The main objective of NRPB advice concerning in utero exposures to ionising radiations is

In addition, advice is meant to help to avoid unnecessary concern or action if an exposure does occur.

Introduction

Scope

The advice covers the risks to the developing embryo and fetus of death, malformation, mental impairment, cancer (solid tumours and leukaemias) and heritable damage from irradiation before the mother could be aware of a pregnancy - an unknown pregnancy*. It also considers two other issues of particular relevance: the possible risks from irradiation of the early conceptus (3-4 weeks gestational age) and from preconception gonadal irradiation. These recommendations should be read along with

• the recommendations of a joint working party of the Royal College of Radiologists (RCR) and NRPB on patient dose reduction¹,

• the NRPB suggested national reference dose levels²,

• the joint Institute of Physical Sciences in Medicine (now the Institute of Physics and Engineering in Medicine), College of Radiographers (CoR) and NRPB protocol for patient dose measurements in diagnostic radiology³,

• the advice of the Administration of Radioactive Substances Advisory Committee (ARSAC)⁴.

This document also replaces the joint RCR and CoR advice of October 1986⁵. As with any use of radiation in medicine, compliance with statutory legislation is mandatory, ie with the Ionising Radiations Regulations 1985 and the Ionising Radiation (Protection of Persons Undergoing Medical Examination or Treatment) Regulations 1988.

Assessment of risk

Risk is assessed on the basis of dose. The doses quoted in this document are taken from the most recent UK surveys of doses for many common examinations and hence represent the latest available data on UK practice^4,6; consequently, some of the doses in this document will not be the same as those in Documents of the NRPB, Volume 4, No. 4⁷. However, the information may not reflect doses in all departments for these examinations and, of course, does not provide data for all possible examinations.

It is therefore essential that all staff institute the guidance in this document on the basis of the mean doses delivered to the fetus in their departments for both radiology and nuclear medicine procedures (for this purpose, fetal dose should be assumed to be equal to uterine dose).

Diagnostic examination of females of reproductive age

Whenever possible, alternative investigation techniques, not involving ionising radiations, should have been considered before a decision is taken to use ionising radiations in female patients of reproductive age.

At diagnostic dose levels, the only adverse effect of radiation on the conceptus which is likely to pose a significant risk is that of cancer induction. None of the other potential hazards (death, malformation, growth retardation, severe mental retardation and heritable effects) presents a significant problem at the low exposures used in diagnostic procedures.

Implementation of guidance

When a female of reproductive age presents for an examination in which the primary beam irradiates the pelvic area, or for a procedure involving radioactive isotopes, she should be asked whether she is or might be pregnant. If the patient cannot exclude the possibility of pregnancy, she should be asked whether her menstrual period is overdue. In line with accepted convention, this action should be recorded in an appropriate place, as required by local rules.

Particular problems may be experienced in obtaining this information from females under the age of 16 years; in such cases staff should refer to the guidance given by the College of Radiographers in The implications for radiographers of the Children Act⁸.

Depending on the answers, patients can then be assigned to one of the following groups.

• No possibility of pregnancy

  Proceed with the examination.

• Patient definitely, or probably, pregnant

  If pregnancy is established, or likely, review the justification for the proposed examination, and decide on whether to defer the investigation until after delivery, bearing in mind that a procedure of clinical benefit to the mother may also be of indirect benefit to her unborn child and that delaying an essential procedure until later in pregnancy may present a greater risk to the fetus. If a procedure is undertaken, the fetal dose should be kept to the minimum consistent with the diagnostic purpose(s).

• Low dose procedure, pregnancy cannot be excluded

  Proceed with the examination, provided that the period is not overdue. If the period is overdue, follow the advice in the previous paragraph.

• High dose procedures (defined as examinations resulting in fetal doses of some tens of milligray)

  In most departments, the only routine examinations in this category will probably be abdominal and pelvic computed tomography. However, any procedure that delivers doses to the fetus of some tens of milligray (eg some barium studies) may carry significant risks - this reinforces the importance of knowing the magnitude of doses in individual departments. The new evidence suggests that these may carry a small risk of cancer induction for the unknown fetus. One of two courses could be adopted:

  • apply the rule that in females of childbearing age these examinations are booked for the first 10 days of the menstrual cycle, when conception is unlikely to have occurred (formerly known as the '10-day rule'),

  • re-book patients who attend for such examinations and are identified to be in the second half of their cycle, of childbearing age and in whom pregnancy cannot be excluded. The number of such patients is likely to be small.

It should be emphasised that although there may be a small risk to the unknown fetus, this risk will increase in the months following the first missed period, and high dose examinations should only be re-booked if they can safely be postponed until after delivery, should the patient prove to be pregnant.

Subsequently, if it becomes obvious that a fetus has been inadvertently exposed, despite the above guidance, the small risk to the fetus of the exposure does not justify the greater risks of invasive fetal diagnostic procedures to the fetus and mother (particularly as they are unlikely to pick up any induced effect), nor does the risk justify those of a termination of the pregnancy to the mother.

References

1. NRPB/RCR.
   Patient dose reduction in diagnostic radiology.
   Doc. NRPB, 1, No. 3, 1-46 (1990).

2. NRPB.
   Medical exposure: guidance on the 1990 recommendations of ICRP.
   Doc. NRPB, 4, No. 2, 43-74 (1993).

3. Dosimetry Working Party of the Institute of Physical Sciences in Medicine. National protocol for patient dose measurements in diagnostic radiology. Chilton, NRPB (1992).

4. ARSAC. Notes for guidance on the administration of radioactive substances to persons for purpose of diagnosis, treatment or research. London, Administration of Radioactive Substances Advisory Committee (1993).

5. RCR/CoR. Guidelines for implementation of ASP8. Exposure to ionising radiation of pregnant women: advice on the diagnostic exposure of women who are, or who may be, pregnant. London, Royal College of Radiologists and College of Radiographers (1986).

6. Hart, D, et al. Doses to patients from medical x-ray examinations in the UK - 1995 review. Chilton, NRPB-R289 (1996).

7. NRPB.
   Diagnostic medical exposures: exposure to ionising radiation of pregnant women.
   Doc. NRPB, 4, No. 4, 5-14 (1993).

8. CoR. The implications for radiographers of the Children Act. London, College of Radiographers (1995).

* For the purposes of this document, an unknown pregnancy is defined as one in which the mother is not aware of her pregnancy because a menstrual period has not been missed.

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