Astatine-211 Targeted Alpha Therapy

Introduction

Astatine-211 (At-211) is a 7.2-hour, cyclotron-produced alpha emitter that delivers essentially one high-LET alpha particle per decay over a range of only a few cell diameters, which makes it well suited to microscopic and residual disease. Its physics, dosimetry, production, and radiation-safety profile differ enough from the other therapeutic alpha emitters that it deserves to be understood on its own terms, not as "just another alpha therapy."

Targeted alpha therapy (TAT) uses radionuclides that emit alpha particles — helium nuclei — to destroy cancer cells. Alpha particles carry high linear energy transfer (LET) and travel only tens of micrometers in tissue, so they concentrate dense, hard-to-repair DNA damage at the target while sparing tissue a fraction of a millimeter away. Among the alpha emitters under study, At-211 is distinctive: it is a radiohalogen, chemically more like iodine than like the metallic emitters actinium-225 and radium-223, and over its decay it produces effectively a single alpha particle rather than a long chain of alpha-emitting daughters.^{6, 7}

That single-alpha behavior is a genuine advantage for dosimetry and for avoiding recoiling free daughters, but the 7.2-hour half-life, the need for a cyclotron with an alpha beam, and astatine's chemical volatility create real constraints. This guide explains how At-211 decays, how its dose is estimated, how it compares with Ac-225 and Ra-223, and what a defensible radiation-safety program must address. DRPS supports facilities exploring alpha-emitter programs through its PET/CT and nuclear medicine physics and radiation safety officer services across Florida, Maryland, Virginia, Washington DC, California, and Nevada.

Topic Explanation

What is astatine-211?

At-211 is a radioactive isotope of astatine, element 85, the heaviest halogen, with a half-life of about 7.214 hours. It is one of the most studied radionuclides for targeted alpha therapy because its decay delivers one alpha particle with no persistent alpha-emitting progeny.^{6, 9, 10}

At-211 decays along two branches that both end, promptly, in an alpha emission:^{9, 10}

• Alpha branch (~41.8%): At-211 emits a 5.87 MeV alpha particle directly, decaying to bismuth-207.
• Electron-capture branch (~58.2%): At-211 decays by electron capture to polonium-211, which has a half-life of only about 0.52 seconds and promptly emits a 7.45 MeV alpha particle, decaying to stable lead-207.

Because essentially every decay produces exactly one alpha particle — either directly or through the very short-lived Po-211 — At-211 behaves, for dose purposes, like a single-alpha emitter.⁷ The electron-capture branch also produces polonium K-shell x-rays in the roughly 77–92 keV range, which are energetic enough to image on a gamma camera and to survey, a practical bonus that pure alpha emitters do not offer.¹⁰

For background on how emission characteristics shape handling decisions across therapy isotopes, see Understanding Common Isotopes in PET & Radiopharmaceutical Therapy.

Why alpha particles, and why this one?

Alpha particles deposit energy along a very short, dense track. For At-211's 5.87 and 7.45 MeV alphas, the range in soft tissue is on the order of 50–70 micrometers — a few cell diameters — with LET near 100 keV/µm. That combination produces clustered double-strand DNA breaks with a high relative biological effectiveness, largely independent of oxygenation and cell-cycle phase. The therapeutic logic is to carry that payload, on an antibody, peptide, or small molecule, to cancer cells and irradiate them and their immediate neighbors while sparing tissue beyond the alpha range.^{5, 6}

At-211 is a particularly clean match for residual or microscopic disease: a surgically created tumor cavity, disseminated microtumors in a body cavity, or circulating and micrometastatic cells. Its single-alpha decay avoids the problem that complicates Ac-225 and Ra-223 — a chain of alpha-emitting daughters that can recoil off the targeting molecule and redistribute in the body.⁶

Key Technical Principles

Decay physics and the single-alpha advantage

The table compares At-211 with the two best-known therapeutic alpha emitters on the properties that drive dosimetry, logistics, and safety.

