Nuclear Medicine Radiation Safety
The framework that governs nuclear medicine practice, with the principles behind it, the diagnostic reference levels you audit against, and the rules for the people who need the most care: the pregnant patient, the breastfeeding mother, and the therapy patient going home.
- Justification, optimisation (ALARA) and dose limitation govern practice; statutory dose limits apply to workers and the public, not to a patient’s own justified, optimised procedure.
- Diagnostic reference levels are audit benchmarks, not dose limits: if your typical administered activity for a study consistently exceeds the DRL, investigate and optimise; local DRLs govern.
- Ask every woman of childbearing potential about pregnancy before administration and document it; justify each diagnostic study and consider alternatives, and I-131 and other therapies are contraindicated in pregnancy.
- Breastfeeding interruption depends on the radiopharmaceutical; with I-131 sodium iodide, diagnostic or therapy, breastfeeding stops for that child.
- After therapy, US NRC release requires the maximally exposed other individual’s dose to be unlikely to exceed 5 mSv (0.5 rem); UK/EU release rests on dose constraints and written instructions.
Explain it simply
- Every scan must do more good than harm and use as little radiation as is reasonable. Legal caps on radiation are for staff and the public, not for a patient’s own needed scan.
- Reference levels are yardsticks for checking the usual amount of radioactive medicine a department gives, not legal caps. If a department keeps going over one, it should find out why and fix it, following its local levels.
- Before giving the radioactive medicine, ask anyone who could be pregnant and write down the answer. Each test must be truly needed, other tests should be considered, and radioactive treatments are not given in pregnancy.
- Whether a mother needs a break from breastfeeding depends on which radioactive medicine she was given. After radioactive iodine (I-131 sodium iodide), she stops breastfeeding that child altogether.
- When a patient goes home after radioactive treatment, the rules depend on the country. They are there to protect family and the public, and patients get written instructions.
A guide only: follow your departmental protocols and regional requirements.
Framework & limits
The three principles (ICRP)
Justification: the benefit of the exposure must outweigh the harm; every request is vetted against clinical value. Optimisation (ALARA/ALARP): keep doses as low as reasonably achievable, using DRLs as a practical benchmark. Dose limitation: statutory limits apply to workers and the public (but not to the dose a patient receives from their own justified, optimised procedure).
Who says what
| Body / instrument | Scope | Role in NM |
|---|---|---|
| ICRP (Pub. 103 + NM-specific) | International recommendations | The principles + dose-coefficient science everyone builds on. |
| IAEA BSS (GSR Part 3) | International basic safety standards | Baseline requirements many national laws adopt. |
| UK: IR(ME)R 2017 + IRR17; ARSAC | Medical exposures / worker protection / practitioner licensing | Referrer–practitioner–operator duties; ARSAC practitioner licence and employer licence to administer (they replaced certificates in February 2018). |
| US: NRC 10 CFR 20 & 35 (or Agreement State) | Worker/public limits + medical use of byproduct material | Authorised User, written directive, patient-release rule. |
| EU: BSS Directive 2013/59/Euratom | Member-state law | DRLs, justification, dose records. |
Dose limits (typical statutory values)
| Group | Effective dose limit | Notes |
|---|---|---|
| Classified radiation worker | 20 mSv/yr (avg; ICRP/IRR17/EU) | US NRC occupational limit is 50 mSv/yr (5 rem). |
| Member of the public | 1 mSv/yr | Basis of patient-release dose constraints for comforters/carers. |
| Eye lens (worker) | 20 mSv/yr (ICRP/IRR17/EU) | Reduced from 150 mSv after ICRP. US NRC limit is still 150 mSv/yr (15 rem). |
| Skin / extremities (worker) | 500 mSv/yr | Relevant to finger doses when drawing up + injecting. |
| Declared-pregnant worker (fetus) | ~1 mSv over the declared pregnancy (EU); US ~5 mSv (0.5 rem) | Restrict handling of high activities/therapies. |
Patient doses are not “limited”; they are justified and optimised. Comforters & carers get a dose constraint (often ~5 mSv per episode), not a limit.
Protecting the worker
Time · Distance · Shielding plus containment. Syringe shields (tungsten) and vial shields; keep fingers off the barrel; work quickly and rehearse; step back between tasks (inverse-square is your friend). Dose-calibrator + survey meter + contamination monitor; personal dosimeter (body ± ring/extremity). Positron emitters (511 keV) need more shielding than Tc-99m (140 keV). Lead aprons help little at 511 keV, so distance and time dominate in PET. Designate + monitor controlled areas; contamination-control surfaces, gloves, spill kit.
