Retarded tumor growth and greater longevity in mice after fetal irradiation by 2450‐MHz microwaves

In the first of two studies, 48 mice of the conventional CFW strain were sham‐radiated or radiated in utero for 20 minutes daily by 2,450‐MHz microwaves at a dose rate, respectively, of 0 or 35 mW/g during days 11–14 of gestation. All 48 mice were implanted with a homogenate of a lymphoreticular cell sarcoma on the 16th day postpartum and then, commencing on the 19th day, they underwent a series of 36 daily exposures to the sham‐ or to the microwave‐radiation. Fetal exposure to radiation, which elevated dams colonic temperatures by an average of 2.24°C, was associated with a lower incidence of tumors (13% vs 46% for fetally sham‐radiated mice) as verified histologically at necropsy on the 93rd day postpartum. In the second study, 84 CFW mice received the four radiation treatments in utero; 60 mice were sham‐radiated in utero and served as controls. Postnatal radiation was not administered. All 144 mice were implanted with the homogenate on the 16th day postpartum and then were observed for nearly 36 months for development of palpable tumors and for longevity. Tumors initially developed at a lower rate in fetally radiated mice and 2.5 months after implantation the respective percentages of “takes” in sham‐ and microwave‐irradiated mice were comparable to those observed at termination of the first study. Subsequently the rate of tumor induction in radiated mice accelerated, and after the fourth month the final percentage of radiated mice with tumors (46%) slightly exceeded that of controls (40%). Both tumor‐bearing and tumor‐free animals that had been radiated as fetuses lived longer on the average than respective controls. Long‐term augmentation of immunocompetency by in utero hyperthermia is believed to be responsible for the delayed induction of tumors and for enhancement of survival.