In utero radionuclide exposures and risk of leukaemia

Ionising radiation is a known leukaemogen and in utero exposure to diagnostic X-rays associates with increased risk of childhood leukaemia (see abstract by R. Wakeford). This paper examines the methodology used by the International Commission on Radiological Protection (ICRP) to estimate doses to the child following in utero radionuclide exposures, considers experimental evidence for in utero haemopoietic sensitivity to alpha irradiation, and compares measurements of naturally-occurring and man-made alpha-emitting radionuclides in human fetal tissues. The ICRP has published dose coefficients for the offspring of women exposed to radionuclides during or before pregnancy. These dose estimates include in utero doses to the embryo and fetus and doses delivered postnatally to the newborn child from radionuclides retained at birth. The models used provide estimates of dose to fetal bone marrow but do not take account of the complexity of haemopoietic tissue development in the yolk sac, liver and skeleton. Simplifying assumptions were also made in the calculation of alpha and beta doses within skeletal tissues. Studies using mice have examined the sensitivity of haemopoietic tissue to in utero irradiation from alpha emitters and tritium beta emissions. Results for the incidence of stable chromosome aberrations in marrow cells from neonatal mice after in utero exposure to X-rays or Pu during embryogenesis or fetal development suggest substantially greater sensitivity to alpha irradiation, and high values of relative biological effectiveness (RBE). Similar studies involving chronic in utero exposure to tritiated water gave results consistent with an RBE of 1 – 2. High in utero sensitivity to alpha irradiation has also been shown in studies of the clonogenic viability of marrow haemopoietic progenitor cells. However, evidence from life-span studies of mice exposed to Pu in utero indicates that these observations of haemopoietic sensitivity are not reflected in an increased incidence of leukaemia. Measurements of alpha emitters in human fetal tissues obtained in the U.K. show that concentrations of naturally-occurring alpha emitters, principally polonium-210, are orders of magnitude greater than concentrations of Pu, attributable to weapons-test fall-out. Measurements at the University of Bristol of concentrations of Po in fetal bone of up to 180 mBq kg compare with Pu concentrations of 5 – 20 μBq kg . Estimates of in utero doses from Po and other alpha emitters show these to be a small proportion of overall in utero dose from natural sources.