Radium-223 (Xofigo) Therapy: Physics & Safety

Radium-223 dichloride (Xofigo) is a bone-seeking, alpha-emitting radiopharmaceutical therapy whose physics — short-range, high-LET alpha particles with very low photon yield — makes contamination control, accurate activity measurement, and correct written-directive and patient-release handling the core radiation-safety tasks, not structural shielding. Because radium is a calcium mimetic, it concentrates at sites of increased bone turnover around metastases and delivers a large, highly localized dose over a few cell diameters.¹⁷

Radium-223 was the first alpha-emitting therapy to show an overall-survival benefit in a phase III trial, and it remains a widely used targeted alpha therapy for metastatic castration-resistant prostate cancer (mCRPC). For the nuclear medicine physicist and radiation safety officer, it is also a useful case study in how alpha-emitter physics reshapes a safety program: the questions that dominate a beta or photon therapy — how much lead, what dose rate at the door — are largely replaced by questions about surveys, body-fluid precautions, dose-calibrator traceability, and written directives.¹⁷¹²

Introduction

Radium-223 delivers therapy through a decay chain that releases most of its energy as alpha radiation within a very short range, which is exactly why it is effective against bone metastases and why its external radiation profile is benign. Alpha particles have high linear energy transfer (LET) and produce dense, difficult-to-repair DNA damage, but they travel less than about 100 micrometers — fewer than about ten cell diameters — so the therapeutic dose is deposited essentially at the site of uptake while sparing adjacent marrow relative to beta emitters.¹⁶⁷

In the pivotal ALSYMPCA trial, radium-223 improved median overall survival to 14.9 months versus 11.3 months with placebo (hazard ratio 0.70), establishing it as the first alpha therapy with a survival benefit in mCRPC.² It is indicated for men with symptomatic bone metastases and no known visceral metastatic disease.¹¹³

This guide walks through the decay physics, the dosing and activity-measurement details that trip up new programs, the dosimetry, the radiation-safety and contamination-control program, and the NRC written-directive and patient-release requirements. DRPS supports radiopharmaceutical therapy programs through PET/CT and nuclear medicine physics, radiation safety officer consulting, and radioactive material license support across Florida, Maryland, Virginia, Washington DC, California, Nevada, Pennsylvania, New York, New Jersey, and Delaware.

Topic Explanation

What is radium-223 dichloride?

Radium-223 dichloride is a simple radioactive salt in which the therapeutic agent is the radium ion itself, which the body handles like calcium. After intravenous injection, radium-223 is rapidly cleared from blood and taken up into newly formed bone matrix (hydroxyapatite) at sites of osteoblastic activity — precisely where prostate-cancer bone metastases stimulate abnormal bone formation. There it acts as a targeted internal alpha source.¹⁷

Unlike beta-emitting bone agents, radium-223 does not require a chelator or targeting antibody; the calcium-mimetic chemistry of radium provides the targeting. This simplicity is clinically attractive, but it also means the physicist cannot rely on a companion imaging surrogate the way a Lu-177 theranostic pair provides — radium-223 imaging is difficult because of its low photon yield (discussed below).⁷¹⁰

For how radium-223 compares with other therapy radionuclides in emission type and handling, see our overview of common PET and radiopharmaceutical-therapy isotopes and the facility-side companion on RPT shielding for Lu-177, Ra-223, and Ac-225.

Where does radium-223 fit clinically?

Radium-223 is used in mCRPC with symptomatic bone metastases and no known visceral metastases. Its value is both a survival benefit and palliation of bone pain, with a favorable myelosuppression profile relative to wide-field beta therapy because of the short alpha range.²¹³ It is administered as an outpatient therapy — a direct consequence of its low external dose rate.¹⁷

Key Technical Principles

The decay chain

Radium-223 (physical half-life about 11.43 days) decays to stable lead-207 through a rapid cascade of daughters. Conserving mass and charge, the parent-to-Pb-207 transition comprises four alpha decays and two beta decays — a six-stage decay chain — with essentially all of the daughters decaying far faster than the parent, so they remain in near-equilibrium with radium-223 in the body.¹³¹¹

Nuclide	Decay mode	Half-life (ICRP 107 / NNDC)	Role
Ra-223	α	~11.43 d	Parent; bone-seeking, rate-limiting step
Rn-219	α	~3.96 s	Short-lived noble-gas daughter
Po-215	α	~1.78 ms	Short-lived
Pb-211	β⁻	~36.1 min	Beta daughter
Bi-211	α	~2.14 min	Alpha daughter
Tl-207	β⁻	~4.77 min	Beta daughter
Pb-207	stable	—	End of chain

