KING EDWARD MEMORIAL HOSPITAL

Seth Gordhandas Sunderdas Medical College

Acharya Donde Marg, Parel,
Mumbai 400 012. India.
Tel.: 91-22-2410 7000 Fax: 91-22-2414 3435

How Safe is Radiation in Pregnancy ?

Dr. Vijay Hanchate

Introduction :

X rays were discovered at the end of nineteenth century and almost immediately employed in medical diagnosis. Enormous benefits to human health have resulted. The detrimental effects of ionising radiation were recognised early on, with the result that the history of radiation hazards and protection is nearly as long as that of X rays themselves.

Medical irradiation is by far the largest man-made contribution to the radiation burden of the population; for example, a study of the average annual dose to the UK population reveals over 90% of radiation dose from artificial sources is due to medical examination.

Effects of Radiation :

Radiation is fundamentally a random process; it can never be predicted exactly which cells in the body will be effected. However, it would be natural to expect that at high doses more cells might be affected and therefore more damage might be done than at low doses – the severity of the damage and/or the frequency of the damaging events will depend on the dose.

Two terms have been introduced to classify the damage due to radiation through two different processes.

A. Non – Stochastic or Deterministic Effects :

Effects that occur only after a minimum dose has been received and then result in increasing severity of damage and increasing number of transformed cells. Since the word stochastic means random, the term non-stochastic can be taken to mean that effects are not random but quite predictable. To a certain degree the effects can be largely predicted for a particular individuals from the dose received.

Examples of deterministic effects are skin erythema and ulceration.

B. Stochastic Effects :

These effects, in contrast, take place even at very low dose levels, and where the number of cells transforming increases with increasing dose. These are chance events and cannot be predicted accurately in any one individual and can be quantified only in terms of probabilities derived from a study of a large affected populations. Thus the probability of radiation inducing leukemia in a person increases with increasing radiation. Cancer affecting the breast, lung, blood, thyroid and other organs are the main stochastic effects of radiation.

The effect of radiation during pregnancy special consideration since the developing embryo which is subjected to radiation forms a special case of “the innocent patient”.

It would be natural to assume that introducing irradiation into the complicated process of cell development in human fetus would produce some disruption. Special precautions are called for in dealing with a pregnant or potentially pregnant patient, this is because of both the increased risk of radiation damage to the developing fetus and the uncertainties in the clinical situation presented.

Effects for Fetal Irradiation :

Radiation exposure has harmful effects on the development of the fetus. Its severity is explained through the fact that the tissues that form the embryo have only a limited number of cells, so that the death of only a few of these can bring about irreparable damage.

The two principal effects are :

  1. Malformation and growth defects, particularly in organs which were developing at the time of irradiation.
  2. Cancer developing during childhood.

These effects are strongly dependent on three factors :

  1. Development Stage.
  2. Radiation Dose.
  3. Radiation Dose-Rate.

1. Development stage :

There are three significant periods in embryonic development which exhibit different degree of radio sensitivity. First, a rather short ‘preimplantation stage’ which lasts from fecundation to the fourteenth day (0-14 days). In this stage, the sensitivity of the embryo to radio-exposure is low. The embryo is ,in fact, composed of a population of cells which have not yet been sufficiently differentiated to repair radiation induced. The damaged cells, when in small numbers, are replaced by intact cells coming from new mitosis of the non-injured cells and the embryo develops normally. During this stage, therefore, radiation exposure will have no effect, or if the dose is high, will prevent implantation and the egg will be rejected.

The ‘organogenesis stage’ runs from 15th day to 50th day is characterised by sudden and marked increase in radiosensitivity. In this stage, certain amount of cellular differentiation has emerged. The injured cells, at this stage, differ from one another. A great many malformations and growth deficiencies may be produced during this period, continuing with lesser likelihood into the fetal stage. The particular type of malformation depends upon the organ system differentiated at the time of irradiation. Radiosensitivity is at its maximum between the third and the fourth week of gestation. Radioexposure even at low doses (0.1 Gy i.e.10 Rad) in this period can cause congenital malformation.

At the ‘fetal Stage’ of the development – the stage of simple growth, the embryo is less radio sensitive.

