Pregnancy and Radiation Protection in Diagnostic Radiology
- Is there a 'safe' level of radiation exposure for a patient during pregnancy?
- What is the 'ten-day rule' and what is its status?
- What if a patient underwent an abdomen CT before realizing that she is pregnant?
- How safe are radiological examinations of chest and extremities in pregnancy?
- Can cardiac catheterization be performed on a pregnant patient?
- Why have there been decisions on termination of pregnancy after radiation exposure?
- Can the patient become sterile after undergoing a diagnostic X ray examination?
- Can a pregnant employee continue to work in the X ray department?
- How high is the chance that a staff member will approach the dose limits of exposure?
1. Is there a 'safe' level of radiation exposure for a patient during pregnancy?
Dose limits do not apply for radiation exposure of patients, since the decision to use radiation is justified depending upon the individual patient situation. When it has been decided that a medical procedure is justified, the procedure should be optimized. This means that the conditions should achieve the clinical purpose with the appropriate dose. Safe limits are determined only for the staff and not for patients.
2. What is the ten-day rule and what is its status?
'Ten day rule' was postulated by ICRP for woman of reproductive age. It states that "whenever possible, one should confine the radiological examination of the lower abdomen and pelvis to the 10-day interval following the onset of menstruation." The original proposal was for 14 days, but this was reduced to 10 days to account for the variability of the human menstrual cycle. In most situations, there is growing evidence that a strict adherence to the "ten-day rule" may be unnecessarily restrictive. When the number of cells in the conceptus is small and their nature is not yet specialized, the effect of damage to these cells is most likely to take the form of failure to implant, or of an undetectable death of the conceptus; malformations are unlikely or very rare. Since organogenesis starts 3 to 5 weeks post-conception, it was felt that radiation exposure in early pregnancy couldn't result in malformation. The main risk is that of abortion if the radiation exposure results in death of the conceptus. It requires a foetal dose of more than 100 mGy for this to occur. Based on this, it was suggested to do away with the 10-day rule and replace it with a 28-day rule. This means that radiological examination, if so justified, can be carried throughout the cycle until a period is missed. Thus the focus is shifted to a missed period and the possibility of pregnancy. If there is a missed period, a female should be considered pregnant unless proved otherwise. In such a situation, every care should be taken to explore other methods of getting needed information by using non-radiological examinations.
3. What if a patient underwent an abdomen CT before realizing that she is pregnant?
Occasionally, a patient will not be aware of a pregnancy at the time of an X ray examination, and will naturally be very concerned when the pregnancy becomes known.
In such cases, the radiation dose to the fetus/conceptus should be estimated, but only by a medical physicist/ radiation safety specialist experienced in dosimetry. The patient can then be better advised as to the potential risks involved. In many cases there is little risk, as the irradiation will have occurred in the first 3 weeks following conception. In a few cases the conceptus will be older and the dose involved may be considerable. It is, however, extremely rare for the dose to be high enough to warrant advising the patient to consider terminating the pregnancy.
If a calculation of radiation dose is required in order to advise the patient, the radiographic factors should be noted if known. Some assumptions may be made in the dosimetry, but it is best to use actual data. The patient's date of conception or date of LMP (last menstrual period) should also be determined.
4. How safe are radiological examinations of chest and extremities in pregnancy?
Medically indicated diagnostic studies remote from the fetus (e.g. radiographs of the chest or extremities) can be safely done at any time during pregnancy if the equipment is in proper working order. Commonly, the risk of not making the diagnosis is greater than the radiation risk involved.
If an examination is typically at the high end of the diagnostic dose range and the fetus is in or near the radiation beam or source, care should be taken to minimize the dose to the fetus while still making the diagnosis. Tailoring the examination and examining each radiograph as it is taken until the diagnosis is achieved and then terminating the procedure can do this.
