In women of childbearing age, the possibility of pregnancy and the justification for the examination should be considered. The recommended precautions to prevent or minimise irradiation of the fetus include the following [ICRP 84]:
IF IT IS POSSIBLE THAT YOU MIGHT BE PREGNANT, NOTIFY THE PHYSICIAN OR TECHNICIAN BEFORE RECEIVING ANY RADIOACTIVE MATERIAL.
As with diagnostic radiology procedures, the pregnant patient can be apprehensive after a procedure has been performed. In the case of nuclear medicine, the patient may be even more apprehensive, realizing that an administered radioactive material has been incorporated into her body, that it will be there for some time and that it potentially may cross the placenta to the fetus. As a result of this, more careful explanation to the patient and her husband or other appropriate persons may be needed to put the potential radiation risks into perspective. In contrast to diagnostic radiography examinations, foetal doses in nuclear medicine depend predominantly upon administered activity, and they are independent of the imaging equipment.
Yes. Pregnancy need not be considered a contradiction for nuclear medicine procedures, in particular diagnostic ones involving short lived radionuclides, provided there are strong clinical justifications and effort has been made to explore alternatives involving non-ionizing radiation.
Since radionuclides in maternal tissues contribute to foetal dose, maternal hydration and frequent voiding can reduce the foetal dose after the administration of a number of radiopharmaceuticals.
Irradiation of the foetus results from placental transfer and distribution of radiopharmaceuticals in the foetal tissues, as well as from external irradiation from radioactivity in the mother's organs and tissues. The physical, chemical, and biological properties of the radiopharmaceuticals are the critical factors in possible placental transfer.
Using smaller administered activities and longer imaging times can reduce the absorbed dose to the foetus. This is feasible if the patient is not too sick and is able to remain still. Occasionally, the sequence of the examinations and choice of radiopharmaceutical can be adjusted to reduce radiation dose.
Typical foetal doses for common radiopharmaceuticals are presented in the following table.
|Radiopharmaceutical||Procedure||Administered activity (MBq)||Early pregnancy (mGy)||Nine months (mGy)|
|99mTc||Bone scan (phosphate)||750||4.6-4.7||1.8|
|99mTc||Lung perfusion (MAA)||200||0.4-0.6||0.8|
|99mTc||Lung ventilation (aerosol)||40||0.1-0.3||0.1|
|99mTc||Thyroid scan (pertechnetate)||400||3.2-4.4||3.7|
|99m||Red blood cell||930||3.6-6.0||2.5|
1) Foetal thyroid doses are much higher than foetal whole body dose, viz. 5-15 mGy/MBq for 123I and 0.5-1.1 Gy/MBq for 131I.
As a rule, a pregnant woman should not be treated with a radioactive substance unless the radionuclide therapy is required to save her life: in that extremely rare event, the potential absorbed dose and risk to the foetus should be estimated and conveyed to the patient and the referring physician. Considerations may include terminating the pregnancy.
Radioiodine therapy is essentially contra-indicated in patients who are known to be pregnant. Radioiodine easily crosses the placenta and the foetal thyroid begins to accumulate iodine at about 10 weeks of gestational age. If radioiodine treatment of thyroid carcinoma is to be performed, it should be delayed until after delivery. If this is done, the physician should also be aware that radioiodine is excreted in breast milk and breast-feeding should be stopped completely after a therapeutic dose. If this is not done the infant may become permanently hypothyroid or be at high risk for subsequent thyroid cancer.
In women, thyroid carcinoma comprises over 80% of cancer of the head and neck diagnosed between the ages of 15-45 years. Thyroid cancers are relatively non-aggressive compared to most other cancers. As a result both surgical and radio-iodine treatment are often delayed until after pregnancy. In general, if any therapy is to be performed during pregnancy, it will be surgery during the second or third trimester.
A major problem occurs when a female, who is not thought to be pregnant, is treated for thyroid carcinoma and is found to be pregnant after the administration of radioiodine. Menstrual history is often not adequate to ensure that a patient is not pregnant. In most developed countries, it is common practice to obtain a pregnancy test prior to high-dose 131I scanning or therapy for women of childbearing age unless there is a clear history of prior tubal ligation or hysterectomy precluding pregnancy. In spite of the above, it still happens that pregnant women are treated, either because of false histories or because the pregnancy is at such an early stage that the pregnancy test is not yet positive.
