituitary tumor incidence.