Effects on radiation exposure in utero
It is unlikely that radiation from diagnostic radiological examinations will result in any deleterious effects on the child, but the possibility of a radiation-induced effect cannot be entirely ruled out. The effects of exposure to radiation on the conceptus depend on the time of exposure with respect to the date of conception and the amount of absorbed dose. The following description is for scientific professionals and the effects described are to be seen only under the situations mentioned. It does not imply that these effects shall occur with doses encountered in common examinations, as they are quite small. For further details, please see ICRP 84.
- Prenatal doses from most properly done diagnostic procedures present no measurable increase in the risk of prenatal death, malformation, or the impairment of mental development over the background incidence of these entities. Higher doses, such as those involved in therapeutic procedures, can, however, result in significant foetal harm.
- There are radiation-related risks throughout pregnancy that are related to the stage of pregnancy and the foetal absorbed dose. Radiation risks are most significant during organogenesis and the early foetal period, somewhat less in the second trimester, and least in the third trimester.
- During the period of ±25 weeks post conception, the central nervous system (CNS) is particularly sensitive to radiation. Foetal doses in excess of about 100 mGy may result in a verifiable decrease of IQ. During the same time, foetal doses in the range of 1,000 mGy (1 Gy) result in a high probability of severe mental retardation. The sensitivity is highest 8±15 weeks post conception. The CNS is less sensitive to these effects at 16±25 weeks of gestational age and rather resistant after that.
- Radiation has been shown to cause leukemia and many types of cancer in both adults and children. Throughout most of pregnancy, the embryo/fetus is assumed to be at about the same risk for potential carcinogenic effects of radiation as are children.
5. Can cardiac catheterization be performed on a pregnant patient?
Yes. There will be many situations where the benefit of performing the procedure is much greater than any small possible harm that might arise from the radiation exposure. However, as always with any medical exposure, each particular procedure must be clinically justified, including in this situation taking into account when the procedure needs to occur and the anticipated radiation dose to the foetus. Once justified, due care is taken to optimise how the procedure is performed so as to minimise radiation exposure to the foetus, consistent with achieving the desired clinical outcome. The radiation exposure to foetus predominantly arises from scattered radiation within the patient. Some of the main methods for minimising the dose to the foetus include: restricting the X-ray beam size to being as small as is necessary for the clinical purpose; choosing the direction of the primary beam so that it is as far away from the foetus as possible; selecting appropriate exposure factors; and ensuring that the overall exposure time is as small as possible. For well performed procedures, estimated foetal doses are typically quite small, and well below the level of concern for radiation effects, but calculations of dose by a knowledgeable medical physicist is desirable. As a final comment, putting a lead apron on the table to cut down any primary beam from the X-ray tube reaching the foetus has very little effect, but it can be reassuring to the patient and staff and thus is recommended provided the use of the apron does not compromise the performance of the procedure.
6. Why have there been decisions on termination of pregnancy after radiation exposure?
According to ICRP 84, termination of pregnancy at foetal doses of less than 100 mGy is not justified based upon radiation risk. At foetal doses between 100 and 500 mGy, the decision should be based upon the individual circumstances.
The issue of pregnancy termination is undoubtedly managed differently around the world. It is complicated by individual ethical, moral, and religious beliefs as well as perhaps being subject to laws or regulations at a local or national level. This complicated issue involves much more than radiation protection considerations and require the provision of counseling for the patient and her partner.
At foetal doses in excess of 500 mGy, there can be significant foetal damage, the magnitude and type of which is a function of dose and stage of pregnancy.
For more information, consult ICRP 84.
7. Can the patient become sterile after undergoing a diagnostic X ray examination?
The gonads are radiosensitive organs in the human body. The threshold radiation dose for permanent sterility in men is 3500 - 6000 mGy, and for women 2500 - 6000 mGy. As diagnostic X ray examinations involve small doses (see Table 1), they imply no risk of sterility.
TABLE I. APPROXIMATE FOETAL DOSES FROM COMMON DIAGNOSTIC PROCEDURES IN THE UNITED
KINGDOM.