The single-alpha behavior is more than a tidy fact. For Ac-225 and Ra-223, the alpha-emitting daughters can detach after the parent decays (nuclear recoil energy far exceeds any chemical bond) and travel to other organs, which complicates both efficacy and normal-tissue dose.⁶ At-211 largely sidesteps this: the only alpha-emitting "daughter," Po-211, lives about half a second and stays essentially where the parent decayed, and its end product, Pb-207, is stable. One practical note for waste: the alpha branch produces bismuth-207, a long-lived (about 31.6-year) contaminant, but only in the small quantities corresponding to the administered activity.

Decay math and the logistics constraint

The 7.2-hour half-life governs everything about distribution. The decay constant is:

The activity remaining after a time $t$ follows:

So after one day of transport and preparation:

Only about 10% of the produced activity survives 24 hours, and roughly half is gone every 7 hours. A dose that leaves the cyclotron at 370 MBq is near 185 MBq after about 7 hours and near 37 MBq after a day. This is why At-211 production must sit close in time and distance to the clinic, and why work on maintaining radiochemical yield soon after purification is an active area: labeling yield itself falls with the accumulated radiation dose to the chemistry after dry-distillation purification, so the practical window is tight.⁸

Production and radiochemistry

At-211 is made by bombarding a bismuth-209 target with an alpha-particle beam of roughly 28 MeV in the reaction 209Bi(α,2n)211At. The beam energy must be controlled: too high an energy co-produces At-210, whose daughter Po-210 is highly radiotoxic, so energy selection is a safety-critical parameter, not just a yield choice.⁶ Relatively few cyclotrons worldwide can deliver the required alpha beam, which has historically limited availability.

After irradiation, At-211 is typically isolated from the target by dry distillation — heating the target so volatile astatine is driven off and trapped — then taken up for labeling chemistry. Its halogen chemistry allows antibody and small-molecule labeling by methods conceptually related to radioiodination (for example, astatodemetalation of tin precursors), though astatine is less electronegative and its bonds can be less stable than iodine's, which is itself a research focus.^{6, 8}

Clinical Impact

What the clinical evidence shows

At-211 TAT is investigational, but the clinical record is informative:

• Recurrent brain tumors. In an early feasibility study, At-211-labeled chimeric antitenascin antibody 81C6 was instilled into a surgically created resection cavity at 71–347 MBq. About 96.7% of decays stayed in the cavity, no dose-limiting toxicity was identified, and median survival was about 52 weeks for glioblastoma patients — proof of concept for regional alpha therapy.¹
• Ovarian cancer. A phase I trial delivered intraperitoneal At-211-MX35 F(ab′)₂ to patients in second remission, escalating to 215 MBq/L with no dose-limiting toxicity and no observed hematologic toxicity; long-term follow-up added individual absorbed-dose estimates and showed no apparent radiation-induced toxicity.² A companion risk analysis estimated a low excess secondary-cancer burden of roughly 1.1 per 100 treated, dominated by dose to the urinary organs.³
• PSMA-targeted prostate cancer. Preclinical At-211 PSMA agents are advancing, with the single-alpha-per-decay property cited as a translational advantage because it simplifies dosimetry and avoids daughter-emitter toxicity.⁷

The pattern is consistent: At-211 is most compelling where the disease is microscopic or confined to a compartment (a resection cavity, the peritoneum), so the short alpha range is an asset and systemic dose stays low.^{1, 2, 4, 5}

Why dosimetry is both easier and harder

The single-alpha decay makes the source term simpler than for chain emitters: one alpha per decay, no migrating alpha-emitting daughters to track.⁷ But micro-scale dosimetry is intrinsically demanding. Because the alpha range is only a few cell diameters, the absorbed dose is highly non-uniform at the cellular level, and whether a given cell is hit depends on the microscopic distribution of the radiopharmaceutical. Meaningful dose estimates therefore combine macroscopic pharmacokinetics with small-scale or cellular dosimetry, as the ovarian trial's individualized dose work illustrates.^{2, 3} For the frameworks behind this, see our overviews of MIRD-schema internal dosimetry and voxel-based dosimetry for radiopharmaceutical therapy.

