Cancer after Intrauterine Exposure to the Atomic Bomb

Abstract

In 1956 Alice Stewart and her associates (1) first reported an excess of cancer under 10 years of age among children whose mothers received diagnostic X-ray exposures of the abdomen during pregnancy. Her subsequent reports indicated that the frequency of each of the main childhood cancers was increased about 1.5-fold (2). This same relative risk for virtually all forms of childhood cancer after very low doses has been a puzzle ever since. MacMahon at first confirmed Stewart's findings (3), but when he and Monson (4) extended their series, the excess of solid cancer disappeared but the 1.5-fold excess of leukemia remained. Stewart's data, according to a calculation by Jablon and Kato (5), led to an estimate of at least 5 children with cancer among A-bomb survivors exposed in utero. Only one was observed, however, as anticipated from Japanese national rates. An excess of cancer in adulthood was expected among the 807 exposed in utero, at least as much as experienced by children who were exposed between birth and 9 years of age. In 1988, when the in utero cohort was 39 years old, a suggestive increase was reported (6). A brief update 5 years later revealed a much diminished excess, because cancers had occurred at a higher rate in the lightly exposed than in the heavily exposed (7). Now a fuller account of the findings through age 46 is given by Delongchamp and his colleagues in this issue (8). For the first time, comparison is made with those just beyond the limit of gestation: children 0-5 years of age. [Data for 0-9 years of age would have allowed easy reference to age-specific effects in previously published reports of the Radiation Effects Research Foundation (RERF).] Among the children exposed at ages 0-5 years who survived the A-bomb, the peak incidence in acute lymphocytic leukemia occurred 5 years after exposure. The rates fell to near normal 30 years later (9). Breast cancer after childhood exposure did not begin to develop until age 25, as it does in the general population (10). In the U.S., after radiotherapy at birth for enlargement of the thymus, the rate of thyroid cancer rose beginning at about 10 years of age (11). Among A-bomb survivors exposed at 0-5 years of age, Delongchamp et al. found an excess of solid tumors in women but not men, due largely to excesses in cancer of the female breast and ovary. The rates for leukemia were also levated. These findings are in accord with those previously made by RERF. The rates for other cancers as given in the report (stomach, liver and female genital) contribute modestly to the excess of solid tumors. One thyroid cancer death is listed in a footnote, but the exposure, if any, is not given. On a relative scale, thyroid cancer is one of the big thre (with leukemia and female breast cancer) induced among A-bomb survivors, but because almost all thyroid ncer is curable, there is rarely a death from this neoplasm. It is odd that an excess of solid cancer was observed