Ra-223 Dichloride Therapy for Prostate Cancer

Introduction

Radium-223 dichloride, sold as Xofigo, is an alpha-emitting, bone-seeking radiopharmaceutical that prolongs overall survival and delays symptomatic skeletal events in men with metastatic castration-resistant prostate cancer (mCRPC) and symptomatic bone metastases. It is given as a weight-based intravenous injection — 55 kBq per kilogram of body weight — once every 4 weeks for six doses, and it is the only approved alpha therapy to show a survival benefit in this setting. ^{1, 4}

Radium-223 occupies an unusual place in radiopharmaceutical therapy. Unlike the beta-emitting bone agents that preceded it and the ligand-targeted beta therapies that followed, it delivers its dose through high-linear-energy-transfer (high-LET) alpha particles that travel less than the width of a few cells. It is not a receptor-targeted drug at all: as a calcium analog, radium-223 follows bone-mineral chemistry to concentrate exactly where osteoblastic prostate metastases are remodeling bone. That mechanism is what makes it effective, and it also shapes its radiation-safety profile — a contamination-control problem far more than a shielding problem.

This guide is written for the clinical and radiation-safety side of a Xofigo program: how the alpha radiobiology works, what the dosing regimen and dose-calibrator measurement require, what the ALSYMPCA trial actually showed, why the ERA-223 combination with abiraterone was restricted, and how patient release, body-fluid precautions, and the medical physicist and radiation safety officer (RSO) roles fit under NRC and Agreement State rules. It is deliberately distinct from our shielding-calculation treatment of the same isotope in the RPT shielding guide for Lu-177, Ra-223, and Ac-225. DRPS supports these programs as part of its PET/CT and nuclear medicine physics services across Florida, Maryland, Virginia, Washington DC, California, and Nevada.

Topic Explanation

What is radium-223 dichloride therapy?

Radium-223 dichloride therapy is a systemic, unsealed-source alpha therapy for mCRPC with symptomatic bone metastases and no known visceral metastatic disease. Radium is a group-2 alkaline-earth metal that behaves biochemically like calcium. When injected intravenously, radium-223 is rapidly cleared from blood and taken up into the bone mineral matrix — hydroxyapatite — preferentially at sites of high bone turnover, which in advanced prostate cancer are the osteoblastic metastases themselves. ⁴ The alpha decays that follow deliver a concentrated cytotoxic dose to the adjacent tumor and stromal cells.

The FDA indication is specific: patients with castration-resistant prostate cancer, symptomatic bone metastases, and no known visceral metastatic disease. ^{4, 5} It is a treatment for bone-dominant disease, not for soft-tissue or visceral spread, and it is given alongside continued androgen-deprivation therapy (an LHRH analog or surgical castration).

For facilities standing up or expanding this service, the therapy touches several DRPS service lines at once — radiation safety officer consulting, radioactive material license support, and PET/CT and nuclear medicine physics — because a Xofigo program is as much a radiation-safety and licensing exercise as it is an oncology one.

Where Ra-223 sits among bone-targeting radionuclides

Radium-223 was not the first radionuclide used to treat painful bone metastases. Strontium-89 chloride (Metastron) and samarium-153 EDTMP (Quadramet) are beta-emitting bone agents used for pain palliation. What sets radium-223 apart is both its emission type — alpha rather than beta — and its documented survival benefit rather than palliation alone. For isotope-selection context across therapy radionuclides, see Understanding Common Isotopes in PET & Radiopharmaceutical Therapy.

Key Technical Principles

Alpha-emitter radiobiology: high LET over a very short range

The therapeutic advantage of radium-223 is that its alpha particles deposit energy densely (high LET) over a range shorter than the diameter of a few cells, producing clustered, difficult-to-repair DNA damage right at the bone-tumor interface while sparing more distant red marrow. ^{1, 4}

Two physical properties drive this. First, alpha particles carry high LET — on the order of tens of keV per micron — so the ionization density along each track is high, causing complex double-strand DNA breaks that are far less repairable than the sparse damage from low-LET beta or photon radiation. Second, the range of these alphas in tissue is extremely short, on the order of a few tens of microns (less than about 100 µm), which is why a bone-surface-seeking radionuclide can irradiate the metastatic lesion intensively while limiting dose to the marrow cavity a short distance away. This short range is also why radium-223 produces relatively low myelosuppression compared with what its cell-killing potency might suggest. ^{1, 4}

The Ra-223 decay chain and half-life

Radium-223 has a physical half-life of 11.43 days and decays through a short chain of daughters to stable lead-207. ⁴ Across the full chain there are several alpha decays and a couple of beta decays, and the great majority of the emitted energy — about 95% — is carried by the alpha particles, with only a small fraction as beta and photon emissions. ⁴ That small photon component is why external dose rates from a treated patient are low and structural shielding is usually not the governing concern.

The decay of any radionuclide follows first-order kinetics:

Over the 4-week (28-day) interval between injections, the fraction of a given administration's radium-223 still present at the time of the next injection is:

so roughly 82% of each dose's radium-223 has physically decayed before the next injection is given — a useful check when planning decay-in-storage of any residual activity, contaminated waste, or unused vials.