Most commonly, the pregnancy is early and the major problem is foetal whole body dose due to gamma emissions from radio-iodine in the maternal bladder. During pregnancy, the whole body dose to the conceptus is in the range of 50-100 mGy/GBq of administered activity. This dose can be reduced by hydrating the patient and by encouraging frequent voiding.
If the conceptus is more than 8 weeks post conception (and the foetal thyroid may accumulate iodine) and the pregnancy is discovered within 12 hours of iodine administration, giving the mother 60-130 mg of stable potassium iodide (KI) will partially block the foetal thyroid and reduce thyroid dose. Twelve hours after radio-iodine administration, this intervention is not very effective.
Patients treated with radioiodine can be a significant radiation source to pregnant family members. The dose to a family member staying at a distance of 0.5 meters from a patient until the radioactivity totally decays (about 10 weeks) is about 1.3 mGy from a hyperthyroid patient and 6.8 mGy from a thyroid cancer patient. Also, these patients must be careful not to transfer radioiodine contamination to family members by direct or indirect means.
Most female patients are advised not to become pregnant for at least 6 months after radiotherapy with radioiodine. This is not based upon potential heritable radiation effects, but rather upon the need to be sure (1) that the hyperthyroidism or cancer is controlled, and (2) that another treatment with radioiodine is not going to be needed when the patient is pregnant. It is also based upon the fact the ICRP has recommended that enough radio-iodine be cleared to ensure that the unborn child does not receive a dose in excess of 1 mGy unless it is medically necessary for the health of the mother. There are occasional circumstances in which 32P, 89Sr, or 131I meta-iodobenzylguanidine are used for therapy. In order to keep the dose to the foetus below 1 mGy, pregnancy should be avoided for 3, 24, and 3 months respectively.
For most diagnostic procedures, there would be no need for pregnant staff to take any additional precautions other than limiting their direct contact to as short as necessary. Because the exposure from patients who have been administered radiopharmaceuticals is quite low, there is no radiological reason not to continue imaging procedures. Moreover, the International Basic Safety Standards require that "notification of pregnancy shall not be considered a reason to exclude a female worker from work" [BSS I.17]. If the institution administers therapeutic activities of 131I for thyroid cancer, should be considered refraining from this particular part of the practice. Not only that there are higher exposures associated with the administration of 131I, but also iodide is volatile, increasing the potential for internal uptake.
Some radiopharmaceuticals cross the placenta freely, e.g., radioactive iodides, and are taken up in foetal tissues, where they irradiate the tissues. Some analogues of natural metabolites (e.g., radiostrontium for calcium and radiocaesium for potassium) are less readily transferred. Radiopharmaceuticals that are retained by the mother, and do not cross the placenta (e.g., radiocolloids), only act as external sources of irradiation to the foetus.
In the case of radiopharmaceuticals that are rapidly eliminated by the maternal kidneys, the urinary bladder, acting as a reservoir, is a major source of foetal irradiation. After the administration of such radiopharmaceuticals, maternal hydration and frequent voiding should, therefore, be encouraged. For those radiopharmaceuticals that have gastrointestinal excretion however, administration of laxatives is only rarely helpful in reducing foetal dose.
A typical example of sequence is a ventilation perfusion lung scan ordered on a pregnant patient to exclude a pulmonary embolus. In routine operation, many laboratories will perform the ventilation scan first and then do the perfusion scan. This has advantages in some situations. In the specific case of a suspected pulmonary embolus, the perfusion scan can be performed first, and if the result is normal, a ventilation scan is not needed at all.
The choice of radiopharmaceuticals for the ventilation portion of the lung scan can also affect foetal dose. If the scan is performed with 133Xe gas, there is very little foetal dose; however, one can also do ventilation scans using 99mTc-DTPA aerosol. This will be absorbed and excreted via the kidneys, and while in the bladder it will contribute to foetal dose.