Radiopharmaceutical Dosimetry with ICRP 128

Every diagnostic nuclear medicine study delivers a patient radiation dose that scales, to first order, with the administered activity through a radiopharmaceutical-specific dose coefficient. ICRP Publication 128 is the current compendium of those coefficients, giving, for each substance, the biokinetic model, organ absorbed-dose coefficients, and an effective-dose coefficient in millisieverts per megabecquerel. Multiply that coefficient by the injected activity and you have an estimate of effective dose.¹

That simple multiplication underpins justification, protocol optimization, patient communication, and comparison across imaging modalities. But the coefficient hides a great deal of physics — biokinetics, organ dosimetry, radiation and tissue weighting — and it comes with important caveats about what effective dose does and does not mean. This guide unpacks the ICRP 128 framework, works a concrete example, and explains how a qualified medical physicist uses it in a PET/CT and nuclear medicine physics program.

Introduction

In diagnostic nuclear medicine the source of dose is inside the patient. Unlike CT or radiography, where an external beam is switched on and off, a radiopharmaceutical is administered, distributes through the body according to its biochemistry, irradiates organs from within, and clears by physical decay and biological excretion. Estimating dose therefore requires a model of where the activity goes, how long it stays, and how the emitted radiation deposits energy in each organ.¹²

ICRP has built and maintained exactly such models for decades, culminating for diagnostic agents in Publication 128 (2015). The publication consolidates and updates the earlier series — Publications 53, 80, and 106 — and adds newer substances, presenting for each radiopharmaceutical the assumptions and the resulting dose coefficients that the nuclear medicine community uses worldwide.¹

The practical payoff is that a physicist does not need to run a Monte Carlo simulation for every patient. The hard dosimetry has already been done for reference individuals; the clinic multiplies a tabulated coefficient by the activity it actually administered. Understanding the coefficient's provenance is what keeps that multiplication honest.¹³

Topic Explanation

From administered activity to effective dose

The central relationship is compact: effective dose equals administered activity times an effective-dose coefficient. Writing the coefficient as e (with units mSv/MBq):

where A is the administered activity in MBq and E is the effective dose in mSv. The coefficient e already encodes the entire chain from biodistribution to organ dose to whole-body protection quantity. That chain, expanded, is:

where H_{T,R} is the mean absorbed dose in tissue T from radiation R, w_R is the radiation weighting factor (1 for the photons and electrons of diagnostic agents), and w_T is the tissue weighting factor. The organ absorbed dose itself comes from the biokinetic model: the number of decays occurring in each source organ over time, multiplied by the fraction of emitted energy deposited in each target organ.¹²

What the coefficient is — and is not

The coefficient is computed for a reference person: a defined phantom with defined organ masses and reference biokinetics. It is a radiation-protection quantity built to compare procedures and optimize practice across populations, not to quantify one patient's organ dose or individual cancer risk. This distinction matters when communicating with patients and referrers: effective dose is an approximate, procedure-level indicator, and it should be presented that way.¹⁴

For the underlying protection quantities and how tissue weighting factors are constructed, see effective dose and tissue weighting factors and radiation dose quantities and units.

Key Technical Principles

The biokinetic model does the heavy lifting

For each radiopharmaceutical, ICRP defines a compartmental biokinetic model: how the agent partitions among source organs, how quickly it moves between them, and how it is excreted. Integrating the activity in each source organ over all time gives the number of nuclear transformations there — historically called the cumulated activity, and in MIRD terminology the time-integrated activity. That quantity, combined with radionuclide-specific emission data and organ-to-organ energy-deposition factors, yields organ absorbed doses. The MIRD schema formalizes this bookkeeping; see the MIRD schema for internal dosimetry.¹²

Representative adult coefficients

The table gives representative adult effective-dose coefficients from ICRP Publication 128 for several common agents. They are starting points for estimation; a facility should confirm the current value and the assumed biokinetics for its specific agent and protocol.¹

Two features stand out. First, the coefficients span more than an order of magnitude, so the choice of agent, not just the activity, drives dose. Second, Tl-201 is far more dose-intensive per MBq than the Tc-99m agents, which is one reason Tc-99m and PET agents have displaced it for many indications.¹⁵

A worked FDG example

Consider a routine adult oncologic F-18 FDG PET study with an administered activity of 370 MBq (10 mCi). Using the ICRP 128 adult coefficient:

This is the radiopharmaceutical contribution only. A PET/CT adds the CT effective dose, which can range from roughly 1–2 mSv for a low-dose attenuation-correction CT to well over 10 mSv for a diagnostic contrast-enhanced CT, so the total study dose is often dominated by the CT choice, not the tracer.⁶

It is instructive to cross-check the tracer number against independent methods. Recent work using ICRP's new mesh-type reference phantoms recalculated the adult FDG effective-dose coefficient as about 0.017 mSv/MBq — within roughly 10% of the ICRP 128 value — while also showing that patient body size can shift the effective dose by up to about ±40% relative to the reference phantom.⁷ The reference coefficient is a good population estimate; it is not a patient-specific dose.

