Is there a safe level of radiation exposure for a patient during pregnancy?
Dose boundaries do not apply for radiation exposure of patients, since the decision to use radiation is reasonable 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. Dose limits are determined only for the staff and not for patients.
What is the ten-day rule and what is its status?
It is important for radiology facilities to have procedures to determine the pregnancy status of female patients of reproductive age before any radiological procedure that could result in a significant dose to the embryo or foetus. The approach is not uniform in all countries and facilities. One approach is the ‘ten day rule,’ which 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. 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 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.
A conservative approach used by some radiology facilities is to apply a 10-day rule only for examinations with the potential to deliver a high dose to the lower abdomen and pelvis, such as barium enemas and CT of the abdomen or pelvis. These facilities use a 28-day rule for all other examinations.
Should pregnancy be terminated 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 counselling 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.
What if a patient underwent an abdomen CT before realizing that she is pregnant?
In some cases, a patient will not be aware of her pregnancy at the time of an X-ray examination. After realizing that she is pregnant, she will be concerned. The radiation dose to the foetus/conceptus should be estimated, but only by a medical physicist/radiation safety specialist experienced in this type of 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.