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Documents of the NRPB
Volume 4 No 4
1993
Statement by the National Radiological Protection Board
Diagnostic
Medical Exposures: Advice on Exposure to Ionising Radiation During
Pregnancy
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).
Diagnostic Medical Exposures: Exposure to Ionising Radiation of Pregnant Women
Biological Basis of the Board's Statement
ABSTRACT
A booklet
summarising NRPB advice has been published, entitled Diagnostic
Medical Exposures: Advice on Exposure to Ionising Radiation during
Pregnancy.
Extracts from the text are
available.
Estimates of Late Radiation Risks to the UK Population
ABSTRACT
This publication provides a review of information for assessing stochastic effects (cancer and hereditary disease) and the effects of irradiation in utero that are likely to arise in the UK population following exposure to external radiation, or as a result of intakes of radionuclides. It updates information in the report NRPB-R226. Since the publication of that report in 1988 more detailed information from follow-up of the Japanese atomic bomb survivors has been released, the 1990 Recommendations of ICRP have been published and the first analysis of theUK National Registry for Radiation Workers, which provides direct evidence on the risks from low dose and low dose rate occupational exposures, has been completed. Important elements in these publications concern: the appropriate choice of projection models and the DDREF for estimating lifetime cancer risks and the assessment of dose-response relationship for mental retardation resulting from in utero exposure to radiation. The development of risk models which take account of mechanisms of carcinogenesis has also received more attention in recent years. Throughout this review the aim has been to present a scientific rather than a conservative assessment, and to provide information about the uncertainties involved. The UK specific risk factors developed in this report for cancer induction are not intended to replace the values that ICRP has developed for setting standards in radiation protection. They are, however, intended for use in calculating late health effects within a UK population, for example, in accident consequence assessments or in determining probability of causation. There is little new information about deterministic effects and earlier Board publications (NRPB-R226 and NRPB-M246) will continue to be available until such time as early effects data are reviewed.Price £15.00
ISBN 0-85951-365-3
