Protecting pregnant patients
Early pregnancy can go undetected, so it is prudent to ensure that patients of childbearing potential are not pregnant before undergoing diagnostic radiology or nuclear medicine studies that provides doses above which the risk of adverse fetal health effects is not considered negligible (1–10 mGy) (). Before Radiation therapy, and in the absence of a documented history of applicable gynaecological surgery (e.g. tubal ligation, hysterectomy) or an established postmenopausal state, serum or urine pregnancy tests should be obtained, ideally 24–72 h prior to treatment ().
There are radiation-related risks to the embryo/fetus during pregnancy that are related to the stage of pregnancy and the absorbed dose to the embryo/fetus. ICRP has evaluated the effects of prenatal irradiation in detail (ICRP Publication 90 Biological Effects after Prenatal Irradiation (Embryo and Fetus)). These effects include lethal effects, malformations, central nervous system effects, leukaemia and childhood cancer. Consideration of these effects is important when pregnant patients undergo diagnostic radiology, interventional procedures, or radiation therapy using ionising radiation. A balance must be attained between the health care of the patient and the potential for detrimental health effects to the embryo/fetus that accompanies the specific radiological procedure (ICRP Publication 84).
For diagnostic radiology, referring physicians should have discussions with imaging specialists to help determine the best tests for their pregnant patients, taking into account non-ionising diagnostic imaging modalities, such as ultrasound or magnetic resonance imaging. If these modalities are not the most appropriate test based on the clinical scenario, or are not available, the most appropriate ionising radiation imaging modality should be utilised in such a way as to achieve the diagnosis required while keeping fetal and maternal doses as low as reasonably possible. Almost always, if a diagnostic radiology examination is medically indicated, the risk to the mother of not performing the procedure is greater than the risk of potential harm to the embryo/fetus.
If the examinations are justified and their performance is optimised, they should not be withheld from pregnant patients. Fetal dose reduction measures will vary depending on the specific test being administered, but may include reducing the dose of injected radiopharmaceutical, limiting the number of images performed, beam collimation, patient shielding, and ensuring that the maternal pelvis (and fetus) is not in the beam path during fluoroscopic procedures unless it is absolutely necessary.
A pregnant patient has a right to know the magnitude and type of potential radiation effects that may result from in-utero exposure. Benefits and risks of the examination should be discussed with the patient, including developmental and cancer risks to the unborn baby. Central nervous system malformations and intellectual deficits have only been reported at fetal doses >100 mSv. Fetal doses from routine medical imaging are generally well below 100 mSv, and are generally below 20 mSv ().
radiation therapy (whether with external beam, brachytherapy, or nuclear medicine) can involve high radiation doses and may cause harm to the fetus. The risk to the fetus is dependent on the stage of fetal development. Lethal risk occurs in the pre-implantation phase, malformation occurs during major fetal organogenesis 3–7 weeks post implantation, mental retardation occurs at 8–15 weeks post implantation, and future cancer risk follows a stochastic model. Although there is variability in threshold radiation doses, it can be generalised that risks become notable at fetal doses of 100 mGy (10 rad) or above. So, it is essential to ascertain whether a female patient is pregnant prior to radiation therapy. In pregnant patients, cancers that are remote from the pelvis can usually be treated with radiation therapy. However, this requires careful planning. Cancers in the pelvis cannot be treated adequately during pregnancy without severe or lethal consequences for the embryo/fetus. radiation therapy in pregnant patients requires pre-therapy fetal dosimetry estimation followed by a comprehensive discussion of the benefits and risks of the procedure, with the patient included as part of the informed consent process.
It should also be noted that 131I used for diagnostic or therapeutic purposes and 32P used for therapeutic purposes, should be avoided in pregnant patients because iodine and phosphor can cross the placental barrier readily. The fetal thyroid is sufficiently mature to concentrate iodine at approximately 10 weeks post implantation, and there is a risk of causing fetal hypothyroidism (ICRP Publication 84 Pregnancy and Medical Radiation). As a rule, a pregnant patient 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 fetus should be estimated and conveyed to the patient and the referring Physician. Considerations may include terminating the pregnancy (ICRP Publication 84 Pregnancy and Medical Radiation).
Termination of pregnancy is an individual decision affected by many factors. Absorbed doses below 100 mGy to the developing embryo/fetus should not be considered a reason for terminating a pregnancy. At doses to the embryo/fetus above this level, informed decisions should be made based upon individual circumstances, including the magnitude of the estimated dose to the embryo/fetus, and the consequent risks of harm to the developing embryo/fetus and risks of cancer in later life.