Practical Optimization Tips

A program handling At-211 should plan around its four defining features: short half-life, alpha emission, volatility, and halogen chemistry.

1. Plan the timeline backward from decay

• Schedule production, labeling, quality control, and administration within the tight time window set by the 7.2-hour half-life.
• Account for radiochemical-yield loss soon after purification, and build decay and yield into the activity ordered versus administered.⁸

2. Engineer against volatility

• Perform production and labeling in well-ventilated enclosures (fume hoods, gloveboxes, or hot cells) with appropriate traps, because astatine can volatilize during heating and handling.
• Treat airborne astatine as a real pathway, not a theoretical one, in the facility's air-handling and effluent plan.

3. Control contamination like an alpha lab

• Use alpha-appropriate contamination surveys and wipe tests; alpha contamination is easy to miss with the wrong instrument. See choosing the right radiation survey meter.
• Use the Po K x-rays (~77–92 keV) for gamma-camera imaging, dose confirmation, and area surveys where a suitable detector is available.¹⁰

4. Protect the thyroid

• Because astatine, like iodine, can concentrate in the thyroid, include thyroid bioassay for workers who handle unsealed At-211 and consider thyroid-blocking strategies in the safety plan.

5. Manage waste for the right nuclides

• Short-lived At-211 and Po-211 decay quickly, but plan storage and documentation for the small quantity of long-lived bismuth-207 produced in the alpha branch.

Common pitfalls to avoid

• Treating At-211 like a metallic alpha emitter. Its volatility and thyroid affinity are hazards Ac-225 and Ra-223 do not pose.
• Underestimating decay losses. Half the activity is gone every 7 hours; logistics must be built around it.⁸
• Ignoring co-produced contaminants. Beam energy must be controlled to limit At-210/Po-210.⁶
• Surveying only for photons. Alpha contamination requires appropriate methods and instruments.
• Assuming uniform dose. Micro-scale dose is highly non-uniform and depends on radiopharmaceutical distribution.^{2, 3}

Regulatory Considerations

At-211 is byproduct material used for medical purposes, so a program must fit within the facility's radioactive material license and the federal or Agreement State rules for medical use. Because the therapy is investigational, it typically proceeds under research or emerging-use authorizations rather than as a routine approved product.

• 10 CFR Part 20 — Standards for Protection Against Radiation set the occupational and public dose limits, airborne concentration limits, contamination controls, and waste provisions that govern an alpha-emitter program.¹¹
• 10 CFR Part 35 — Medical Use of Byproduct Material governs authorized users, written directives, and the radiation safety officer's responsibilities; a novel therapy radionuclide generally falls under the emerging-medical-use provisions (10 CFR 35.1000) with NRC-reviewed procedures, or under a research pathway.¹²
• NRC NUREG-1556, Volume 9 — program-specific guidance for medical-use licenses frames expectations for facility design, surveys, and radiation safety procedures that an At-211 service would extend to alpha and volatility hazards.¹³
• 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. Verify which authority issues the license and which requirements apply before relying on any assumption. Nuclear decay data for dosimetry and shielding should be drawn from authoritative compilations such as ICRP Publication 107.⁹

Because At-211 adds volatility, airborne-pathway, thyroid-uptake, and alpha-contamination concerns on top of ordinary nuclear medicine practice, a program should coordinate the license, written procedures, air handling, bioassay, and waste plan with radioactive material license support and RSO guidance before the first production run.

Frequently Asked Questions (FAQs)

What is astatine-211?

Astatine-211 (At-211) is a radioactive isotope of the halogen astatine with a half-life of about 7.2 hours that is used for targeted alpha-particle therapy. It is produced in a cyclotron by bombarding bismuth-209 with alpha particles, and over its decay it releases essentially one high-energy, short-range alpha particle, which makes it attractive for destroying microscopic or residual cancer while sparing nearby normal tissue.

How is astatine-211 different from Ac-225 and Ra-223?