Worked example: weight-based administered activity

The administered activity is not fixed; it scales with body mass. The current label prescribes 55 kBq/kg per injection. ^{4, 5} For an 80 kg patient:

Across the full course of six injections, the cumulative administered activity is:

These are small activities by radiopharmaceutical-therapy standards — three orders of magnitude below a typical 7.4 GBq Lu-177 administration — which is a central reason external exposure is modest and patient release is straightforward. The physicist should confirm the dose calibrator has a validated radium-223 setting and that the measured activity, decay-corrected to the time of administration, matches the prescription in the written directive before injection. ⁴

How Ra-223 compares with other bone-targeting radionuclides

The table below places radium-223 alongside the beta-emitting bone agents and a modern ligand-targeted beta therapy. Values are representative, established radionuclide properties and should be verified against primary decay-data compilations for any calculation. ^{4, 12}

The practical message is that radium-223's alpha emission and sub-100-µm range are what let it be a treatment rather than only a palliative agent, and are also why its radiation-safety program looks different from that of the beta emitters: the hazard shifts from external photon dose toward internal-contamination control. ⁴

Clinical Impact

Patient selection and the ALSYMPCA evidence

Radium-223's approval rests on the phase 3 ALSYMPCA trial, which randomized 921 men with symptomatic mCRPC and bone metastases to six injections of radium-223 or placebo plus best standard of care and showed a significant overall-survival benefit. ¹ In the updated analysis, median overall survival was 14.9 months with radium-223 versus 11.3 months with placebo (hazard ratio 0.70; P < 0.001). ¹ The trial was stopped early for efficacy at the prespecified interim analysis, and radium-223 was associated with low rates of myelosuppression and fewer adverse events overall. ¹

Beyond survival, radium-223 delayed skeletal morbidity. A dedicated ALSYMPCA analysis reported that the time to first symptomatic skeletal event was longer with radium-223 than placebo (median 15.6 versus 9.8 months; hazard ratio 0.66; P = 0.00037), with reductions in the need for external-beam radiation for bone pain and in spinal cord compression. ² These symptomatic-skeletal-event endpoints — clinically apparent events rather than radiographic-only findings — are what make the benefit meaningful to patients, and radium-223 is now positioned as a life-prolonging option best used before visceral spread develops. ^{2, 7}

The ideal candidate mirrors the trial population and the label: castration-resistant disease, two or more symptomatic bone metastases, and no known visceral metastases. ^{1, 4} Adequate baseline hematologic function is required, and blood counts are checked before each dose. ⁴

The ERA-223 fracture signal and the combination restriction

A major post-approval lesson came from ERA-223, which tested radium-223 concurrently with abiraterone acetate plus prednisone/prednisolone and found no benefit but a substantial excess of fractures — leading to a label warning and a 2018 EMA restriction against the combination. ^{3, 6}

ERA-223 randomized 806 chemotherapy-naive, asymptomatic or mildly symptomatic mCRPC patients to radium-223 (55 kBq/kg) or placebo, each added to abiraterone plus prednisone/prednisolone. ³ The trial was unblinded early after more fractures and deaths were seen in the radium-223 arm. At the primary analysis it did not improve symptomatic skeletal event-free survival (hazard ratio 1.12; P = 0.26), and fractures of any grade occurred in about 29% of the radium-223 group versus about 11% of the placebo group. ³ The authors concluded the combination should not be used. ³

The regulatory response was direct. The FDA prescribing information warns that safety and efficacy of Xofigo in combination with cancer therapies other than LHRH analogues have not been established and that an increased risk of fractures and mortality is possible, and it is not indicated in combination with abiraterone plus prednisone/prednisolone. ⁵ In 2018 the EMA restricted Xofigo to patients who have had two prior treatments for mCRPC or who cannot receive other treatments, and specified it must not be used together with abiraterone and prednisone/prednisolone or in asymptomatic patients. ⁶ The practical clinical takeaways are to avoid the abiraterone combination and to pair radium-223 with appropriate bone-health management and fracture-risk assessment. ^{3, 6}

Administration, dose measurement, and QC

Radium-223 is supplied as a ready-to-use single-dose vial and is given as a slow intravenous injection, typically over about a minute, followed by a saline flush. ⁴ The activity per patient is calculated from body weight and decay-corrected to the administration date. Two measurement points matter for the physicist: verifying the dose-calibrator radium-223 setting (an appropriate dial/response factor, established and periodically checked as part of the instrument's quality-control program) and confirming the measured activity matches the written directive before administration. For the broader instrument program that underpins this, see our guide to dose calibrator quality control. Because the photon yield is low, a standard well-type dose calibrator — not a survey meter — is the appropriate assay instrument, and the calibrator's radium-223 accuracy should be validated rather than assumed.

Practical Optimization Tips

A defensible radium-223 program is mostly about disciplined contamination control, accurate dose measurement, and clean documentation. The following practices help.

