What is the difference between in vivo and in vitro bioassay?

In vivo (direct) bioassay measures radioactivity in the body with external detectors, such as a thyroid uptake probe for iodine, a lung counter for insoluble aerosols, or a whole-body counter for penetrating gamma emitters. In vitro (indirect) bioassay measures radioactivity excreted from the body, most commonly in urine or feces, and is the method of choice for alpha and pure beta emitters that cannot be counted externally.

How is a bioassay measurement converted into dose?

In three steps. First, divide the measured quantity by the biokinetic model's predicted retention or excretion fraction at the time of measurement to estimate the intake. Second, multiply the intake by the committed effective dose coefficient, or compare it to the annual limit on intake, to get the committed effective dose equivalent. Third, add that internal committed dose to the external deep-dose equivalent to obtain the total effective dose equivalent.

What is the annual limit on intake (ALI)?

The ALI is the intake of a radionuclide in a year that produces either a committed effective dose equivalent of 5 rem (0.05 Sv) or a committed dose equivalent of 50 rem (0.5 Sv) to any single organ, whichever is smaller. For iodine-131 the thyroid organ limit governs, giving an inhalation ALI of 50 microcuries. Comparing an estimated intake to the ALI is a direct way to express dose as a fraction of the occupational limit.

Why does the timing of a bioassay measurement matter so much?

Because the body clears and redistributes radionuclides over time, the fraction of an intake that remains measurable changes rapidly, especially in the first days. A thyroid or urine measurement taken on day one implies a very different intake than the same reading on day ten. The biokinetic model that converts measurement to intake is strongly time-dependent, so the measurement time relative to the intake must be known and documented.

Who should design and interpret a bioassay program?

A qualified medical physicist or health physicist should design the monitoring program — choosing the method, frequency, and detection sensitivity — and interpret results using accepted biokinetic models and NRC guidance. The radiation safety officer integrates bioassay into the dose-of-record. DRPS provides both the physics interpretation and radiation safety officer support.

Internal Dose Assessment from Bioassay

Q: What is bioassay and why is it done?

Bioassay is the measurement of radioactive material in the body or in biological samples such as urine, used to estimate intake and internal dose. It is performed when a worker is likely to receive a committed effective dose equivalent that is a meaningful fraction of the occupational limit — under NRC rules, when intake is likely to exceed 10% of the applicable annual limit on intake — or after a suspected contamination event.

A bioassay result is a measurement, not a dose. A thyroid count reads in counts per minute; a urine sample reads in becquerels per liter. Turning either into a committed effective dose equivalent requires a biokinetic model to estimate the intake, dose coefficients or the annual limit on intake to convert intake to dose, and a monitoring program designed well enough that the measurement means something. This guide walks through that full chain, with a worked iodine-131 example.¹⁵⁶

Internal dose cannot be read off a dosimeter the way external dose can. When a worker inhales, ingests, or absorbs radioactive material, the resulting dose is delivered over time as the material distributes through and clears from the body. Bioassay — measuring the activity still in the body or already excreted — is the evidence from which that internal dose is reconstructed.¹⁹

Introduction

The central problem of internal dosimetry is that we can measure what is in or leaving the body, but we must infer the dose, which depends on the intake, the radionuclide's biokinetics, and its radiation emissions. A bioassay program exists to make that inference defensible: to decide who to monitor, how, and how often, and to convert the numbers that come back into a dose of record that stands up to regulatory review.¹³⁵

This guide covers the two families of bioassay (in vivo and in vitro), the three-step chain from measurement to committed dose, the role of the intake retention function and dose coefficients, the annual limit on intake as a practical conversion tool, a worked I-131 thyroid example, program design considerations, and the regulatory framework under 10 CFR Part 20. Every numeric value is anchored to NRC regulations or published guidance so the method rests on citable ground.¹²⁵