(Adapted from Sharp, Shrimpton, and Buiy, 1998).
| Conventional X ray examinations | Mean (mGy) | Maximum (mGy) |
|---|---|---|
| Abdomen | 1.4 | 4.2 |
| Chest | < 0.01 | < 0.01 |
| Intravenous urogram | 1.7 | 10 |
| Lumbar spine | 1.7 | 10 |
| Pelvis | 1.1 | 4 |
| Skull | < 0.01 | < 0.01 |
| Thoracic spine | < 0.01 | < 0.01 |
| Fluoroscopic examinations | Mean (mGy) | Maximum (mGy) |
| Barium meal (UGI) | 1.1 | 5.8 |
| Barium enema | 6.8 | 24 |
| Computed tomography | Mean (mGy) | Maximum (mGy) |
| Abdomen | 8.0 | 49 |
| Chest | 0.06 | 0.96 |
| Head | < 0.005 | < 0.005 |
| Lumbar spine | 2.4 | 8.6 |
| Pelvis | 25 | 79 |
8. Can a pregnant employee continue to work in the X ray department?
A pregnant worker can continue working in an X ray department as long as there is reasonable assurance that the foetal dose can be kept below 1 mGy during the pregnancy. In interpreting this recommendation, it is important to ensure that pregnant women are not subjected to unnecessary discrimination. There are responsibilities for both the worker and the employer. The first responsibility for the protection of the conceptus lies with the woman herself, who should declare her pregnancy to management as soon as the condition is confirmed. The following recommendations are taken from ICRP 84:
- Restricting dose to the conceptus does not mean that it is necessary for pregnant women to avoid work with radiation or radioactive materials completely, or that they must be prevented from entering or working in designated radiation areas. It does, however, imply that the employer should carefully review the exposure conditions of pregnant women. In particular, their working conditions should be such that the probability of high accidental doses and radionuclide intakes is insignificant.
- When a medical radiation worker is known to be pregnant, there are three options that are often considered in medical radiation facilities: 1) no change in assigned working duties; 2) change to another area where the radiation exposure may be lower; or 3) change to a job that has essentially no radiation exposure. There is no single correct answer for all situations, and in certain countries there may even be specific regulations. It is desirable to have a discussion with the employee. The worker should be informed of the potential risks, local policies, and recommended dose limits.
- Change to a position where there is no radiation exposure is sometimes requested by pregnant workers who realize that risks may be small but do not wish to accept any increased risk. The employer may also arrange for this in order to avoid future difficulties in case the employee delivers a child with a spontaneous congenital abnormality (which occurs at a rate of about 3 in every 100 births). This approach is not required on a radiation protection basis, and it obviously depends on the facility being sufficiently large and flexibility to easily fill the vacated position.
- Changing to a position that may have lower ambient exposure is also a possibility. In diagnostic radiology, this may involve transferring a technician from fluoroscopy to CT scanning or some other area where there is less scattered radiation to workers. In nuclear medicine departments, a pregnant technician can be restricted from spending a lot of time in the radiopharmacy or working with radioiodine solutions. In radiotherapy with sealed sources, pregnant technicians or nurses might not participate in manual brachytherapy.
- An ethical consideration is involved in both of these last two alternatives since another worker will have to incur additional radiation exposure because a co-worker became pregnant.
- There are many situations in which the worker wishes to continue doing the same job, or the employer may depend on her to continue in the same job in order to maintain the level of patient care that the work unit is customarily able to provide. From a radiation protection point of view, this is perfectly acceptable providing the foetal dose can be reasonably accurately estimated and falls within the recommended limit of 1 mGy foetal dose after the pregnancy is declared. It would be reasonable to evaluate the work environment in order to provide assurance that high-dose accidents are unlikely.
- The recommended dose limit applies to the foetal dose and it is not directly comparable to the dose measured on a personal dosimeter. A personal dosimeter worn by diagnostic radiology workers may overestimate foetal dose by about a factor of 10 or more. If the dosimeter has been worn outside a lead apron, the measured dose is likely to be about 100 times higher than the foetal dose. Workers in nuclear medicine and radiation therapy usually do not wear lead aprons and are exposed to higher photon energies. In spite of this, foetal doses are not likely to exceed 25 percent of the personal dosimeter measurement.