The daughter half-lives are drawn from evaluated nuclear decay data (ICRP Publication 107 and the NNDC databases); they are short enough that, for practical dosimetry and safety, the chain is treated as decaying with the parent.³¹¹ Because about 95% of the total emitted energy is carried by alpha particles, with only small beta and photon contributions, the external radiation field is dominated by a small number of low-abundance photons while the therapeutic effect is delivered by the alphas.¹¹⁰¹²

Why alphas make this a contamination problem, not a shielding problem

Alpha particles from the radium-223 chain deposit a total energy on the order of ~28 MeV over a range of less than ~100 micrometers in tissue.⁶ They cannot penetrate the unbroken skin, a glove, or the vial wall, so external exposure to staff is negligible and structural shielding is essentially never the controlling concern. The safety problem inverts relative to a photon emitter: the hazard is internal contamination — radium-223 or contaminated body fluid entering the body through ingestion, inhalation, or a wound — and the spread of removable contamination from spills, blood, and excreta.¹⁷

Excretion is predominantly fecal, with a smaller urinary component, so body-fluid precautions and bathroom hygiene are central to the release instructions.¹⁷

Dosing and the weight-based activity calculation

The approved regimen is 55 kBq/kg (1.49 µCi/kg) intravenously every 4 weeks for six injections.¹ The administered activity for a given patient is:

A_{admin} = 55 kBq/kg \times m (kg)

For an 80 kg patient:

A_{admin} = 55 \times 80 = 4400 kBq = 4.4 MBq

Over the full six-cycle course, an 80 kg patient receives roughly 26 MBq total (a 70 kg patient receives about 23 MBq), which is a very small activity by nuclear-medicine standards and reflects the potency of high-LET alpha radiation per unit activity.¹⁶

The volume to draw is obtained from the vial's activity concentration at its reference date (1100 kBq/mL) with a decay correction to the administration date:

V = \frac{A _{admin}}{C _{ref} \cdot e ^{- λ Δ t}}, λ = \frac{ln 2}{T _{1/2}} = \frac{ln 2}{11.43 d} \approx 0.0606 d^{- 1}

At the reference date ( $Δ t = 0$ ), the 80 kg example gives $V = 4400/1100 = 4.0$ mL. If the dose is administered several days later, the concentration falls by $e^{- λ Δ t}$ and the required volume increases accordingly. The written directive should record the intended activity, and the administered activity should be verified before injection.¹

Measuring radium-223 activity: a traceability trap

Because radium-223 emits so few photons, dose-calibrator measurement is unusually sensitive to the calibration setting. Accurate assay requires a validated, radionuclide-specific dial setting or calibration factor traceable to a national metrology standard. This is not a hypothetical concern: a 2015 revision of the U.S. national standard for radium-223 corrected the previous transfer standard by about −9.5%, and this is exactly why the labeled dosing changed from 50 kBq/kg (as used in ALSYMPCA) to 55 kBq/kg — the physical activity administered is the same; only the standard's calibration changed.²⁴

Practically, the physicist and RSO should:

Confirm the dose calibrator uses the manufacturer- and standard-consistent setting for radium-223.
Cross-check against the vendor's stated activity and reference date on receipt.
Apply decay correction from the reference date to the assay/administration time.
Document constancy, and treat any large discrepancy between the calibrator reading and the vendor certificate as a measurement problem to resolve before administration.¹⁴

For the broader instrument program that underpins this, see dose calibrator quality control.

Clinical Impact

Dosimetry: high local dose, spared marrow

Alpha therapy concentrates dose at the microscopic scale. Published biokinetic-model dosimetry estimates that a full six-cycle course delivers on the order of ~17 Gy to the bone endosteum (the osteogenic target near bone surfaces) and ~1.7 Gy to the red marrow for a representative 70 kg patient — an order-of-magnitude difference that reflects the short alpha range depositing dose at bone surfaces while sparing marrow.⁵ Lesion-level absorbed doses measured with quantitative imaging are highly heterogeneous, reported across roughly 0.6–44.1 Gy depending on the metastasis, which underscores that a fixed weight-based activity does not translate to a uniform tumor dose.⁸¹⁰

Because alpha radiation is high-LET, its biological effectiveness exceeds that of photons or betas per unit absorbed dose; cellular dosimetry studies of radium-223 have used relative biological effectiveness (RBE) values on the order of 5.5, which is why very small absorbed doses in gray produce meaningful biological effect.⁹ The relationship between absorbed dose and equivalent dose is the standard weighting:

H_{T} = R \sum w_{R} \cdot D_{T, R}

with a large radiation weighting factor for alpha particles — a reminder that alpha absorbed dose and its biological impact must not be equated with the same number of gray from photons. For the full framework, see radiation dose quantities and units.

