Preskorn SH, Edwards WD, Justesen DR
There were two separate experiments in this paper. In the first, mice were radiated or sham-radiated in utero for 20 minutes daily by 2450 MHz microwaves during days 11-14 of gestation. On the 16th day postpartum all mice were implanted with lymphoreticular cell sarcoma. From the 19th day, they had daily exposure to further radiation, or to sham radiation. Those exposed to radiation in utero had a lower incidence of tumours (13% vs 46%) at necropsy at 93 days postpartum.

In the second study, the same radiation was given in utero, but no postnatal radiation was given. The mice were followed for 36 months. Initially tumours developed at a similar rate to the first study, but after the 4th month the percentage of radiated mice that had tumours was greater than the controls. Both tumor-bearing and tumour-free animals that had been irradiated as fetuses lived longer on average than the controls.

The authors note that the radiation in utero increased the colonic temperatures of the dams by an average of 2.24 degrees centigrade, and they speculate that this led to long-term augmentation of immunocompetency, which may have been responsible for the delayed induction of tumours and the increased survival.
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