Mar. 24 -- FRIDAY, March 14 (HealthDay News) -- Among Japanese survivors of the atomic bombs dropped by the U.S. on Hiroshima and Nagasaki, those exposed to radioactive fallout as young children appear to face a greater risk of developing adult cancers than those exposed while still in the womb, new research suggests.
To date, the risk posed by radiation exposure while in the womb has been a little-studied subject, even though many pregnant women worldwide face radiation exposure through their work or as patients.
"Clearly there's an increased risk for adult cancer among all those exposed to radiation, but risk following exposure in utero [in the womb] seems to be quite a bit smaller than risk among those exposed as young children," said study author Dale Preston, a principal scientist with the Hirosoft International Corp., a California-based consulting and software development company.
Preston conducted his study in association with researchers from the U.S. National Cancer Institute and scientists from the Radiation Effects Research Foundation (RERF), located in Hiroshima and Nagasaki. The findings are published in the March 19 issue of the Journal of the National Cancer Institute.
The RERF -- where Preston had previously worked for approximately two decades -- was founded five years after the dropping of the Hiroshima and Nagasaki bombs on Aug. 6 and Aug. 9, 1945, respectively.
Initially called the Atomic Bomb Casualty Commission, the foundation's mission is to examine the long-term impact of radiation exposure among Japan's 120,000-plus survivors of the bombings.
Cancer incidence rates weren't available for the years from 1945 to 1957, so the new study focused on survivors who were between the ages of 12 and 55 between 1958 and 1999. Nearly 2,500 of the male and female participants were in utero when the bombs fell, while nearly 15,500 were below the age of 6 at the time. None had any history of cancer prior to 1958. Radiation dosage exposures were determined, as was proximity of the child and/or pregnant mother to the location of each explosion epicenter.
Preston and his team found that 336 men and 407 women had developed cancer during the study period, with diagnosis rates increasing dramatically after the age of 40. Cancers of the digestive system were most common, accounting for 70 percent of male and 30 percent of female malignancies. Tumors of the breast and reproductive system accounted for 48 percent of malignancies in women.
Among those who developed cancer, the researchers determined that nine of the survivor patients developed their disease as a result of radiation exposure while in the womb, compared with 87 whose illness was linked to childhood exposure.
Preston and his colleagues concluded that early childhood atomic bomb exposure was linked to a greater risk for adult cancers than exposure in the womb.
"This is a subject of much interest, and these studies are ongoing," Preston said. "And in another five or six years the number of cancer cases among survivors will probably double, because these people are relatively young -- in their 50s -- and cancer rates go up in general with age. And I think -- if the trends we found continue as expected -- that we will see that the risk differences between childhood and in utero exposure will grow wider."
Dr. Gerald Crabtree, a professor of pathology and developmental biology at Stanford University School of Medicine, said a number of factors might ultimately account for the lower exposure risk while in the womb.
"It's possible that in utero the developing embryo has a better DNA repair process," Crabtree said. "Or it could be that embryos are better at removing damaged cells altogether, because cell death is a normal part of the developmental process."
"Another possibility," he added, "would be that there's enough maternal wall to offer a small amount of shielding around the placenta -- though I would think this is less likely because this kind of radiation pretty much penetrates body tissue without a problem. So perhaps it could also be that cancerous tumors were not produced at the instant of the bomb explosion but later, and that the mother's placental barrier perhaps protected the unborn baby from exposure. But all this is all just theorizing because we don't really know."
For more on radiation exposure, visit the U.S. Nuclear Regulatory Commission.
SOURCES: Dale Preston, Ph.D., principal scientist, Hirosoft International Corp., Eureka, Calif.; Gerald Crabtree, M.D., professor, pathology and developmental biology and investigator, Howard Hughes Medical Institute, Stanford University School of Medicine, Stanford, Calif; March 19, 2008, Journal of the National Cancer Institute