Radiation effects on embryonic and fetal development
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Radiation effects on embryonic and fetal development are generally considered low risk compared to the normal risks of pregnancy. Most diagnostic x-ray and nuclear medicine examinations are <50 mSv and have not been demonstrated to produce any significant impact on fetal growth and development. For doses up to 100 mSv, any radiation risks are deemed to be low compared with the normal risks of pregnancy.
Human abnormalities, explicitly identified as due to radiation, are impossible to isolate given the spontaneous incidence of all effects is ~6%. Any evidence comes from the International Commission on Radiological Protection (ICRP).
Sources of ICRP data
Data on the effects of ionizing radiation on humans is predominately from studies (Life Span Cohort Study) of Japanese survivors of the 1945 atomic bomb blasts and their assessments by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) 3,8 and National Academy of Sciences Biologic Effects of Ionizing Radiation (BEIR) 4. Although several updates have been issued in light of further research and continued cohort follow-up, these studies initially estimated lifetime cancer risk by considering accumulated data to 1985 and making projections of lifetime risk by a multiplicative model for high dose and dose rate exposure. The BEIR report supports the linear no-threshold model, which extrapolates directly observable effects of exposure at higher ionizing radiation doses, to suggest that all radiation exposure is harmful. This has faced criticism in some of the interpretations of the literature 5, which suggest a non-linear dose-response relationship, and there is some evidence of a potential protective effect at very low doses 6,7. The Life Span Cohort Study is ongoing and continues to publish updates that can better inform our understanding about long term risks of radiation exposure.
Other data on carcinogenic effects of ionizing radiation exposure:
Marshall Islanders who inhaled/ingested iodine-131 following US nuclear weapon testing
Chernobyl nuclear disaster
patients who received significant diagnostic exposures: multiple fluoroscopies for tuberculosis (Massachusetts, USA and Nova Scotia, Canada), prenatal diagnostic x-rays
radium watch dial painters
civil aviation personnel
populations with high natural background exposures, e.g. Guarapari, Brazil
Effects on embryonic and fetal development
Due to rapid cell proliferation, migration and differentiation, the developing embryo is extremely radiosensitive. Additionally, as with ionizing radiation exposure in children compared to adults; the potential time available for stochastic effects to manifest is typically greater.
The response after exposure to ionizing radiation depends on:
total dose, dose rate and radiation quality
stage of development at the time of exposure
Radiation effects related to the stage of development
Radiation risks to the fetus are dependent on the developmental period the embryo/fetus is at upon exposure. The risks are most significant during organogenesis and the early fetal stage.
pre-conception radiation of either parents' gonads has not been shown to result in an increased risk of malformations or cancer in children
fertilisation to day 9
"all-or nothing‟ phenomenon, i.e. either intrauterine death and resorption (usually undetected) or normal fetal risk
threshold dose ≈ 100 mSv
3rd-8th week after conception
risk of fetal death decreases substantially, whereas the risk of congenital malformation coincides with the peak developmental periods of various organ systems
growth retardation can also occur if irradiated >4 weeks gestation
deterministic effect with a probable threshold of >100 mSv
Fetal growth stage
begins after the end of organogenesis (>8 weeks) and continues until term
poses little risk of congenital malformations, but CNS abnormalities (reduced intelligence quotient (IQ) is the main risk at 8-25 weeks) 2, growth retardation and risk of childhood cancer (main risk after 25 weeks) can be significant
may occur if the embryo irradiated during organogenesis (3rd-8th week), especially at 20-40 days
greatest probability of malformation in a specific organ system (critical period) exists when the radiation exposure is received during the period of peak differentiation of that system
response of each organ system depends on gestational age, radiation quantity, quality and dose rate, oxygen tension and cell type
probable threshold of ≥100 mSv: these doses are rarely, if ever reached with routine diagnostic examinations, but may be reached with interventional pelvic procedures or radiation therapy
most malformations at the 100 mSv threshold will be CNS-related