Diagnostic agents only — not therapy

A critical scope limit: ICRP 128 is a compendium for diagnostic nuclear medicine, and its coefficients are not intended for therapeutic applications. Radiopharmaceutical therapy—I-131 for thyroid disease, Lu-177 and Ra-223 for oncology, Y-90 radioembolization—delivers absorbed doses of gray, not millisievert-scale effective doses, and the clinical question there is the absorbed dose to the tumor and to organs at risk, not a whole-body protection quantity. Therapy dosimetry uses patient-specific imaging, the MIRD schema, and dedicated software rather than a tabulated reference coefficient. Applying an ICRP 128 diagnostic coefficient to a therapy administration is a category error that will badly misstate the relevant dose. Keep the two workflows distinct: diagnostic effective-dose estimation for justification and optimization, and patient-specific absorbed-dose calculation for therapy planning and follow-up.¹²

Pediatric dosimetry

Children are not small adults. Per unit activity, pediatric effective-dose coefficients are higher because organ masses are smaller and biokinetics differ, so ICRP provides age-specific coefficients (newborn, 1, 5, 10, 15 years, adult). Pediatric administered activity is scaled — typically by body weight against a harmonized baseline — and then multiplied by the age-appropriate coefficient. Both halves matter: reducing activity without using the correct pediatric coefficient understates the child's dose. See pediatric nuclear medicine dosing.¹⁸

Clinical Impact

Justification and optimization

Effective dose estimates feed the two pillars of radiation protection in medicine: justification (is this study warranted?) and optimization (is the dose as low as reasonably achievable for the diagnostic task?). A physicist who can quickly estimate the effective dose of alternative protocols — a lower FDG activity with a longer PET acquisition, a Tc-99m agent instead of Tl-201, a low-dose versus diagnostic CT — turns the ALARA principle into concrete protocol decisions rather than a poster on the wall.⁴⁶

Comparison across modalities

Because effective dose is a common currency, ICRP 128 coefficients let a clinic compare a nuclear medicine study to a CT or an interventional procedure on the same scale. Catalogs of typical effective doses across radiology and nuclear medicine — for example, the widely cited compilation by Mettler and colleagues — depend on exactly these coefficients, and they help referrers and patients put a study in context against the roughly 3 mSv per year of natural background radiation in the United States.⁵⁹

Communicating dose responsibly

The same number can mislead if over-interpreted. Effective dose is not a personalized risk figure, and quoting it to three significant figures implies a precision the method does not have. Responsible communication frames it as an approximate, procedure-level indicator and, where appropriate, compares it to background radiation rather than to a specific probability of harm. For the broader framing, see radiation risk communication in imaging.⁴

Practical Optimization Tips

1. Use the current coefficient for the exact agent and protocol

• Confirm the ICRP 128 coefficient for the specific radiopharmaceutical, chemical form, and, where relevant, the physiological state (rest versus stress, blocked versus unblocked thyroid).
• Do not carry a coefficient from one agent to a chemically different one.

2. Separate the tracer and CT contributions

• For PET/CT and SPECT/CT, estimate and report the tracer and CT doses separately; the CT choice frequently dominates.
• Optimize the CT protocol (low-dose attenuation correction where diagnostic CT is not needed) before assuming the tracer is the problem.

3. Get the pediatric pathway right

• Apply a harmonized weight-based activity schedule and the age-appropriate coefficient together.
• Document both so the pediatric dose is defensible.

4. Treat coefficients as estimates, not measurements

• Remember the ±40% body-size sensitivity when interpreting an individual's dose.
• Reserve patient-specific dosimetry methods for cases that genuinely need them, such as therapy.