Topic Explanation

What bioassay measures

Bioassay is the determination of the kind, quantity, or concentration of radioactive material in the body (in vivo) or excreted from the body (in vitro).¹⁹ The choice of method follows the radionuclide's emissions and biokinetics:

In vivo (direct) bioassay measures radioactivity inside the body using external detectors. Examples include a thyroid probe for radioiodine, a lung counter for insoluble inhaled aerosols, and a whole-body counter for penetrating gamma emitters such as cesium-137. In vivo methods require the radionuclide to emit photons that escape the body.¹
In vitro (indirect) bioassay measures radioactivity in biological samples — most often urine, sometimes feces or breath. It is the method of choice for alpha emitters (plutonium, americium) and pure beta emitters (tritium, carbon-14) that cannot be counted externally.¹⁹

When monitoring is required

Internal monitoring is not performed on everyone. Under 10 CFR 20.1502, the licensee must monitor the occupational intake of radioactive material and assess committed effective dose equivalent for adults likely to receive, in a year, an intake exceeding 10% of the applicable annual limit on intake (ALI), and for declared pregnant workers, minors, and others under stricter criteria.⁵ The 10%-of-ALI threshold is the practical trigger that decides who enters a routine bioassay program, and a suspected contamination or uptake event triggers special (incident) bioassay regardless.³⁵

The quantities of record

The regulatory endpoints are defined in 10 CFR 20.1003:⁵

Committed dose equivalent (CDE, $H_{T, 50}$ ) — the dose equivalent to a specific organ or tissue over the 50 years following an intake.
Committed effective dose equivalent (CEDE, $E_{50}$ ) — the sum of organ CDEs weighted by tissue weighting factors.
Total effective dose equivalent (TEDE) — the sum of the external deep-dose equivalent and the internal CEDE, and the quantity compared to the 5 rem annual limit.

Key Technical Principles

Step 1 — From measurement to intake

The measured quantity is only a fraction of what was taken in, because the body has been clearing and redistributing it since the intake. That fraction is given by a biokinetic model. For an in vivo measurement of body or organ content $M$ at time $t$ after intake, the intake $I$ is:

I = \frac{M}{m ( t )}

where $m (t)$ is the intake retention function — the fraction of the intake predicted to remain in the measured compartment at time $t$ .¹⁶ For an in vitro measurement, the daily excretion rate $E$ is divided by the excretion function $e (t)$ (the fraction of intake excreted per day at time $t$ ):

I = \frac{E}{e ( t )}

These functions come from the ICRP biokinetic models adopted by the NRC, and they are strongly time-dependent — which is why the measurement time relative to the intake must be known.¹⁶⁸

Step 2 — From intake to committed dose

Once intake is estimated, committed dose follows one of two equivalent routes.

Dose-coefficient route. Multiply the intake by the committed effective dose coefficient $e_{50}$ (dose per unit intake) for that radionuclide, intake pathway, and chemical form:

E_{50} = I \times e_{50}

Dose coefficients for workers are tabulated in ICRP Publication 68 and the more recent Occupational Intakes of Radionuclides series.⁷⁸

ALI route. Equivalently, because the ALI is by definition the intake that produces 5 rem CEDE (or 50 rem to the limiting organ), the committed doses can be written as fractions of the ALI:⁵

E_{50} (rem) = \frac{I}{ALI _{stochastic}} \times 5, H_{T, 50} (rem) = \frac{I}{ALI _{non-stochastic}} \times 50

Step 3 — Combine into TEDE

Internal committed effective dose is added to the external deep-dose equivalent for the same period:

TEDE = H_{E, external} + E_{50, internal}

and the TEDE is what is compared to the 5 rem (0.05 Sv) annual occupational limit.⁵

The ALI, DAC, and DAC-hours

The annual limit on intake is the smaller of the intake giving 5 rem (0.05 Sv) CEDE or 50 rem (0.5 Sv) CDE to any organ.⁵ The derived air concentration (DAC) is the airborne concentration that, breathed by reference man for a 2,000-hour working year at an inhalation rate of 1.2 m³/h (about 2,400 m³/year), yields one ALI.⁵ This lets airborne monitoring cross-check bioassay: 2,000 DAC-hours equal one ALI, which equals 5 rem CEDE, so:

E_{50} (rem) = \frac{DAC-hours}{2000} \times 5

Worked example: I-131 thyroid bioassay

Consider a nuclear-pharmacy worker with a routine thyroid count after a suspected inhalation of iodine-131. The relevant occupational values from 10 CFR Part 20, Appendix B (Class D, 1 µm AMAD aerosol) are an inhalation ALI of 50 µCi (thyroid-limited, non-stochastic), a stochastic value of 200 µCi, and an inhalation DAC of 2 × 10⁻⁸ µCi/mL.⁵

Assumptions:

A thyroid count at a known time post-intake, divided by the model retention fraction $m (t)$ , yields an estimated intake $I = 6 μ Ci$ (2.2 × 10⁵ Bq).

Step 2 — committed doses, via the ALI route:

H_{thyroid, 50} = \frac{6}{50} \times 50 rem = 6 rem = 60 mSv

E_{50} = \frac{6}{200} \times 5 rem = 0.15 rem = 1.5 mSv

Interpretation: the estimated intake is 12% of the 50 µCi ALI, which exceeds the 10%-of-ALI monitoring threshold, so the intake and the resulting CEDE must be assessed and recorded.⁵ The thyroid committed dose equivalent of 6 rem is well below the 50 rem organ limit, and the 0.15 rem CEDE is a small fraction of the 5 rem annual limit — but both become part of the worker's dose of record and are added to any external dose to form the TEDE.

As an independent cross-check, suppose a breathing-zone air sampler indicated 0.5 DAC for 30 hours, i.e., 15 DAC-hours. The implied committed effective dose equivalent would be:

E_{50} = \frac{15}{2000} \times 5 rem = 0.0375 rem \approx 0.4 mSv

The air-monitoring estimate and the bioassay estimate should be reconciled; large discrepancies point to a wrong intake time, an unmodeled chemical form, or a measurement problem.¹¹¹

Summary of the assessment chain

Step	Input	Tool	Output
1. Intake	Measured body content or excretion	Intake retention / excretion function $m (t)$ , $e (t)$	Intake $I$ (Bq or µCi)
2. Committed dose	Intake $I$	Dose coefficient $e_{50}$ or ALI	$E_{50}$ (CEDE), $H_{T, 50}$ (organ CDE)
3. Dose of record	CEDE + external DDE	Summation per 10 CFR 20.1202	TEDE

Clinical Impact

For a medical facility, internal dose assessment matters most for radioiodine and other unsealed therapy and imaging radionuclides handled in the hot lab and therapy suite. Radiopharmacy staff who draw and dispense I-131, nuclear medicine technologists who administer therapy activities, and personnel who care for radioiodine therapy inpatients are the workers most likely to approach the 10%-of-ALI monitoring threshold.²⁵

A credible bioassay program protects both the worker and the facility. For the worker, it confirms that intakes are being controlled and that the dose of record is accurate. For the facility, it demonstrates to the regulator that internal exposure is monitored, assessed, and recorded as required — the absence of which is a recurring inspection finding. Because iodine clears the thyroid over days to weeks, the window to obtain a meaningful post-incident thyroid count is short; a program that is not ready to measure promptly loses the ability to reconstruct the intake at all.²³ For the iodine-specific program, see our guide to thyroid bioassay for I-131 workers, and for the broader context, our overview of occupational exposure monitoring.

