Synergistic induction of tumours in NMRI mice by combined foetal X-irradiation with low doses and ethylnitrosourea administered to juvenile offspring.

Mice were X-irradiated on day 15 of gestation with 0.2, 0.4, 0.8 or 1.6 Gy. Offspring were reared by their mothers and divided into two subgroups at an age of 21 days, one subgroup receiving a single dose (45 mg/kg) of ethylnitrosourea (ENU). All animals were kept ultimately until 22 months to register the long-term tumour pattern. The carcinogenic effects of ENU alone were also studied in two separate experiments. Prenatal X-irradiation with 1.6 Gy mostly abolished the carcinogenic late effects of ENU, with the exception of an almost constant leucosis incidence and an unchanged lung tumour frequency. Lowering the prenatal X-ray dose to 0.8 Gy resulted in a significantly increased rate of liver tumours and ovary tumours. Synergistic effects on various tissues were observed after both 0.4- and 0.2-Gy foetal X-irradiation treatment in combination with postnatal application of ENU. These effects mainly involved a significant increase in the frequency of leucosis and of tumours of the liver, intestine, uterus and ovaries. The greater-than-additive effect in the case of these tumours suggests that low-level prenatal X-irradiation leads to a lasting sensitivity of some tissues towards a subsequent carcinogenic stimulus