Breastfeeding Interruption After Radiopharmaceuticals

Some radiopharmaceuticals are secreted into breast milk, so a nursing infant can receive an internal radiation dose by ingestion; breastfeeding interruption recommendations are chosen to keep that infant's effective dose below about 1 mSv, following NRC Regulatory Guide 8.39 and ICRP Publication 106. The required action ranges from no interruption for most Tc-99m agents and F-18 FDG, to a defined interruption of hours to weeks for agents such as Tc-99m pertechnetate, Ga-67, and Tl-201, to complete cessation for that child after I-131 sodium iodide.¹⁴⁶

This is a distinct radiation-safety question from general patient release limits and from close-contact external exposure. It requires the authorized user, medical physicist, and radiation safety officer to combine radionuclide-specific biokinetics with the regulatory instruction and recordkeeping requirements of 10 CFR 35.75.²

Introduction

Lactation interruption after nuclear medicine is a radionuclide-specific dose assessment, not a one-size-fits-all "wait 24 hours" rule. Whether a nursing mother must interrupt breastfeeding, and for how long, depends on how much of the administered activity is secreted into milk, how quickly it clears, and the committed effective dose the ingested activity would deliver to the infant.⁵⁶

Two facts drive the topic. First, radiopharmaceuticals differ enormously in the fraction that reaches milk, so the infant effective dose per unit administered activity spans more than seven orders of magnitude across agents.⁶ Second, the milk pathway is separate from the external, close-contact pathway: a mother can be cleared to hold her infant while still being advised to interrupt breastfeeding, or the reverse, depending on the radionuclide.⁹

This guide explains why certain radiopharmaceuticals appear in milk, the 1 mSv infant-dose framework that sets interruption times, radionuclide-specific guidance anchored to Regulatory Guide 8.39 and ICRP Publication 106, the difference between the ingestion and external-dose problems, and the documentation and RSO responsibilities that make the practice defensible. DRPS supports nuclear medicine programs on exactly these questions as part of its PET/CT and nuclear medicine physics and radiation safety officer services across Florida, Maryland, Virginia, Washington DC, California, and Nevada.

Topic Explanation

Why do radiopharmaceuticals appear in breast milk?

A radiopharmaceutical reaches breast milk when the radionuclide or its labeled molecule distributes into the lactating breast and is secreted into milk, or when a free radionuclide separates from its carrier molecule and follows the body's normal handling of that element. The two most important mechanisms are direct secretion of small, soluble species and in-vivo breakdown of the radiolabel.⁵

Free ionic species are the classic problem. Iodide as I-131 or I-123 sodium iodide, and free Tc-99m pertechnetate, behave chemically like the body's own iodide and pertechnetate: both concentrate in the mammary gland and are secreted into milk. Radioiodide is then further concentrated in the infant thyroid after ingestion, which is why radioiodine dominates the high end of infant-dose estimates.⁴⁵⁶

Labeled compounds vary. A tightly bound, rapidly cleared agent such as a Tc-99m diphosphonate bone tracer delivers very little activity to milk, whereas an agent that releases free pertechnetate in vivo, or that distributes into the breast, delivers more. This is why two Tc-99m agents can carry very different recommendations even though they share the same radionuclide and 6-hour physical half-life.⁶

Understanding the agent, not just the isotope, is the core of the assessment. For background on how these tracers are chosen and dosed, see our overview of pediatric nuclear medicine dosing and radiopharmacy aseptic technique and QC.

What is breastfeeding interruption meant to prevent?

Interruption is meant to prevent the nursing infant from receiving an internal committed effective dose above the accepted objective from ingesting activity in milk. It is a control on the ingestion pathway. The infant swallows milk containing the radionuclide; the radionuclide is absorbed, distributes, irradiates tissues, and decays. Interruption removes the most active early milk from the feeding schedule so that only lower-activity milk is consumed after decay and biological clearance have reduced the concentration.⁵⁶

The separate close-contact pathway is external: photons emitted from the mother's body irradiate the infant while she holds, feeds, or sleeps near the child. That pathway is not solved by discarding milk; it is solved by time and distance. Keeping the two pathways distinct avoids two common errors: assuming that "safe to hold the baby" means "safe milk," and assuming that a milk interruption also handles the external dose. It often does not.⁹