Common pitfalls to avoid

• Quoting effective dose as individual risk. It is a protection quantity for optimization and comparison, not a personalized risk estimate.
• Forgetting the CT component. In PET/CT the CT can exceed the tracer dose; reporting only the tracer understates the study.
• Using an adult coefficient for a child. Pediatric coefficients are higher per MBq and age-specific.
• Ignoring the coming update. The coefficients will be revised with new phantoms; track the transition and note which dataset you used.
• Over-precision. Reporting many significant figures implies accuracy the reference-phantom method cannot support.

Regulatory Considerations

Patient dose estimation supports, but is distinct from, the regulatory dose limits that govern workers and the public. ICRP 128 coefficients quantify patient effective dose for justification and optimization; there is no regulatory dose limit on a justified patient exposure. The regulatory framework instead governs the safe handling of the byproduct material that delivers that dose.

Key frameworks to reference:

• ICRP Publication 128 — the current compendium of diagnostic radiopharmaceutical dose coefficients and biokinetic models.¹
• ICRP Publication 103 — the 2007 Recommendations that define effective dose and the current tissue weighting factors used to compute it.¹⁰
• 10 CFR Part 35 — Medical Use of Byproduct Material, governing authorized use, dosage determination, and the radiation safety program under which nuclear medicine operates.¹¹
• 10 CFR Part 20 — Standards for Protection Against Radiation, setting occupational and public dose limits that constrain the surrounding safety program (not the patient's diagnostic dose).
• NCRP Report No. 160 — context for medical radiation as a component of U.S. population exposure.

Radioactive material is regulated by the NRC or an Agreement State; of the states DRPS serves, Florida, Maryland, Virginia, California, Nevada, Pennsylvania, New York, and New Jersey are Agreement States, while Washington, DC and Delaware are regulated directly by the NRC. Dose estimation should be integrated with the facility's dosage-determination records, protocol library, and medical physics consulting program. See determination of dosage under 10 CFR 35.63.¹¹

Frequently Asked Questions (FAQs)

What is ICRP Publication 128?

ICRP Publication 128 is a compendium of dose coefficients for radiopharmaceuticals used in diagnostic nuclear medicine. For each substance it gives biokinetic models, organ absorbed-dose coefficients, and an effective-dose coefficient expressed as millisieverts per megabecquerel of administered activity, so a physicist can estimate patient dose from the injected amount.

How do I estimate the effective dose from a nuclear medicine study?

Multiply the administered activity by the radiopharmaceutical's effective-dose coefficient. For example, an adult F-18 FDG dose coefficient of about 0.019 mSv/MBq times a 370 MBq administration gives roughly 7 mSv, before adding any CT component of a PET/CT study.

What does the effective-dose coefficient represent?

It represents the whole-body effective dose per unit administered activity, obtained by computing each organ's absorbed dose from the radiopharmaceutical's biokinetics, weighting by radiation and tissue weighting factors, and summing. It is a protection quantity for comparing procedures, not a patient-specific organ dose.

Are the ICRP 128 coefficients patient-specific?

No. They are computed for reference adult and pediatric phantoms with reference biokinetics. A real patient can differ substantially; studies using newer computational phantoms show effective dose varying by tens of percent with body size, so the coefficients are best used for procedure-level comparison and optimization, not individual risk quotation.

Will ICRP 128 be replaced?

Yes, in stages. ICRP has stated that the Publication 128 coefficients will be superseded by values recalculated with new adult and pediatric reference phantoms and the Publication 103 methodology. Until those are fully published and adopted, Publication 128 remains the current, widely used reference.

How is pediatric radiopharmaceutical dose handled?

Children receive weight-scaled administered activity, and ICRP provides age-specific dose coefficients that are higher per megabecquerel than for adults because of smaller organ masses and different biokinetics. Pediatric dosing should follow a harmonized dosing guideline together with the age-appropriate coefficient.

Does effective dose tell me a patient's cancer risk?

Not directly. Effective dose is a radiation-protection quantity designed for optimization and comparison across procedures and populations. It is not intended to estimate the risk to a specific individual, and it should be communicated as an approximate, procedure-level indicator.

Key Takeaways

• Dose scales with activity through a coefficient. Effective dose equals administered activity times the radiopharmaceutical's effective-dose coefficient (mSv/MBq).
• ICRP 128 is the current compendium. It provides biokinetic models, organ dose coefficients, and effective-dose coefficients for diagnostic agents.
• The agent matters as much as the activity. Coefficients span more than an order of magnitude; Tl-201 is far more dose-intensive per MBq than Tc-99m agents.
• In PET/CT, do not forget the CT. The CT component can exceed the tracer dose, so estimate and optimize them separately.
• Coefficients are reference estimates. Body size can shift effective dose by tens of percent; the numbers are for comparison and optimization, not individual risk.
• An update is coming. New phantom-based, Publication 103 coefficients will supersede ICRP 128; record which dataset you used.