Imaging is difficult by design

The same low photon yield that makes radium-223 safe to handle makes it hard to image. Quantitative gamma-camera imaging is possible but demanding, typically using a low-abundance emission near 82 keV with a medium-energy collimator and careful scatter/attenuation handling to recover lesion activity.¹⁰ Most programs therefore rely on the fixed weight-based regimen rather than image-based dose personalization, though research dosimetry continues to characterize lesion response.⁸¹⁰

Practical Optimization Tips

Build the program around contamination control

For a radium-223 service, the RSO's radiation-safety plan should emphasize:

Personal protective equipment — gloves (double-gloving for administration), lab coat, and eye protection; treat the syringe and any body fluid as potentially contaminated.
Absorbent, contained work surfaces for preparation and administration.
Survey strategy that suits alpha emitters — external dose-rate surveys will read low; removable-contamination wipe surveys counted on an appropriate system are the meaningful check. Do not conclude "no contamination" from a low exposure-rate reading alone.
Spill and body-fluid procedures — blood, urine, and especially feces can carry contamination; have a written spill response and decontamination plan.
Waste handling — segregate and decay-in-storage or dispose per license conditions; verify there are no long-lived impurity concerns for the disposal pathway.

For survey-instrument selection and cleanup practice, see nuclear medicine decontamination best practices.

Get the activity measurement right the first time

Lock down the dose-calibrator setting and traceability before the first patient, and document it. Confirm the vendor certificate, reference date, and decay correction on every unit dose. Because the therapeutic activities are tiny, a calibration error is proportionally large and directly affects the delivered dose.⁴

Standardize the written directive and time-out

Radium-223 requires a written directive; make the directive, the independent activity verification, and a pre-administration time-out part of a standard workflow to prevent wrong-patient or wrong-activity events. See written directives in nuclear medicine.

Common pitfalls to avoid

Treating alpha therapy as "no radiation-safety concern." External dose is low, but internal-contamination control is the whole game.
Using a generic or unverified dose-calibrator setting. Low photon yield makes this the single most common measurement error.
Judging safety by exposure-rate readings alone. Removable-contamination wipe surveys are the meaningful metric.
Copying a Lu-177 or I-131 SOP. The excretion routes, imaging, and contamination emphasis differ.
Neglecting fecal-route precautions. Excretion is predominantly fecal; release instructions must reflect this.

Regulatory Considerations

Radium-223 is used under 10 CFR Part 35, Subpart E, as an unsealed byproduct material for which a written directive is required (10 CFR 35.300), and the program must satisfy NRC or Agreement State medical-use requirements. The key regulatory anchors are:

Written directive (10 CFR 35.40 and 35.41). A written directive signed by the authorized user is required before administration, and the licensee must have procedures to ensure administrations conform to it.¹³
Authorized user training (10 CFR 35.390 / 35.396). Radium-223 is administered parenterally; 10 CFR 35.396 provides the training pathway for parenteral administration of unsealed byproduct material requiring a written directive. Confirm the authorized user's credentials against the applicable section.
Safety precautions and surveys (Subpart E, 10 CFR 35.315) and Part 20 dose limits. Contamination control, surveys, and ALARA fall under the general Part 35 Subpart E precautions and 10 CFR Part 20.
Patient release (10 CFR 35.75 and NRC Regulatory Guide 8.39, Rev. 1, 2020). Radium-223 patients are releasable because the dose to any other individual is far below the 5 mSv (0.5 rem) limit; the licensee provides written instructions emphasizing hygiene and body-fluid precautions for a short period after each injection.
NRC licensing guidance (FSME-13-002). The NRC's 2013 licensing guidance for radium-223 dichloride establishes the Part 35 Subpart E framework and remains the foundational program guidance; align the license application and procedures with it and with the facility's specific license conditions.

Agreement States administer their own 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 for byproduct material. Confirm which authority issues your license and which written-directive, survey, and release requirements apply before relying on any single assumption. For the patient-side detail, see patient release after radiopharmaceutical therapy.

Frequently Asked Questions (FAQs)

What is radium-223 dichloride (Xofigo)?

Radium-223 dichloride, marketed as Xofigo, is a radiopharmaceutical therapy for men with castration-resistant prostate cancer that has spread to the bones and is causing symptoms, without known visceral (organ) metastases. Radium behaves chemically like calcium, so it concentrates in areas of increased bone turnover around metastases, where its short-range alpha particles deliver a high, localized radiation dose.

Why is radium-223 mainly a contamination-control problem rather than a shielding problem?

About 95% of the energy released by radium-223 and its daughters is carried by alpha particles, which travel less than about 100 micrometers in tissue and cannot penetrate the skin or the vial wall. The external photon dose rate is very low, so structural shielding is rarely the issue. The real hazards are internal contamination and spread of contamination from body fluids, so gloves, surveys, spill procedures, and body-fluid precautions dominate the safety program.

