Medical application of radiation during pregnancy

If a pregnant woman is exposed to radiation, then malformations and developmental disorders may occur in the unborn child. In addition, the child has an increased risk of developing cancer or leukaemia. For this reason, there are appropriate provisions for protecting the unborn child in the German X-Ray Ordinance and the Radiation Protection Ordinance. Accordingly, before applying ionising radiation in medical diagnostics or therapy, the examining physician has to ask every woman of reproductive age whether she is or may be pregnant.

Choosing alternative techniques

If pregnancy has been established or cannot be completely excluded, the necessity of applying radiation has to be evaluated considering an especially careful balancing of the risks and benefits. The examination should be postponed until the end of pregnancy, if possible, or alternative techniques (with lower or no radiation dose, for example ultrasound) should be considered

Biological radiation effects

A distinction is made between two categories of biological radiation effects: deterministic and stochastic radiation effects.

• Deterministic effects (also termed “tissue reactions”) occur through the massive killing of cells in an organ or tissue system: If the killing of too many tissue cells leads to an imbalance between cell replenishment and cell loss and if this imbalance exceeds a critical threshold level, the affected organ or tissue is damaged. For deterministic effects, threshold doses are assumed below which the number of cells killed is too low to permanently impair the function of organs and tissues. The severity of deterministic effects increases with increasing dose.

• Stochastic effects result from changes in the genetic information of the cells (DNA). Cellular control mechanisms may thus be disturbed. These effects can subsequently lead to malignant diseases such as cancer or leukaemia. Between the radiation exposure and the onset of cancer or leukaemia, there is a so-called latency period which can last several years or even decades. The probability that stochastic effects will occur increases with increasing dose. A threshold dose - as with deterministic effects - is not assumed.

Effects

Malformations and developmental disorders in the unborn child belong to the deterministic effects of radiation exposure. Their occurrence does not only depend on the level of radiation dose, but also on the developmental stage of the unborn child and thus on the time of the radiation exposure in the course of the pregnancy.

• Early phase of pregnancy: radiation exposure may lead to failure of implantation or death of the fertilized egg. The dose threshold value for this effect is at least 50 to 100 millisieverts (mSv) (uterus dose).
• Between the 4th and 10th week of pregnancy (counted from the first day of the last period): during the so-called organogenesis, the cells divide and differentiate. Embryonic organ primordia, for example for the heart and nervous system, are formed. During this phase, there is a risk of malformations. Dose thresholds have been observed for this in animal experiments. For humans, dose threshold values of at least 50 to 100 mSv are assumed.

• From the 10th week of pregnancy: from this period onwards, radiation exposures may lead to brain maldevelopments. In the case of the atomic bomb survivors of Hiroshima and Nagasaki, mental retardation was observed more frequently in children who had been exposed in utero to the atomic bombings during this phase of pregnancy. A threshold dose of about 300 mSv is assumed for this radiation effect.

With regard to an individual examination in the scope of standard radiological and nuclear medical diagnostics, the lowest estimated value for the threshold dose of 50 mSv for the unborn child is generally not exceeded. In a standard radiological or nuclear medicine examination the dose for the unborn child is generally well below 50 mSv, i.e. the lowest estimated value for the threshold.

The probability of the occurrence of stochastic late effects does not depend on the developmental stage of the unborn child. It is considered an established fact that the risk of cancer, especially the risk of leukaemia, is increased in children who have been exposed to radiation in utero. However, the corresponding risk estimates are subject to considerable uncertainties.