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In 1964, the National Health and Medical Research Council (NHMRC)
recommended that “In women of childbearing age, non-urgent x-ray
examinations that entailed pelvic irradiation should be restricted to the
first ten days of the menstrual cycle." This practice is commonly known as
the 10 Day Rule.
In
1984, the International Commission on Radiological Protection (ICRP)
reported that there was little, if any, risk of damaging the foetus during
the first two weeks of gestation i.e. before the first menstrual cycle was
missed1. The National Radiological Protection Board (NRPB)
issued advice based on this statement2. The College of
Radiographers and Royal College of Radiologists followed up with their
joint guidelines3. Before an examination of an area where the
uterus is within or close to the irradiated area is conducted, the
radiographers and the radiologists are required to ask the patient if
there is any possibility that the patient may be pregnant. If the patient
replies in the negative (NO) then the radiographers and the radiologists
must ask the date of the patient’s last period. If the menstrual cycle
is overdue, then the examination may be postponed. This practice is
commonly known as 28 Day Rule.
In
1993, the NRPB published its latest statement on diagnostic medical
exposure to ionising radiation during pregnancy4. For adverse
effects (foetal death or malformation, severe mental retardation and
inherited diseases in future generation), the influence of diagnostic
doses of radiation was found to be insignificant in relation to natural
incidence of these disorders. However, for induction of cancer the
position is not clear. Evidence now exists that some carcinogenic
mutations, even at the very early stage of life, are compatible with
continuing development of the foetus and may result in an additional risk
of fatal childhood cancer before the age of 15 of 1 in 13004.
This represents a doubling of the incidence in unexposed population. The
procedures that concern NRPB are those that entail doses of “some tens
mGy”. In routine practice, this means abdominal or pelvic computed
tomography and barium enemas. The NRPB suggests that one way to avoid
irradiation of an early foetus is to restrict these high dose procedures
to the first ten days of the menstrual cycle i.3 a limited return to the
10 Day Rule.
Based
on the current knowledge, the following guidelines should be adhered to;
I.
For most of the routine examinations, except those falling into the
dose category, which will result in irradiation to the uterus, radiology
departments should apply the 28Day Rule.
II.
For non-urgent examinations involving high doses to uterus in
patients who are at risk of pregnancy but not yet overdue, the
examinations should be delayed until the first ten days of the next
menstrual cycle. High dose examinations include computed tomography of the
abdomen and pelvis and barium enema.
III.
Radiation exposure of the lower abdomen and pelvis of women of
childbearing potential should be kept to a minimum. During pregnancy,
radiation to these regions should only occur if the radiological
examination cannot be postponed because of the urgent nature of the
investigation
IV.
The risk of radiation damage to a foetus, even at the relatively
high doses resulting from abdominal or pelvic computed tomography or
barium enema, is small and inadvertent exposure in early pregnancy will
not of itself be an indication for termination or for the use of invasive
diagnostic procedures such as amniocentesis.
References
1.
International Commission on Radiological Protection. Statement from
the 1983 Washington meeting of ICRP. Annals of International Commission on
Radiological Protection 1984:14
2.
National Radiological Protection Board. Exposure to ionising
radiation of pregnant women: advice on the diagnostic exposure of women
who are, or who may be pregnant. ASP8.NRPB, 1985.
3.
College of Radiographers and Royal College of Radiologists.
Guidelines for implementation of ASP8.NRPB, 1986.
4.
National Radiation Protection Board. Board statement on diagnostic
medical exposure to ionising radiation during pregnancy and estimates of
late radiation risks to the UK population. Documents of the NRPB 1993;
4(4):1-14.
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