Key Technical Principles

The 1 mSv infant-dose framework

Breastfeeding interruption recommendations are constructed to keep the nursing infant's committed effective dose below approximately 1 mSv, using radionuclide-specific measurements of activity secreted into milk. NRC Regulatory Guide 8.39, Revision 1 (issued April 2020) states that the duration of interruption is selected to reduce the dose to a nursing infant to less than 1 mSv (0.1 rem), while noting that the regulatory limit under the patient-release rule is 5 mSv (0.5 rem); the physician may adjust the interruption as long as the infant dose stays below the regulatory limit.¹² ICRP Publication 106 and the peer-reviewed dosimetry behind it use the same 1 mSv infant objective.⁴⁷

The infant committed effective dose from the ingestion pathway can be written as the ingested activity multiplied by an age-appropriate dose coefficient:

where $A_{\text{ingested}}$ is the activity taken in through milk (MBq) and $e(g)$ is the ingestion committed effective dose coefficient for the infant age group (mSv/MBq). In practice, the published series report a combined coefficient that folds the milk-excretion fraction, feeding schedule, and infant biokinetics into a single effective dose to the infant per unit activity administered to the mother, $e_{\text{m}}$ (mSv per MBq administered):

Leide-Svegborn and colleagues measured $e_{\text{m}}$ ranging from about $6.7\times 10^{-6}$ mSv/MBq for Tc-99m-labeled red blood cells to about $3.6\times 10^{-2}$ mSv/MBq for Tc-99m pertechnetate, and up to roughly $106$ mSv/MBq for I-131 sodium iodide — a span that explains why recommendations range from none to complete cessation.⁶

Radionuclide-specific interruption recommendations

The table below summarizes recommended breastfeeding actions. Values are anchored to NRC Regulatory Guide 8.39, Revision 1 and ICRP Publication 106, with peer-reviewed dosimetry from Leide-Svegborn et al. (2016). Interruption periods are activity-dependent: below an agent-specific administered-activity threshold, Regulatory Guide 8.39 indicates no instruction is required, and above it the listed action applies. Confirm the current recommendation and the specific administered activity for each patient.¹⁴⁶⁸

Two structural points matter. First, ICRP Publication 106 groups its timed recommendations: roughly three weeks after I-125 compounds (except labeled hippurate), Na-22, Ga-67, and Tl-201; about 12 hours after iodine-labeled hippurates and most Tc-99m compounds; and about 4 hours after Tc-99m red blood cells, phosphonates, and DTPA — with complete cessation for I-131 sodium iodide.⁴ Second, US practice under Regulatory Guide 8.39 often applies a single 24-hour interruption for Tc-99m agents when an instruction is triggered, rather than an agent-by-agent value.¹ Where the two frameworks differ, use the more conservative recommendation and document the basis.

Worked example: interruption interval for free Tc-99m pertechnetate

Consider a lactating patient administered free Tc-99m pertechnetate. Using the Leide-Svegborn effective-dose coefficient for pertechnetate, the infant dose with no interruption (feeding resumed immediately) is:

This exceeds the 1 mSv objective, so an interruption is warranted. The activity available in milk falls with an effective decay constant ${\lambda }_{\text{eff}}$ that combines Tc-99m physical decay ($T_{1/2}=6.0$ h) with biological clearance. Modeling the decline as exponential, the interruption interval that brings the projected infant dose down to the target $E_{\text{target}}=1$ mSv is:

Taking an illustrative effective half-life of $T_{\text{eff}}\approx 3.5$ h for the milk activity (bracketed by the 6.0 h physical half-life and faster biological clearance):

This illustrative result of roughly 12 to 24 hours brackets the anchored recommendations: about 12 hours from ICRP Publication 106 and Leide-Svegborn et al., and a single 24-hour interruption under Regulatory Guide 8.39's generic Tc-99m guidance.¹⁴⁶ The administered activity and coefficient here are anchored, while $T_{\text{eff}}$ is framed as illustrative; a patient-specific calculation should use measured or agent-specific clearance data rather than a generic effective half-life. Individual milk measurements are encouraged when a case is borderline, because measured excretion fractions vary substantially between patients.⁵⁶

Clinical Impact

The practical consequence of getting lactation guidance right is that the infant's dose stays below the objective while the mother avoids an unnecessarily long, distressing interruption or an unnecessary permanent weaning. Both over-restriction and under-restriction are real failure modes.⁵⁷