Practical Optimization Tips

Choose the method to match the radionuclide

Use in vivo counting (thyroid probe, whole-body counter) for photon emitters, and in vitro urinalysis for alpha and pure beta emitters. Trying to force the wrong method — for example, attempting to count tritium externally — guarantees a non-result.¹⁹

Fix and record the intake time

The single most important data point for interpretation is when the intake occurred, because the retention and excretion functions change rapidly. For routine programs, interpret measurements by assuming an intake at a defined point in the monitoring interval; for incidents, establish the time as precisely as possible.¹⁶

Set the measurement frequency from the detection goal

The monitoring interval should be short enough that an intake of regulatory concern is detected before the body clears it below the method's minimum detectable activity. A sensitive method measured too infrequently can miss a reportable intake entirely.³¹¹

Know and document your minimum detectable activity

Every bioassay method has a detection limit. Report results against a stated minimum detectable activity, and design the program so that the dose corresponding to that detection limit is an acceptably small fraction of the limit.¹¹

Reconcile multiple data sources

When air sampling, surface contamination surveys, and bioassay are all available, reconcile them. Agreement increases confidence; disagreement flags an error in intake time, chemical form, or technique before it becomes a dose-of-record mistake.¹¹¹

Keep the records

Internal dose assessments, the models and parameters used, and the supporting measurements must be retained as part of the occupational dose record. Document the assumptions, not just the answer.⁵

Regulatory Considerations

Internal dose assessment is governed by 10 CFR Part 20 and interpreted through NRC regulatory guides and ICRP biokinetic models. Because the subject is radioactive material, the governing authority is the NRC or the Agreement State program — not the FDA.

Determination of internal exposure. 10 CFR 20.1204 sets out how licensees determine the committed effective dose equivalent from intakes, including the use of bioassay and air-sampling data and the ALI comparison.⁵
Monitoring requirement. 10 CFR 20.1502 requires monitoring of occupational intake when it is likely to exceed 10% of the applicable ALI (and under stricter criteria for minors and declared pregnant workers).⁵
Methods and models. NRC Regulatory Guide 8.9 provides acceptable concepts, models, equations, and assumptions for a bioassay program; Regulatory Guide 8.20 addresses applications of bioassay for iodine-125 and iodine-131; Regulatory Guide 8.34 covers monitoring criteria and methods to calculate occupational doses.¹²³ ICRP Publications 78, 68, and the Occupational Intakes of Radionuclides series supply the individual-monitoring framework and dose coefficients; NCRP Report No. 87 remains a standard reference on bioassay for internal deposition.⁶⁷⁸⁹
Records. Intake assessments feed the dose of record retained under 10 CFR Part 20.⁵
State jurisdiction. Across the states DRPS serves — Florida, Maryland, Virginia, Washington DC, California, Nevada, Pennsylvania, New York, New Jersey, and Delaware — radioactive-material programs are administered by the NRC or the Agreement State program (Washington DC and Delaware are direct-NRC). Always confirm requirements with the authority having jurisdiction.

For the derived-limit context behind these calculations, see our guide to ALI, DAC, and internal dose limits and the broader NRC occupational dose limits under Part 20.

Frequently Asked Questions (FAQs)

What is bioassay and why is it done?

Bioassay measures radioactive material in the body or in excreta to estimate intake and internal dose. It is required when intake is likely to exceed 10% of the applicable ALI, and after suspected contamination events.¹⁵

In vivo versus in vitro?

In vivo measures activity in the body with external detectors (thyroid probe, lung or whole-body counter) and needs photon emitters; in vitro measures excreted activity (urine, feces) and is used for alpha and pure beta emitters.¹⁹

How is a measurement turned into dose?

Estimate intake by dividing the measurement by the model retention or excretion fraction, convert intake to committed dose with a dose coefficient or the ALI, then add the external deep-dose equivalent to get the TEDE.¹⁵⁶

What is the ALI?

The intake giving either 5 rem CEDE or 50 rem to a limiting organ, whichever is smaller. For I-131 the thyroid governs, giving a 50 µCi inhalation ALI.⁵

Why does measurement timing matter?