Time Table of Embryo/Fetal Stages :

Stage

Times in Days After Conception

Pre-implantation

0-14

Organogenesis

15-50

Fetal

51-280

 

Figure 1 illustrates schematically the sensitivity to various adverse effects following a fetal dose of 100 rems delivered at different times during gestation.

Figure 1

Principal Birth Defects :

 

ORGAN KIND OF BIRTH DEFECT
BRAIN
  • ANENCEPHALY
  • HYDROCEPHALUS
  • CEREBRAL ATROPHY
  • MENTAL RETARDATION
EYE
  • ANOPHTALMIA
  • MICROPHTALMIA
  • RETINOBLASTOMA
SKELETON
  • DWARFISM
  • CRANIOSYNOSTOSIS
  • SPINA BIFIDA
  • MALFORMATION OF THE EXTREMITIES

2. Radiation Dose :

The probability of malformation and growth retardation falls steeply as the dose is reduced. In its most recent review, the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) has proposed an upper limit of combined radiation risk for several fetal effects (mortality, induction of malformation, mental retardation and childhood cancer) of 3 chances per 100 children for each rem of fetal dose. The estimate for each, of these four effects separately would be somewhat lower. The normal fetal risk for these conditions in absence of radiation was estimated as 60 chances per 1000 children .

3. Radiation Dose Rate:

Reduction of dose-rate generally reduces or eliminates the likelihood of malformation or growth retardation.

Normal Risks for Irradiation in Utero for Absorbed Doses in the Embryo or Fetus :

Time for Conception Nominal Risks Per Milligray
First two weeks Minimal
3rd through 8th weeks Potential for malformation of organs
8th through 15th weeks Severe mental retardation 1 in 2,500
16th through 25th weeks Severe mental retardation in 10,000
Throughout pregnancy Childhood cancer 1 in 50,000

 

(These nominal risks do not take into account the possible presence of a threshold dose below which severe mental retardation would not occur).

Guidelines for Radiation protection in Pregnancy :

The majority of practicing physicians, at some time in their career, will be faced with a patient who has discovered in retrospect that she was pregnant at a time when extensive x-ray procedures were performed that involved the pelvis or lower abdomen. To make matters worse, as likely as not, the dose will have been delivered during early postconception weeks, because it is unlikely that a pregnancy would remain unsuspected beyond that time and this is the period in which the most disastrous consequences may result from absorption of a given dose of radiation.

Russell, in 1986, introduced the ‘ten day rule’ to prevent untoward effects of radiation in pregnancy. The ‘ten day rule’ states that radiological procedures in women of child bearing age that involve the abdomen or pelvis should only be carried out during first 10 days after a menstrual period of normal duration and intensity. This rule means that x-ray procedures on fertile women who present during the later half of the menstrual cycle should be deferred and an appointment made for after the start of the menstrual cycle to avoid the risk of irradiating an unsuspected concepts. Procedures to be confined to this ‘safe’ period include most nuclear medicine examinations and radiography or fluoroscopy of the abdomen, hip, lumbosacral spine etc.

Elective radiological procedures that should be subject to the 10-day rule include x-ray examinations before employment , annual check ups, and follow up examinations for a previous injury or illness or to monitor the course of a disease, such as a benign breast condition, which warrants ‘regular mammography Xray examinations of this kind, which monitor the patients’ health or existing disease, can certainly be delayed without prejudice until it is definite that the woman is not pregnant.

On other hand, delaying a procedure that is needed to evaluate a new symptom or diagnose a new disease is not justified if the delay could prove detrimental to the health or welfare of the patient. Considering the need for diagnostic test for the mother, the risk to the mother of waiting for the next period, according to the ten-day rule, would be greater than danger to the fetus. This is especially true if conception was found to have occurred, in which case the test should not be carried out al all. These were the considerations behind the 1984 ICRP revision of the rule which now involves no special limitation on exposure in the four weeks following menstruation.