How is radium-223 dosed?

The FDA-approved regimen is 55 kBq per kilogram of body weight (1.49 microcuries per kilogram) given by slow intravenous injection once every 4 weeks for a total of six injections. The administered activity is calculated from the patient's weight, the vial's activity concentration at the reference date, and a decay correction to the day of administration.

Why is measuring radium-223 activity in a dose calibrator difficult?

Radium-223 emits very few photons, so dose calibrators must use a validated, radionuclide-specific calibration setting or factor traceable to a national standard. A well-known 2015 revision of the U.S. national standard corrected the earlier value by about 9.5%, which is why the labeled activity concentration changed from 50 to 55 kBq/kg for the same physical activity. Facilities should confirm their dose-calibrator setting matches the current standard and the manufacturer's reference calibration.

Can a radium-223 patient be released after treatment?

Yes. Because the external dose rate is very low, radium-223 patients are essentially always releasable under 10 CFR 35.75 and NRC Regulatory Guide 8.39, with the dose to any other individual well below the 5 mSv (0.5 rem) limit. Written instructions focus on hygiene and body-fluid precautions — handwashing, flushing the toilet, managing incontinence and spills — for a short period after each injection.

What NRC requirements apply to a radium-223 program?

Radium-223 is used under 10 CFR Part 35 Subpart E as an unsealed byproduct material requiring a written directive. A written directive is required before administration (10 CFR 35.40/35.41), the authorized user must meet the applicable training requirements (for example, 10 CFR 35.396 for parenteral administration), and the program should follow NRC licensing guidance in FSME-13-002 and the facility's radioactive material license conditions.

Key Takeaways

Radium-223 is a bone-seeking alpha therapy. Its calcium-mimetic chemistry targets sites of osteoblastic bone metastasis, where short-range alphas deliver a high local dose.¹⁷
The chain is four alpha + two beta decays to stable Pb-207, with ~95% of the energy released as alpha radiation.¹¹¹
Contamination control — not shielding — is the safety priority. Alphas cannot penetrate skin or vial; the hazards are internal contamination and body-fluid spread.¹⁷
Dosing is 55 kBq/kg every 4 weeks × 6. The 50→55 kBq/kg change reflected a 2015 national-standard recalibration, not a dose increase.¹⁴
Activity measurement is the classic pitfall. Low photon yield demands a validated, traceable dose-calibrator setting.⁴
Patients are releasable under 10 CFR 35.75 / RG 8.39, with hygiene-focused instructions; use is under Part 35 Subpart E with a written directive.¹¹³

Conclusion

Radium-223 dichloride is a compact lesson in alpha-emitter medical physics. Its physics — a short-range, high-LET alpha cascade with minimal photon output — is what makes it clinically effective against bone metastases and what makes its external radiation profile benign enough for outpatient use. That same physics shifts the radiation-safety program away from lead and dose rates toward contamination control, traceable activity measurement, written directives, and hygiene-based patient-release instructions. Programs that respect those priorities — and that lock down the dose-calibrator traceability before the first patient — can deliver radium-223 safely and defensibly while meeting NRC and Agreement State requirements.

How DRPS Can Help

Diagnostic Radiation Physics Services (DRPS) supports nuclear medicine and radiopharmaceutical therapy programs with PET/CT and nuclear medicine physics, dose-calibrator and instrument QC, contamination-control and survey program development, written-directive and patient-release procedure review, radiation safety officer consulting, and radioactive material license support delivered by board-certified medical physicists.

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

A strong alpha-therapy program is not about adding shielding. It is about making the contamination-control, measurement, and documentation steps the routine, easy path for the clinical team.

Related Resources

References

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Sánchez-Jiménez J, López-Montes A, Núñez-Martínez L, et al. 223Ra-dichloride spectrometric characterization: searching for the presence of long-lived isotopes with radiological protection implications. Physica Medica. 2017;35:97-101. doi:10.1016/j.ejmp.2017.02.006. PubMed
International Commission on Radiological Protection. ICRP Publication 107: Nuclear Decay Data for Dosimetric Calculations. Annals of the ICRP. 2008;38(3). icrp.org
U.S. Nuclear Regulatory Commission. FSME-13-002: Licensing Guidance for Radium-223 Dichloride. January 2013. nrc.gov
U.S. Nuclear Regulatory Commission. 10 CFR Part 35: Medical Use of Byproduct Material. ecfr.gov
U.S. Nuclear Regulatory Commission. Regulatory Guide 8.39, Revision 1: Release of Patients Administered Radioactive Material. April 2020. nrc.gov