- Finally, factors other than radiation exposure should be considered in evaluating pregnant workers' activities. In a medical setting there are often requirements for lifting patients and for stooping or bending below knee level. There are a number of national groups that have established non-radiation related guidelines for such activities at various stages of pregnancy.
- Occasionally, there are situations where family members provide essential medical care, either in the hospital or at home, to patients who have received radionuclides. In such circumstances, public dose limits do not apply to the family member. Efforts should optimally be directed at not involving females who are or may potentially be pregnant. If it is essential to involve the help of a pregnant female, it should be done in such a way that the foetal dose from this involvement does not exceed 1 mGy.
9. How high is the chance that a staff member will approach the dose limits of exposure?
Radiation doses to occupationally exposed staff working with radiological equipment are generally low and it is unlikely that the equivalent dose limit recommended by the ICRP (see Table II) will be approached. However, for some fluoroscopic examinations there is a potential for higher radiation doses to staff. During interventional radiology procedures, particular radiation protection problems arise from the extended fluoroscopy times and from the use of certain radiological equipments, which may not have lead rubber protective curtains. Consequently, the implications of the ICRP recommendations on the radiation exposure of the fetus of staff performing fluoroscopy procedures should be assessed.
TABLE II. ICRP 60 RECOMMENDATIONS
| Assumed radiation risks | ICRP Publication 60 |
|---|---|
| Workers | 4.0 x 10-2 Sv-1 for fatal cancer 0.8 x 10-2 Sv-1 for non-fatal cancer detriment 0.8 x 10-2 Sv-1 for severe genetic effects |
| Members of the public | 5.0 x 10-2 Sv-1 for fatal cancer 1.0 x 10-2 Sv-1 for non-fatal cancer 1.3 x 10-2 Sv-1 for severe genetic effects |
| Embryo-foetus | Not specifically stated |
| Occupational dose limits | ICRP Publication 60 |
| Based on stochastic effects | 50 mSv annual effective dose limit and 100 mSv in 5 y cumulative effective dose limit |
| Based on deterministic effects | 150 mSv equivalent dose to lens of eye and 500 mSv annual equivalent dose limit to the skin, hands and feet |
| Public dose limits | ICRP Publication 60 |
| Based on stochastic effects | 1 mSv annual effective dose limit and, if needed, higher values provided that the annual average over 5 y does not exceed 1 mSv |
| Based on deterministic effects | 15 mSv annual effective dose limit to lens of eye and 50 mSv annual equivalent dose limit to skin, hands, and feet |
Counseling patients on the dose and risk assessment procedures
Patients who have received diagnostic studies while pregnant are often alarmed because of emotional perceptions surrounding radiation. The health professionals should advise patients about the steps that will be taken for risk assessment and provide appropriate information regarding the risk associated with diagnostic (and therapeutic) radiation exposure during pregnancy. The following points should be considered:
- It is unlikely that radiation from diagnostic radiological examinations will result in any deleterious effects on the child, but the possibility of a radiation-induced effect cannot be entirely ruled out.
- The patient should be counseled that the risk assessment is being done not because there is reason to believe there is great risk in her circumstance but because it is one of the precautions normally taken whenever a pregnant woman receives certain diagnostic studies. (Note: this applies only to diagnostic studies. The risk from therapeutic studies may be severe, such as foetal thyroid ablation.)
- Each case must be assessed according to the gestational age when exposed and the radiation levels received by the conceptus from each exposure.
- A precise foetal dose assessment requires numerous pieces of information about the X ray system, the examinations conducted, the patient size, etc. Therefore, 'typical' foetal dose numbers should be used with the understanding that there may be a significant difference between the 'typical' dose numbers and the dose numbers resulting from an actual dose assessment.
- The dose evaluation may take up to a week to complete.
- When all the information is acquired, the radiation risk will be assessed and will be reviewed along with other possible risks of pregnancy so that the physician, the patient, and other involved persons understand the circumstances and can thus make a reasonable decision regarding the management of the pregnancy.
References
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