Also in 1984, the National Radiological Protection Board (NRPB) of UK made the following recommendations :

  • That a woman who has had a missed or overdue period or if, in response to the question ‘are you, or might you be, pregnant?’ cannot answer definitely not, be treated as though she were pregnant. The question must be asked properly and a notice at reception is not sufficient, since language or reading difficulties might lead to its being ignored.
  • If a fetus is known to exist, then efforts have to be made to minimize the number of exposures and the dose from any exposure that directly irradiated the fetus, as long as diagnostic value of the test is not compromised.
  • Other x-ray procedures, such as x-rays of the chest, skull, and extremities, may be carried out at any time in the pregnancy if the fetus is properly shielded and the x-ray beam is properly collimated.

The ICRP in 1984 recommended that a pregnancy should be allowed to proceed if the embryo were exposed to less than 100mGy.

A total dose equivalent limit of 0.5 rem (5mSv) is recommended for the embryo or fetus. Once a pregnancy becomes known, exposure of the embryo or fetus shall be limited to less than 0.05 rem (0.5 mSv) in any month.

Female radiation workers who are pregnant should not exceed the dose limit to the surface of abdomen of 2mSv and a radionucliide intake of 1/20th of the annual limit.

All the means available for reducing doses should be used. These include :

In radiology : high voltages, short exposure times, diaphragm localizer, lead shielding of the abdomen, limitations on the number of films per examination, and number of examinations.
In nuclear medicine : Limitations of the administered activity, choice of radionucliides with short effective half life and pure X-ray emitters.

Procedure to be followed in the case of radio-exposure of the onset of pregnancies :

Doses in mGy

Extent of Pregnancy

Less than Ten Weeks Greater than Ten Weeks
Less than 1.5

No intervention

No intervention.
From 1.5 to 15 Discussion based on the desirability of pregnancy No intervention
From 15 to 50 Abortion No intervention except in the case of complications.
Above 50

Abortion.

Abortion

Absorbed radiation doses for typical examinations :

Examination Effective Dose (mSv/examination)
Simple X-ray :
Lumbar 2-40
Chest 0-05
Skull 0-11
Abdomen 1-40
Thoracic spine 0-90
Pelvis 1-80
Procedures :
IVU 4-20
Barium meal 3-40
Barium enema 7-90
Cholecystography 0-90
CT (head & body) 1-10
Nuclear medicine :
99 m Tc Bone 5
99 m Tc Liver 1
99 m Tc Brain 7
99 m Tc Lung perfusion 1
99 m Tc Kidney (DTPA) 3

Flow Chart for Deciding Whether to go in for an X Ray or not :

Flow Chart


Summary :

The risks of radiation effects on children before birth should be reduced by a policy that balances the necessity and benefit of performing the irradiation with the real risks to the developing child or fetus without causing any compromise of diagnostic study or any delay in diagnosis in mother.

There should be clear local policies on:

  1. Avoiding the irradiation of a pregnant or potentially pregnant woman, while still obtaining essential information.
  2. Keeping the dose limits in radiation as low as possible subject to the principle of ALARA i.e. as low as reasonably achievable.

Glossary:

  • Gray (Gy) & Rad: Gray and rad are units of absorbed dose and depend upon the nature.
  • of radiation and the properties of the absorbing body.
  • 1 Gray = 100 rads
  • Sieverts (Sv) and Rem are the units of dose equivalent. It is a product of the absorbed dose and the quality factor which for body tissues is one.
  • 1 Sievert = 100 rems

Suggested Reading:

  1. Russell, J.G.B. (1986) : The rise and fall of the ten-day rule BJR, 59, 3-6.
  2. BJR 1981, Aug. 54 (644) 697-8. Hazards and uses of prenatal diagnostic X radiation.
  3. ICRP 26 (1977) Recommendations of the ICRP. ICRP publication 26. Annals of ICRP 1 (3).
  4. ICRP, 39 (1989) Exposure of women to Ionising Radiation. ICRP Publication 39. Annals of the ICRP 14 (1).
  5. ICRP 49 (1986). Developmental effects of Irradiation on Bram of the Embryo and Fetus. ICRP Publication 49, Annals of ICRP 16 (4).
  6. Evic J. Hall; Radiobiology for the Radiologist; Third Edition. Copyright (c) 1988; by J.B. Lippmcott Company.
  7. Robert, Denis J.G.; Applied Radiobiology and Radiation Protection. First Edition 1990.
  8. Wootton R., Radiation protection of patients; first Edition (1993) Post Graduate Medical Science.

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