Over-restriction happens when a facility applies a blanket "stop breastfeeding" or a long interruption to agents that need none. For most Tc-99m bone, cardiac, and renal studies and for F-18 FDG, the milk itself is not the concern, and an unnecessary multi-day interruption can compromise supply, raise engorgement and mastitis risk, and push a mother toward premature weaning for no dosimetric benefit.⁶⁹ In the published series behind current guidance, only a minority of the radiopharmaceuticals studied required any milk interruption at all.⁵⁶

Under-restriction is the more serious safety failure. Missing the I-131 sodium iodide case is the clearest example: because radioiodide concentrates in the infant thyroid and the effective dose per unit activity is very high, continued breastfeeding after I-131 therapy could deliver a substantial thyroid and effective dose to the infant. Therapeutic I-131 sodium iodide therefore calls for complete cessation for that child, not a timed pause.⁵⁶

The close-contact pathway adds a parallel obligation for penetrating photon emitters. After F-18 FDG the milk is generally acceptable, yet product labeling and radiation-safety guidance advise limiting prolonged close contact for a few hours to reduce the infant's external dose from 511 keV annihilation photons.⁹¹⁰ A mother told only "your milk is fine" may still cuddle the infant continuously and receive an avoidable external dose. Programs that treat these as one instruction get one of them wrong. For the external-dose side of nuclear medicine practice generally, see our guide to occupational exposure monitoring.

Practical Optimization Tips

Screen for lactation before administration

The single most effective control is asking. Every female patient of childbearing potential scheduled for a radiopharmaceutical should be screened for current breastfeeding before dosing, in the same workflow that screens for pregnancy. If the patient is lactating, the authorized user and medical physicist should confirm the agent-specific recommendation before the dose is drawn, while substituting a lower-excretion agent or deferring an elective study is still possible.¹⁷

Match the instruction to the agent and activity

• Identify the exact radiopharmaceutical and chemical form, not just the isotope. Free pertechnetate and a Tc-99m diphosphonate are not the same lactation problem.⁶
• Compare the administered activity to the Regulatory Guide 8.39 threshold for that agent; below the threshold no instruction may be required, above it apply the listed interruption.¹
• For I-131 sodium iodide, recommend complete cessation for that child; do not offer a short timed interruption.¹⁴
• Where US (RG 8.39) and international (ICRP 106) values differ, choose the more conservative period and document why.¹⁴

Manage pumping-and-discard logistics

Advise the mother to express milk during the interruption to maintain supply and comfort, discarding the expressed milk (or storing it for decay only if a written facility protocol permits, which is practical only for very short-lived nuclides). Explain clearly that pumping does not speed decay — it protects lactation, not the infant's dose. Provide a concrete restart time, and where clinically indicated, offer the option of measuring milk activity before resuming to confirm the projected dose is acceptable.⁵⁶

Address close contact separately

For photon-emitting agents such as F-18 FDG, give a distinct close-contact instruction (limit prolonged holding for a defined period) alongside the milk instruction. State plainly which pathway each instruction addresses so the mother is not confused into treating one as the other.⁹

Common pitfalls to avoid

• Treating every Tc-99m agent alike. Free pertechnetate and MAA warrant interruption; most bound, rapidly cleared agents do not.⁴⁶
• Applying a timed interruption to therapeutic I-131 sodium iodide. This case is cessation for that child.¹⁶
• Confusing the milk instruction with the close-contact instruction. They control different pathways.⁹
• Assuming pumping accelerates the safe-to-feed time. It maintains supply only.⁵
• Skipping the record. The release record must capture that breastfeeding instructions were given when required.²

Regulatory Considerations

Breastfeeding interruption sits inside the NRC patient-release framework: 10 CFR 35.75 requires instructions, and in some cases records, whenever a released patient's activity could deliver more than the stated dose thresholds to others, including a nursing infant. The medical physicist and RSO translate the dosimetry into compliant instructions.²

Key frameworks:

• 10 CFR 35.75 — Release of individuals containing unsealed byproduct material. A licensee may release a patient if the dose to any other individual is not likely to exceed 5 mSv (0.5 rem). The licensee must provide written instructions when the dose to another individual is likely to exceed 1 mSv; and specifically, if the dose to a nursing infant or child could exceed 1 mSv without interruption, the instructions must include guidance on interruption or discontinuation of breastfeeding and the consequences of not following it.²
• 10 CFR 35.75 recordkeeping (10 CFR 35.2075). A record that breastfeeding instructions were provided must be retained when the dose to the infant from continued breastfeeding could exceed 5 mSv (0.5 rem).²¹²
• NRC Regulatory Guide 8.39, Revision 1 (April 2020). Provides the accepted methodology and the breastfeeding-interruption table (Table 3) keyed to the 1 mSv infant objective, and updated patient-instruction guidance.¹
• 10 CFR Part 20 — Standards for Protection Against Radiation. Sets the public-dose framework and the ALARA philosophy that motivate the more protective 1 mSv infant objective relative to the 5 mSv release ceiling.³
• 10 CFR Part 35 — Medical Use of Byproduct Material. Governs the authorized user, the RSO, and the written-directive and release program within which these instructions are issued.¹¹
• ICRP Publication 106. The international dosimetric basis for the agent-specific interruption periods, harmonized with the 1 mSv infant objective.⁴

Agreement States administer equivalent programs. Of the states DRPS serves, Florida, Maryland, Virginia, California, Nevada, Pennsylvania, New York, and New Jersey are NRC Agreement States that license medical use under their own radiation-control rules, while Washington, DC and Delaware are regulated directly by the NRC. A facility should confirm which authority issues its license and whether the state has adopted the federal release provisions or its own variant before relying on any interruption value. For related worker-facing radioiodine practice, see thyroid bioassay for I-131 workers, and for the pregnancy analog, managing the pregnant radiation worker.

Frequently Asked Questions (FAQs)

Do all radiopharmaceuticals require breastfeeding interruption?

No. Most Tc-99m-labeled agents and F-18 FDG require little or no interruption of breastfeeding for the milk itself. Interruption is driven by how much activity reaches the milk and the resulting infant dose. In the published series behind current guidance, only a minority of studied radiopharmaceuticals required any milk interruption, while I-131 sodium iodide requires complete cessation for that child.

Why is I-131 sodium iodide different from other agents?

Radioiodine as sodium iodide concentrates strongly in breast tissue and is secreted in milk, and the ingested iodide then concentrates in the infant's thyroid. The effective dose per unit administered activity is very high, so both NRC Regulatory Guide 8.39 and ICRP Publication 106 recommend complete cessation of breastfeeding for that child rather than a timed interruption. A future child may be breastfed normally.

What dose target sets the interruption times?

Interruption periods are selected to keep the nursing infant's effective dose below approximately 1 mSv. Under 10 CFR 35.75, the regulatory ceiling for patient release is 5 mSv to any other individual, but Regulatory Guide 8.39 and the underlying dosimetry papers use the more protective 1 mSv infant objective when constructing interruption recommendations.

What is the difference between the milk (ingestion) issue and the close-contact (external) issue?

They are two separate pathways. The ingestion pathway is activity secreted into breast milk and swallowed by the infant, which interruption and pump-and-discard control. The close-contact pathway is external radiation from photons emitted by the mother's body while she holds the infant, which is controlled by time and distance. For F-18 FDG the milk is generally not the concern, but limiting close contact for a few hours is advised because of the penetrating 511 keV photons.

Does pumping and discarding speed up the return to breastfeeding?

Pumping and discarding maintains milk supply and comfort during the interruption but does not meaningfully accelerate radioactive decay or biological clearance in the mother. The activity in milk declines according to physical decay and biological clearance regardless of pumping. Expressed milk collected during the interruption should be discarded or stored for decay only if a facility protocol specifically allows it.

Who decides and documents the breastfeeding instructions?

The authorized user, supported by the medical physicist and the radiation safety officer, determines the interruption recommendation and ensures written instructions are given. Under 10 CFR 35.75, when the infant dose could exceed 1 mSv the licensee must provide breastfeeding guidance, and a record must be retained when continued breastfeeding could exceed 5 mSv. Documentation of the instruction is part of the patient release record.

Key Takeaways

• The objective is roughly 1 mSv to the infant. Interruption times in Regulatory Guide 8.39 and ICRP 106 are chosen to keep the nursing infant's committed effective dose below about 1 mSv, well under the 5 mSv release ceiling in 10 CFR 35.75.¹²⁴
• The agent, not just the isotope, sets the recommendation. Free Tc-99m pertechnetate and Tc-99m MAA warrant interruption; most bound, rapidly cleared Tc-99m agents need little or none.⁴⁶
• I-131 sodium iodide is cessation, not a pause. Radioiodide concentrates in the infant thyroid, so breastfeeding for that child should stop.¹⁴⁶
• Ga-67 and Tl-201 need long interruptions. These run to multiple weeks, in contrast to the hours typical of Tc-99m agents.¹⁴⁸
• Milk and close-contact are separate problems. F-18 FDG generally needs no milk interruption but does warrant limiting close contact for external dose.⁹
• Instructions and records are regulatory. 10 CFR 35.75 requires written breastfeeding guidance above 1 mSv and a retained record above 5 mSv.²