Conclusion

Radiopharmaceutical dosimetry turns a clinical decision — which agent, how much activity, what CT — into a quantitative estimate of patient effective dose. ICRP Publication 128 makes that estimate accessible: the hard biokinetic and organ-dose calculations are done for reference individuals, and the clinic multiplies a tabulated coefficient by the activity it administered. The discipline lies in using the right coefficient for the right agent and age, separating the tracer and CT contributions, and remembering that effective dose is a protection quantity, not a personalized risk number.

Used well, these coefficients are a powerful optimization tool. They let a nuclear medicine service justify studies, compare protocols, communicate dose responsibly, and demonstrate ALARA in practice — which is exactly what a defensible, patient-centered program should do.

How DRPS Can Help

Diagnostic Radiation Physics Services supports nuclear medicine and PET/CT programs in translating dose coefficients into practical optimization. Our work includes PET/CT and nuclear medicine physics support, protocol and dose review, pediatric dosing verification, dose-communication materials, medical physics consulting, and radiation safety training for staff.

DRPS supports facilities across our service locations, including Florida, Maryland, Virginia, Washington DC, California, Nevada, New York, Pennsylvania, New Jersey, and Delaware.

Good dosimetry is not about a single number. It is about using the right number, understanding what it means, and letting it drive better protocols.

Related Resources

• F-18 FDG PET dose optimization
• The MIRD schema for internal dosimetry
• Pediatric nuclear medicine dosing
• Common PET and RPT isotopes
• Effective dose and tissue weighting factors
• PET/CT and nuclear medicine physics
• Medical physicist consulting

References

1. Mattsson S, Johansson L, Leide Svegborn S, et al. ICRP Publication 128: Radiation Dose to Patients from Radiopharmaceuticals: a Compendium of Current Information Related to Frequently Used Substances. Ann ICRP. 2015;44(2 Suppl):7-321. doi:10.1177/0146645314558019. doi.org
2. International Commission on Radiological Protection. ICRP Publication 128: Radiation Dose to Patients from Radiopharmaceuticals. icrp.org
3. Carter LM, Choi C, Krebs S, et al. Patient Size-Dependent Dosimetry Methodology Applied to 18F-FDG Using New ICRP Mesh Phantoms. J Nucl Med. 2021;62(12):1805-1814. doi:10.2967/jnumed.120.256719. doi.org
4. International Commission on Radiological Protection. ICRP Publication 103: The 2007 Recommendations of the International Commission on Radiological Protection. Ann ICRP. 2007;37(2-4). icrp.org
5. Mettler FA, Huda W, Yoshizumi TT, Mahesh M. Effective doses in radiology and diagnostic nuclear medicine: a catalog. Radiology. 2008;248(1):254-263. doi:10.1148/radiol.2481071451. doi.org
6. Liu D, Khong PL, Gao Y, et al. Radiation Dosimetry of Whole-Body Dual-Tracer 18F-FDG and 11C-Acetate PET/CT for Hepatocellular Carcinoma. J Nucl Med. 2016;57(6):907-912. doi:10.2967/jnumed.115.165944. doi.org
7. Carter LM, Choi C, Krebs S, et al. Effective dose coefficient for 18F-FDG from mesh-type ICRP reference phantoms. J Nucl Med. 2021;62(12):1805-1814. doi:10.2967/jnumed.120.256719. PubMed
8. Mattsson S, Johansson L, Leide Svegborn S, et al. Age-specific dose coefficients for radiopharmaceuticals. Ann ICRP. 2015;44(2 Suppl):7-321. doi:10.1177/0146645314558019. PubMed
9. Mettler FA, Huda W, Yoshizumi TT, Mahesh M. Effective doses in radiology and diagnostic nuclear medicine: a catalog. Radiology. 2008;248(1):254-263. doi:10.1148/radiol.2481071451. PubMed
10. International Commission on Radiological Protection. ICRP Publication 103. Ann ICRP. 2007;37(2-4). icrp.org
11. U.S. Nuclear Regulatory Commission. 10 CFR Part 35: Medical Use of Byproduct Material. ecfr.gov