INTRODUCTION

1. The Board issued ASP8 in 1985 giving advice on the diagnostic exposure of women who are or who may be pregnant. While the current advice draws upon data published since 1985 the main objectives of Board advice are unchanged: to minimise the likelihood of inadvertent exposure of the conceptus before pregnancy is declared, and to prevent unnecessary exposure of the fetus when medical diagnostic procedures involving ionising radiation are indicated during pregnancy. In addition, the current advice should help to avoid unnecessary concern or action if an exposure does occur.

2. In providing this advice the Board has considered the risks to the developing embryo and fetus of death, malformation, mental impairment, cancer (solid tumours and leukaemias) and genetic damage from irradiation after the first missed menstrual period (1). The possible risks from irradiation of the early (up to 3-4 weeks) conceptus and from gonadal irradiation of patients is also covered in the present advice (2).

3. Detailed information underlying this advice is given elsewhere.

DIAGNOSTIC EXAMINATION OF WOMEN OF REPRODUCTIVE CAPACITY

4. When a woman of reproductive age presents for diagnostic radiology in which the primary beam irradiates the pelvic area, or for procedures involving radioactive isotopes, appropriate steps should be taken to determine whether she is or may be pregnant, principally by requesting the date of the last menstrual period. If pregnancy is established or likely, justification for the proposed examination needs to be reviewed including whether the examination can be deferred, bearing in mind that a procedure of clinical benefit to the mother may also be of indirect benefit to her unborn child. In any resulting diagnostic examination it is important to keep the dose to the minimum consistent with diagnostic requirements.

5. For most diagnostic radiation exposures of the early conceptus the risks of cancer will be small; however, those few procedures yielding doses of some tens of milligray should be avoided, if possible, in early pregnancy. When the possibility of early pregnancy cannot be reasonably excluded, one way of avoiding such risks would be to restrict the use of high dose diagnostic procedures, such as barium enema, pelvic computed tomography (CT) or abdominal CT to the early part of the menstrual cycle when pregnancy is unlikely.

RISKS OF INDUCTION OF DEATH, GROSS MALFORMATION AND MENTAL IMPAIRMENT IN THE FETUS

6. In the light of the likely dose thresholds ascribed to these effects the Board considers that the doses resulting from most conventional diagnostic procedures have no substantial effect on the risk for the individual pregnancy regarding the incidence of fetal death, malformation or the impairment of mental development.

RISKS OF INDUCTION OF GENETIC DISEASE AND CANCER IN THE FETUS

7. The induction of genetic disease and cancer by ionising radiation is believed to show no dose threshold. The risks of these effects are judged relative to their natural incidences.

Genetic disease

8. For radiation-induced genetic disease expressing in the descendants of the unborn child the risk for any individual pregnancy following fetal irradiation from medical diagnostic procedures is judged by the Board to be small relative to the natural risk of genetic disease; thus, there is no indication for termination of pregnancy or for the use of invasive fetal diagnostic techniques (such as amniocentesis).

Cancer

9. For the expression of childhood cancer following irradiation of the fetus the Board considers that for most diagnostic procedures giving doses up to a few milligray, the associated risks are acceptable when compared with the natural risk. Therefore, exposure of the fetus in these circumstances is not considered to be a reason for termination of the pregnancy or for the use of invasive fetal diagnostic procedures.

10. For exposure of pregnant women to the higher doses (some tens of milligray) associated with, for example, pelvic CT, there may be more than a doubling of the natural cancer risk in the unborn child. This level of excess risk is about one in one-thousand for the individual fetus and is unlikely to be a reason for termination of the pregnancy or for the use of invasive fetal diagnostic procedures.

PRECONCEPTION RISKS TO PATIENTS

11. For gonadal exposure of the patient, dose minimisation through correct alignment, collimation and the use of gonadal shields whenever practical will minimise possible genetic effects. This advice applies to both female and male patients before and within the reproductive period. With appropriate gonadal dose minimisation, the risk of new mutations resulting from medical diagnostic exposures expressing as genetic disease in the descendants of patients is judged by the Board to be small, when compared for example with the risk of those arising naturally. The Board has also considered the question of possible cancer risk to offspring following parental gonadal irradiation. The current state of knowledge in this area is judged neither to provide grounds for recommending any restriction on post-exposure reproduction in patients having undergone medical diagnostic procedures nor to provide any reason for termination of resulting pregnancies or employing invasive fetal diagnostic procedures. Nevertheless, gonadal dose minimisation is recommended as a matter of simple prudence.

REFERENCES

(1) NRPB. Estimates of late radiation risks to the UK population: Chapter 6 - Irradiation in utero. Doc. NRPB, 4, No. 4, 105-125 (1993).
(2) NRPB. Diagnostic medical exposures: Exposure to ionising radiation of pregnant women. Doc. NRPB, 4, No. 4, 5-14 (1993).