Astatine-211 emits one alpha particle per decay with no long alpha-emitting decay chain, which simplifies dosimetry and avoids the free recoiling daughters that complicate actinium-225 and radium-223. Its half-life is only hours rather than days, so it must be produced and used near the treatment site. Chemically it is a volatile halogen that, like iodine, can concentrate in the thyroid, which creates a handling hazard the metallic alpha emitters do not share.

Why is the short range of the alpha particle useful?

The alpha particles from astatine-211 travel only about 50 to 70 micrometers in tissue, roughly a few cell diameters, and deposit very high linear energy transfer along that short path. This means the radiation dose is concentrated at the targeted cells and immediate neighbors, producing dense, hard-to-repair DNA damage while sparing tissue even a fraction of a millimeter away, which is ideal for micrometastatic and residual disease.

What makes astatine-211 hard to produce and distribute?

Astatine-211 requires a cyclotron capable of delivering an alpha-particle beam of roughly 28 MeV onto a bismuth target, and relatively few facilities worldwide have this capability. Combined with the 7.2-hour half-life, which causes the usable activity to fall to about half every 7 hours and to roughly a tenth within a day, production must be close in time and distance to the clinic, so distribution is a central practical constraint.

What are the main radiation-safety concerns with astatine-211?

Because it is an alpha emitter, external exposure is low and the dominant concerns are contamination control and internal uptake. Astatine's volatility means it can escape as vapor during production, labeling, and handling, and as a halogen it can be taken up by the thyroid, so engineering controls, air handling, contamination and thyroid bioassay, and careful waste management are essential, alongside the Po K x-rays that allow gamma-camera imaging and surveys.

Is astatine-211 therapy approved for clinical use?

As of 2025, astatine-211 targeted alpha therapy is investigational and studied under early-phase clinical trials and research authorizations rather than being a routinely approved product. Radium-223 dichloride is the approved alpha-emitting therapy in widespread use, while astatine-211 agents for brain tumors, ovarian cancer, and prostate-specific membrane antigen targets remain in clinical and preclinical development.

Key Takeaways

• One alpha per decay. At-211 delivers essentially a single high-LET alpha per decay, with the only alpha-emitting "daughter," Po-211, lasting about half a second and ending at stable Pb-207.^{7, 9}
• Short range, high LET. Alphas travel ~50–70 µm with LET near 100 keV/µm, matching micrometastatic and residual disease.^{5, 6}
• Half-life drives logistics. Activity halves every ~7 hours and falls to ~10% in a day, forcing near-site production.⁸
• Cyclotron-made, energy-sensitive. Produced by 209Bi(α,2n)211At; beam energy must limit At-210/Po-210.⁶
• Distinctive safety profile. Volatility and thyroid affinity add airborne and internal-uptake hazards beyond ordinary alpha-contamination control.
• Imageable and surveyable. Po K x-rays (~77–92 keV) allow gamma-camera imaging and surveys.¹⁰
• Investigational but promising. Early trials in brain and ovarian cancer show feasibility and low toxicity.^{1, 2, 3}

Conclusion

Astatine-211 is one of the most promising radionuclides for targeted alpha therapy precisely because its physics is clean: one alpha particle per decay, a short high-LET track, no persistent alpha-emitting daughters, and even a photon signature for imaging. Those properties make it a natural fit for microscopic and compartmental disease, and early clinical work supports its feasibility and tolerability.

But At-211 is not interchangeable with Ac-225 or Ra-223. Its 7.2-hour half-life demands near-site production and tight scheduling; its cyclotron production requires careful beam-energy control; and its chemical volatility and thyroid affinity create airborne and internal-uptake hazards that a metallic alpha emitter does not. A facility that approaches At-211 as its own problem — building the dosimetry, air handling, bioassay, contamination program, and license around these specific features — will be positioned to explore alpha therapy safely and defensibly.