Diagnostic reference levels
Common diagnostic activities & UK ARSAC DRLs (adult)
| Study | Radiopharmaceutical | Typical activity / DRL |
|---|---|---|
| Bone scan | Tc-99m MDP/HDP | ~600 MBq |
| Myocardial perfusion (Tc) | Tc-99m MIBI/tetrofosmin | ~370 rest / ~1110 stress MBq (protocol-dependent) |
| Lung perfusion | Tc-99m MAA | ~100–200 MBq |
| Dynamic renal | Tc-99m MAG3 | ~100 MBq |
| Cortical renal | Tc-99m DMSA | ~80–110 MBq |
| Thyroid | Tc-99m pertechnetate | ~80–200 MBq |
| Hepatobiliary | Tc-99m mebrofenin | ~150 MBq |
| Parathyroid | Tc-99m sestamibi | ~740–900 MBq |
| DaTscan | I-123 ioflupane | ~111–185 MBq |
| Whole-body FDG PET | F-18 FDG | ~3.5 MBq/kg (DRL); ≈ 400 MBq |
| PSMA PET | Ga-68 PSMA-11 | ~1.8–2.2 MBq/kg (≈150 MBq) |
| DOTATATE PET | Ga-68 DOTATATE | ~100–200 MBq |
Approximate effective dose (patient)
| Procedure | Effective dose (approx.) | Everyday comparison |
|---|---|---|
| Tc-99m bone scan (600 MBq) | ~3 mSv | ~1.3 yr natural background (UK ~2.3 mSv/yr) |
| Tc-99m MPI (rest+stress) | ~6–11 mSv | Protocol-dependent |
| F-18 FDG PET (400 MBq) | ~7–8 mSv (+ CT) | CT adds several mSv more |
| Ga-68 DOTATATE / PSMA | ~3–5 mSv | Lower than most F-18 studies |
| V/Q scan | ~1.5–2.5 mSv | Lower fetal dose than CTPA; relevant in pregnancy |
| Thyroid uptake I-123 | ~1–4 mSv | — |
Effective dose ≈ administered activity × ICRP dose coefficient. Use these for patient communication, not for individual risk quotation.
Special populations & release
Pregnancy
Ask every woman of childbearing potential before administration; document. Most diagnostic studies deliver a fetal dose well below the ~100 mGy threshold associated with deterministic effects, but justify each one and consider alternatives (e.g. reduced-particle perfusion-only V/Q, or ultrasound/MRI). I-131 and other therapies are contraindicated in pregnancy. The fetal thyroid concentrates iodine from ~10–12 weeks. For diagnostics, reduce activity where possible, hydrate and encourage voiding to cut bladder-to-uterus dose.
Breastfeeding
Depends on the radiopharmaceutical (secretion into milk + infant dose). Express and store milk beforehand where a longer interruption is expected; a period of close contact restriction may also apply. Representative guidance:
| Agent | Breastfeeding action (typical) |
|---|---|
| Most Tc-99m–labelled agents | No interruption required by ICRP for the tagged compound; many centres advise a short precaution. |
| Tc-99m pertechnetate, Tc-99m MAA, free pertechnetate | Interrupt ~12 h (ACMUI simplifies most Tc-99m to 12 h). |
| F-18 FDG | No interruption of feeding needed for the milk itself; brief close-contact restriction (proximity to infant) is the main measure. |
| I-123 agents / I-131 iodohippurate | Interrupt ~12–24 h (agent-specific). |
| Ga-67, I-131 NaI (diagnostic or therapy) | Stop breastfeeding for this child (cease/complete cessation). |
Follow SNMMI / ICRP Publication 128 and local policy; the ABM Clinical Protocol #31 is a useful lactation reference.
Patient release after therapy
US (NRC 10 CFR 35.75): release is permitted when the dose to the maximally exposed other individual is unlikely to exceed 5 mSv (0.5 rem). This can be based on activity or measured dose rate; I-131 NaI is commonly releasable up to ~1221 MBq (33 mCi) or a dose rate ~0.07 mSv/h at 1 m, with written instructions. Records + instructions are required when the dose could exceed 1 mSv.
UK/EU: release governed by dose constraints to the public and comforters/carers; departments issue written radiation-protection instructions and, where needed, restrict contact durations and arrange inpatient stay.
| Take-home instructions typically cover |
|---|
| Distance + time limits with others, especially pregnant partners and young children; separate sleeping for a defined period. |
| Hygiene: hydrate, void frequently sitting down, flush twice, wash hands; launder separately for a few days. |
| Avoid public transport / long journeys close to others for the first day where advised. |
| Airport/border radiation-detector card (isotope, activity, date, department contact). |
| Return-to-work + carer arrangements; contact number for the RSO/medical physics. |
Sources
- ICRP Publication 103 (system of protection) & Publication 128 (radiopharmaceutical dose coefficients); Publication 135 (DRLs). ICRP DRL review (PDF)
- ARSAC Notes for Guidance (UK DRLs, release). gov.uk
- Interpretation and implementation of adult DRLs in NM and PET in the UK, 2025. PMC11964192
- US NRC 10 CFR 35.75: Release of individuals containing unsealed byproduct material. nrc.gov
- IAEA GSR Part 3 (Basic Safety Standards). iaea.org
- ABM Clinical Protocol #31: Radiology and Nuclear Medicine Studies in Lactating Women. PDF
- SNMMI dosimetry & breastfeeding guidance; Duration of Breastfeeding Interruption. JNMT 2023
High Yield Imaging · Radiation Protection & Safety. Educational summary. National regulations, your ARSAC licences/NRC licence, and local rules are authoritative. Confirm all limits, DRLs and release criteria against current jurisdiction-specific guidance.
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Educational reference for imaging professionals. It does not replace local policy, manufacturer labelling or clinical judgement.