Conclusion

Lactation interruption after radiopharmaceutical administration is a small but high-stakes piece of a nuclear medicine radiation-safety program. It rewards precision: the correct action depends on the specific agent, its chemical form, the administered activity, and a clear separation of the ingestion pathway from the close-contact pathway. Anchoring every recommendation to NRC Regulatory Guide 8.39, ICRP Publication 106, and the peer-reviewed excretion dosimetry keeps infant doses below the 1 mSv objective without imposing unnecessary interruptions on mothers.

The authorized user, medical physicist, and radiation safety officer share responsibility for screening lactating patients, matching the instruction to the agent, managing pump-and-discard logistics, and documenting the guidance under 10 CFR 35.75. A program that treats breastfeeding interruption as a deliberate, radionuclide-specific calculation — rather than a generic waiting period — protects the infant, respects the mother's lactation, and remains defensible on inspection.

How DRPS Can Help

Diagnostic Radiation Physics Services helps nuclear medicine and PET/CT programs build practical, documented lactation-interruption workflows. This can include agent-specific interruption tables keyed to your formulary, patient-instruction and screening templates aligned with 10 CFR 35.75, patient-release dose calculations, radiation safety officer program support, medical physicist consulting, PET/CT and nuclear medicine physics, and radiation safety training for technologists and authorized users.

DRPS supports facilities across our service locations, including Florida, Maryland, Virginia, Washington DC, California, Nevada, New York, Pennsylvania, New Jersey, and Delaware. To discuss a lactation-interruption protocol or a broader patient-release review, contact our team.

A strong program makes the safe choice the easy choice: the technologist screens, the instruction matches the agent, and the record writes itself.

Related Resources

• Patient release after radiopharmaceutical therapy
• Managing the pregnant radiation worker
• Thyroid bioassay for I-131 workers
• Pediatric nuclear medicine dosing
• Radiopharmacy aseptic technique and QC
• Radiation Safety Officer consulting
• PET/CT and nuclear medicine physics

References

1. U.S. Nuclear Regulatory Commission. Regulatory Guide 8.39, Revision 1: Release of Patients Administered Radioactive Materials. April 2020. ADAMS Accession No. ML19232A081. nrc.gov
2. U.S. Nuclear Regulatory Commission. 10 CFR 35.75: Release of individuals containing unsealed byproduct material or implants containing byproduct material. ecfr.gov
3. U.S. Nuclear Regulatory Commission. 10 CFR Part 20: Standards for Protection Against Radiation. ecfr.gov
4. International Commission on Radiological Protection. Radiation Dose to Patients from Radiopharmaceuticals. Addendum 3 to ICRP Publication 53. ICRP Publication 106. Ann ICRP. 2008;38(1-2). PMID 19154964. icrp.org
5. Stabin MG, Breitz HB. Breast milk excretion of radiopharmaceuticals: mechanisms, findings, and radiation dosimetry. J Nucl Med. 2000;41(5):863-873. PMID 10809203. PubMed
6. 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(5):808-821. doi:10.1007/s00259-015-3286-0. doi.org
7. 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(3-4):339-348. doi:10.1093/rpd/ncab041. doi.org
8. National Library of Medicine. Gallium Citrate Ga 67 — Drugs and Lactation Database (LactMed). Bethesda, MD: NLM. ncbi.nlm.nih.gov
9. National Library of Medicine. Fludeoxyglucose F 18 — Drugs and Lactation Database (LactMed). Bethesda, MD: NLM. ncbi.nlm.nih.gov
10. International Atomic Energy Agency. Preventing Unnecessary Exposure in Diagnostic Nuclear Medicine — Radiation Protection of Patients (RPOP). iaea.org
11. U.S. Nuclear Regulatory Commission. 10 CFR Part 35: Medical Use of Byproduct Material. ecfr.gov
12. U.S. Nuclear Regulatory Commission. 10 CFR 35.2075: Records of the release of individuals containing unsealed byproduct material or implants containing byproduct material. ecfr.gov