Pregnancy & Breastfeeding

Every study in a known or possible pregnancy needs justification by the practitioner, ideally with the referring team, and optimisation that does not compromise the diagnosis. A study that benefits the mother often benefits the fetus indirectly. For a lactating patient, choose the tracer and timing with milk excretion in mind, and give written, agent-specific instructions that are recorded in the notes.

Pregnancy: identify and justify

• UK: IR(ME)R requires every employer to have a written procedure for establishing pregnancy and breastfeeding status, with a stated age range (ARSAC gives 12–55 years as an example) and inclusive wording. The answers are documented.
• Graded approach for diagnostic studies (ARSAC): where the fetal dose would be below 10 mGy, ask whether the period is overdue and test if it is. Where it would exceed 10 mGy, perform the study only in the first 10 days of the menstrual cycle.
• Therapy: menstrual history alone is not enough; a pregnancy test is required and recorded, with reasonable exceptions such as intra-articular treatments.
• If a patient is or may be pregnant, the practitioner decides on justification with the multidisciplinary team; alternatives without ionising radiation are considered first.

Reducing fetal dose

• Use the lowest activity that keeps the diagnostic outcome, lengthening acquisition time rather than risking a non-diagnostic study.
• For renally excreted tracers, the maternal bladder is the main fetal dose source: encourage 1–2 L of extra fluid and frequent voiding for 1–2 days.
• Suspected pulmonary embolism: perfusion-only SPECT with 50 MBq ⁹⁹ᵐTc-MAA on day 1, with ventilation on day 2 only if needed; after the first trimester the standard 1-day protocol may be considered (EANM 2019). See PE in pregnancy.
• Radioiodide given before pregnancy was recognised: if it is found within 12 h, repeated oral potassium iodide (60–130 mg) to the mother may reduce the fetal thyroid dose.
• ¹³¹I therapy is absolutely contraindicated in pregnancy: fetal thyroid doses of several hundred gray can ablate the gland.

Breastfeeding: how to interrupt

• Before the study: express and store at least one feed if possible, and feed the infant just before the administration (ARSAC).
• At the next feed, or 3 h after administration if sooner, express as much milk as possible. Discard it, or freeze it and keep it until it has decayed. Continue expressing regularly during the interruption.
• Even where the interruption time is zero, expressing and discarding one feed is appropriate under ALARP.
• The published times assume intravenous injection and mature milk. In early lactation (colostrum), measure milk samples before resuming. Interstitial or eye-drop routes may need no interruption.
• Close contact matters as well as ingestion. UK guidance singles out patients who give most of the close care to a baby after more than 10 MBq ¹¹¹In-leucocytes, 150 MBq ²⁰¹Tl, 800 MBq ⁹⁹ᵐTc myocardial perfusion agent or 400 MBq ¹⁸F-FDG. ¹²³I should be given to a breastfeeding patient only if it is free of ¹²⁴I and ¹²⁵I.

Interruption by agent: three rule books

ICRP counts only the infant's ingestion dose. ARSAC adds external dose from close contact, which explains its longer times for pertechnetate and MAA. The NRC uses one 24-h period for every ⁹⁹ᵐTc agent to keep the advice simple, and asks for it only above the activity that triggers instructions.

Therapy and lactation

• Breastfeeding is contraindicated after therapeutic radiopharmaceuticals that enter milk; after ¹³¹I it must stop completely for the current child, but can resume after a later pregnancy.
• Even after breastfeeding stops, the lactating breast concentrates ¹³¹I-iodide: about 1.3 × 10⁻⁹ Sv/Bq, roughly 20 times the non-lactating breast. The ATA advises deferring therapy until at least 3 months after breastfeeding or pumping stops; involution of lactation can take more than 8 weeks, and a lactation specialist helps weigh the delay against lactation-suppressing drugs (ARSAC).
• See thyroid cancer for the wider radioiodine pathway.

Pitfalls

• Not asking about pregnancy or breastfeeding before the dose is drawn up.
• Forgetting the bladder as the main fetal dose source.
• Applying one generic interruption time, or quoting one country's table to a patient treated under another's rules.
• Missing the colostrum phase, when published interruption times do not apply.

Sources

1. International Commission on Radiological Protection. Pregnancy and medical radiation. ICRP Publication 84. Ann ICRP. 2000;30(1).
2. International Commission on Radiological Protection. Radiation dose to patients from radiopharmaceuticals: a compendium of current information related to frequently used substances. ICRP Publication 128. Ann ICRP. 2015;44(2S).
3. Leide-Svegborn S, Ahlgren L, Johansson L, Mattsson S. Excretion of radionuclides in human breast milk after nuclear medicine examinations: biokinetic and dosimetric data and recommendations on breastfeeding interruption. Eur J Nucl Med Mol Imaging. 2016;43:808–821.
4. Mattsson S, Leide-Svegborn S, Andersson M. X-ray and molecular imaging during pregnancy and breastfeeding — when should we be worried? Radiat Prot Dosimetry. 2021;195:339–348.
5. Administration of Radioactive Substances Advisory Committee. Notes for guidance on the clinical administration of radiopharmaceuticals and use of sealed radioactive sources. UK Health Security Agency; July 2026.
6. US Nuclear Regulatory Commission. Release of patients administered radioactive material. Regulatory Guide 8.39, Revision 1. 2020.
7. Bajc M, Schümichen C, Grüning T, et al. EANM guideline for ventilation/perfusion single-photon emission computed tomography (SPECT) for diagnosis of pulmonary embolism and beyond. Eur J Nucl Med Mol Imaging. 2019;46:2429–2451.
8. Haugen BR, Alexander EK, Bible KC, et al. 2015 American Thyroid Association management guidelines for adult patients with thyroid nodules and differentiated thyroid cancer. Thyroid. 2016;26:1–133.