EXPOSURE TO IONISING RADIATION IN PREGNANCY

Ionising radiation is energy in the form of waves or atomic particles which either carries enough kinetic energy or sufficient charge to displace atomic electrons resulting in molecular ionisation. Key factors which influence the degree of ionising radiation exposure and the subsequent biological effects include the duration of exposure, the distance from the source, the degree of shielding and the type of radiation emitted.

Adverse fetal effects following ionising radiation exposure during gestational development can include dose-related deterministic effects which arise as a result of damage at the cellular level, as well as non-dose-dependent stochastic effects which arise following irradiation and damage of genetic material. Fetal deterministic effects include embryo/fetal loss, congenital malformations (microcephaly, microphthalmia and cataracts), neurobehavioural abnormalities and fetal growth restriction, whereas stochastic effects include genetic disease, germ cell mutation, and/or childhood cancer.

National guidelines advise that pregnant women should not be exposed to doses of radiation in excess of 50 mGy. Results from preclinical animal studies and epidemiological human surveillance together provide evidence that exposure to total absorbed doses of less than 100 mGy are unlikely to result in increased risks of deterministic effects. Although not associated with a threshold dose, it is likely that the risk of stochastic events will increase as the dose of radiation to which the fetus is exposed increases. Following in utero exposure to radiation there is an approximate 1 in 13,000 attributable risk of childhood cancer per mGy of ionising radiation exposure and 1 in 200,000 per mGy risk of germ cell mutation.

There are no published guidelines on the treatment of acute radiation sickness (ARS) in pregnancy. ARS in the mother is likely to confer additional risks to fetal wellbeing, independent of those resulting from the direct effects of radiation. The severity of the maternal symptoms experienced is likely to be a major determinant of any possible adverse fetal effects, independent of the effects of direct fetal irradiation. It is therefore important to treat the mother appropriately to reduce additional risks of adverse pregnancy outcome. Pregnant patients with ARS should be managed as for the non-pregnant patient; where a specific treatment is indicated it should not be withheld on account of pregnancy. Discussion with UKTIS is recommended in all such cases.

Exposure to total absorbed doses of <50 mGy ionising radiation at any stage in pregnancy would not usually be regarded as medical grounds for termination of pregnancy or any additional fetal monitoring. However, other risk factors may be present in individual cases which may independently increase the risk of adverse pregnancy outcome. Clinicians are reminded of the importance of consideration of such factors when performing case-specific risk assessments. Additional fetal monitoring may be required with exposure to higher total absorbed doses (>50 mGy). In all such cases discussion with UKTIS is advised.