Retention and excretion fractions change rapidly, so the same reading implies very different intakes at different times after exposure; the intake time must be known.¹⁶

Key Takeaways

A bioassay result is a measurement; dose is inferred through a three-step chain: intake, committed dose, then TEDE.¹⁵
In vivo bioassay suits photon emitters; in vitro bioassay suits alpha and pure beta emitters.¹⁹
Intake is the measurement divided by the biokinetic model's retention or excretion fraction at the measurement time, so timing is critical.¹⁶
Committed dose follows from a dose coefficient or, equivalently, from comparison to the ALI; for I-131 the inhalation ALI is 50 µCi (thyroid-limited).⁵⁷
Monitoring is required when intake is likely to exceed 10% of the ALI; the worked example (6 µCi intake, 12% of ALI) crosses that threshold.⁵
The program's method, frequency, and minimum detectable activity must be designed together, and all assumptions documented.³¹¹

Conclusion

Internal dose assessment is where a measurement becomes a dose of record, and the credibility of that conversion depends on getting three things right: the biokinetic model that turns a count into an intake, the dose coefficient or ALI that turns an intake into committed dose, and the program design that makes the measurement meaningful in the first place. Done well — with the right method for the radionuclide, a known intake time, a documented detection limit, and reconciled data sources — bioassay gives workers an accurate dose of record and gives the facility a defensible internal-dosimetry program.¹⁵⁶

How DRPS Can Help

Diagnostic Radiation Physics Services (DRPS) supports nuclear medicine and radiopharmacy programs across Florida, Maryland, Virginia, Washington DC, California, Nevada, Pennsylvania, New York, New Jersey, and Delaware with radiation safety officer services, bioassay program design and interpretation, radioactive material license support, and radiation safety training by board-certified medical physicists. Contact us to review or build your internal-dosimetry program.

Related Resources

References

U.S. Nuclear Regulatory Commission. Acceptable Concepts, Models, Equations, and Assumptions for a Bioassay Program. Regulatory Guide 8.9, Revision 1. Washington, DC: NRC; 1993. nrc.gov
U.S. Nuclear Regulatory Commission. Applications of Bioassay for I-125 and I-131. Regulatory Guide 8.20, Revision 2. Washington, DC: NRC; 2014. nrc.gov
U.S. Nuclear Regulatory Commission. Monitoring Criteria and Methods to Calculate Occupational Radiation Doses. Regulatory Guide 8.34. Washington, DC: NRC; 1992. nrc.gov
U.S. Nuclear Regulatory Commission. 10 CFR Part 20, Standards for Protection Against Radiation (§§ 20.1003, 20.1202, 20.1204, 20.1502, Appendix B). nrc.gov
U.S. Nuclear Regulatory Commission. 10 CFR Part 20, Appendix B, Annual Limits on Intake (ALIs) and Derived Air Concentrations (DACs) — Iodine-131. nrc.gov
International Commission on Radiological Protection. Individual Monitoring for Internal Exposure of Workers. ICRP Publication 78. Ann ICRP. 1997;27(3-4). icrp.org
International Commission on Radiological Protection. Dose Coefficients for Intakes of Radionuclides by Workers. ICRP Publication 68. Ann ICRP. 1994;24(4). icrp.org
International Commission on Radiological Protection. Occupational Intakes of Radionuclides: Part 1. ICRP Publication 130. Ann ICRP. 2015;44(2). icrp.org
National Council on Radiation Protection and Measurements. Use of Bioassay Procedures for Assessment of Internal Radionuclide Deposition. NCRP Report No. 87. Bethesda, MD: NCRP; 1987. ncrponline.org
Boice JD, Leggett RW, Ellis ED, et al. A comprehensive dose reconstruction methodology for former Rocketdyne/Atomics International radiation workers. Health Phys. 2006;90(5):409-430. doi:10.1097/01.HP.0000183763.02247.7e. doi.org
Davesne E, Casanova P, Chojnacki E, Paquet F, Blanchardon E. Integration of uncertainties into internal contamination monitoring. Health Phys. 2010;99(4):517-522. doi:10.1097/HP.0b013e3181cd3d47. doi.org