How DRPS Can Help

Diagnostic Radiation Physics Services helps nuclear medicine and theranostics programs translate the physics of alpha-emitter therapy into practical, documented workflows. For an emerging radionuclide such as At-211, that support can include radionuclide-specific radiation-safety planning, air-handling and contamination-control review, survey and bioassay program design, internal and small-scale dosimetry guidance, waste planning, radioactive material license support, and RSO program guidance aligned with NRC and Agreement State requirements.

DRPS supports facilities across our service locations, including Florida, Maryland, Virginia, Washington DC, California, Nevada, New York, Pennsylvania, New Jersey, and Delaware, through PET/CT and nuclear medicine physics and medical physics consulting. A strong alpha-therapy program is built isotope-by-isotope, around the real hazards of the nuclide in hand.

Related Resources

• Ac-225 targeted alpha therapy
• Pb-212 targeted alpha therapy
• Radium-223 dichloride therapy: physics and safety
• Common PET & RPT isotopes
• Cyclotron F-18 production
• Inpatient I-131 therapy radiation safety
• PET/CT and nuclear medicine physics
• Radioactive material license support

References

1. Zalutsky MR, Reardon DA, Akabani G, et al. Clinical experience with alpha-particle-emitting 211At: treatment of recurrent brain tumor patients with 211At-labeled chimeric antitenascin monoclonal antibody 81C6. J Nucl Med. 2008;49(1):30-38. doi:10.2967/jnumed.107.046938. PubMed
2. Hallqvist A, Bergmark K, Bäck T, et al. Intraperitoneal α-Emitting Radioimmunotherapy with 211At in Relapsed Ovarian Cancer: Long-Term Follow-up with Individual Absorbed Dose Estimations. J Nucl Med. 2019;60(8):1073-1079. doi:10.2967/jnumed.118.220384. PubMed
3. Leidermark E, Hallqvist A, Jacobsson L, et al. Estimating the Risk for Secondary Cancer After Targeted α-Therapy with 211At Intraperitoneal Radioimmunotherapy. J Nucl Med. 2023;64(1):165-172. doi:10.2967/jnumed.121.263349. PubMed
4. Frost SHL, Bäck T, Chouin N, et al. Comparison of 211At-PRIT and 211At-RIT of ovarian microtumors in a nude mouse model. Cancer Biother Radiopharm. 2013;28(2):108-114. doi:10.1089/cbr.2012.1281. PubMed
5. Kraeber-Bodéré F, Bodet-Milin C, Rousseau C, et al. Radioimmunoconjugates for the treatment of cancer. Semin Oncol. 2014;41(5):613-622. doi:10.1053/j.seminoncol.2014.07.004. PubMed
6. Kozempel J, Mokhodoeva O, Vlk M. Progress in Targeted Alpha-Particle Therapy. What We Learned about Recoils Release from In Vivo Generators. Molecules. 2018;23(3):581. doi:10.3390/molecules23030581. PubMed
7. Feng Y, Meshaw RL, Finch SW, et al. A third generation PSMA-targeted agent [211At]YF2: Synthesis and in vivo evaluation. Nucl Med Biol. 2024;134-135:108916. doi:10.1016/j.nucmedbio.2024.108916. PubMed
8. Hansson E, Timperanza C, Jensen H, et al. Radiolabeling yield dependency of post-dry distillation decay of astatine-211. Nucl Med Biol. 2025;144-145:108999. doi:10.1016/j.nucmedbio.2025.108999. PubMed
9. International Commission on Radiological Protection. ICRP Publication 107: Nuclear Decay Data for Dosimetric Calculations. Annals of the ICRP. 2008;38(3). icrp.org
10. PRISMAP — The European Medical Radionuclides Programme. Radionuclide portfolio: astatine-211. prismap.eu
11. U.S. Nuclear Regulatory Commission. 10 CFR Part 20: Standards for Protection Against Radiation. ecfr.gov
12. U.S. Nuclear Regulatory Commission. 10 CFR Part 35: Medical Use of Byproduct Material. ecfr.gov
13. U.S. Nuclear Regulatory Commission. NUREG-1556, Volume 9, Revision 3: Program-Specific Guidance About Medical Use Licenses. nrc.gov