1. Treat it as a contamination problem, not a shielding problem

Because radium-223 is predominantly an alpha emitter with low photon output, external dose rates near the patient and the vial are low, and structural shielding is rarely the driver. The real risk is internal contamination from body fluids and spills. Design the workflow around gloves, absorbent coverings, contained handling, and surveys — not lead. ⁴

2. Emphasize fecal-route precautions

Radium-223 is excreted predominantly in feces, with only a small urinary fraction; roughly 63% of administered radioactivity leaves the body within about 7 days, and fecal excretion is the major route. ⁴ Patient instructions and staff precautions should reflect this: careful toilet hygiene, flushing, handwashing, prompt cleanup of any fecal, urinary, blood, or vomit contamination, and laundering of soiled clothing separately for the instructed period.

3. Standardize dose assay and written-directive checks

Lock in a validated dose-calibrator radium-223 setting, decay-correct to the administration time, and require a documented match between the measured activity and the written directive before every injection. Build the weight-based activity calculation (55 kBq/kg) into a checked worksheet to avoid arithmetic errors. ^{4, 5}

4. Plan waste decay-in-storage around the 11.43-day half-life

Contaminated absorbents, gloves, needles, syringes, and any residual vial activity are handled as radioactive waste. With an 11.43-day half-life, decay-in-storage is practical: roughly 82% decays over four weeks, and after about ten half-lives (a few months) the activity is negligible. Survey and document before disposal per license conditions. See radioactive waste management in nuclear medicine and decontamination best practices.

5. Train staff specifically on alpha-emitter handling

Alpha contamination is difficult to detect with common survey instruments and delivers no meaningful external dose, so staff intuition calibrated on photon emitters can mislead. Train the team that "low meter reading" does not mean "no contamination," and reinforce glove changes, surface checks, and spill response.

Common pitfalls to avoid

• Assuming an alpha emitter needs no radiation-safety attention. External dose is low, but body-fluid contamination control is the whole point.
• Combining with abiraterone plus prednisone. ERA-223 showed harm; this combination is restricted. ^{3, 6}
• Using the wrong ALSYMPCA-era activity. The current label value is 55 kBq/kg, not the original 50 kBq/kg. ⁴
• Skipping dose-calibrator validation for Ra-223. A generic setting can misreport a low-photon alpha emitter.
• Under-instructing patients on toilet and hygiene precautions given the dominant fecal excretion route. ⁴

Regulatory Considerations

Radium-223 is byproduct material regulated under NRC 10 CFR Parts 20 and 35 (or the equivalent Agreement State program), and its administration requires an authorized user, a written directive, dose-calibrator measurement, and a patient-release evaluation. ^{8, 9}

Key frameworks:

• 10 CFR Part 35 — Medical Use of Byproduct Material governs authorized-user status, the written directive required for therapeutic administrations, dosage determination and records, and the RSO's responsibilities. Radium-223 is administered under the parallel/other-radionuclide therapy provisions of Part 35. ⁹
• 10 CFR Part 20 — Standards for Protection Against Radiation sets occupational and public dose limits, contamination and survey expectations, and waste-disposal (including decay-in-storage) requirements that frame the program. ⁸
• Patient release is governed by 10 CFR 35.75, which permits release when the projected dose to any other individual is within the regulatory criterion (0.5 rem / 5 mSv), with instructions where appropriate; NRC Regulatory Guide 8.39 provides the methodology and record expectations. ^{9, 10} Because radium-223 activities are low and photon emissions limited, patients are generally released immediately with written body-fluid and hygiene precautions, and the supporting release record should be retained. Our companion piece on patient release after radiopharmaceutical therapy walks through the calculation logic.
• NRC FSME-13-002 provides program-specific guidance for radium-223 dichloride relevant to licensing a Xofigo service. ¹¹

Agreement States administer their own equivalent programs. Of the jurisdictions 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. A facility should confirm which authority issues its license and which written-directive, survey, release, and reporting requirements apply before finalizing procedures. Programs that are new, expanding, or adding radium-223 should coordinate these items with medical physics consulting, RSO program support, and radioactive material license support. ^{8, 9}

Frequently Asked Questions (FAQs)

What is radium-223 dichloride (Xofigo) and how does it work?

Radium-223 dichloride, marketed as Xofigo, is an alpha-emitting radiopharmaceutical for metastatic castration-resistant prostate cancer with symptomatic bone metastases and no known visceral disease. As a calcium mimetic, radium-223 is taken up into newly formed bone matrix (hydroxyapatite) at sites of high osteoblastic turnover around metastases. Its alpha particles deposit high-LET energy over a very short range — on the order of a few cell diameters — so the cytotoxic dose is concentrated in and immediately around the metastasis while sparing more distant marrow.

What is the Xofigo dosing regimen?

The current FDA label prescribes 55 kBq (1.49 microcurie) per kilogram of body weight, given as a slow intravenous injection once every 4 weeks for a total of 6 injections. The activity is patient-weight-based rather than fixed. Blood counts are checked before initiation and before each dose. Safety and efficacy beyond 6 injections have not been established. The original ALSYMPCA trial used 50 kBq/kg; the labeled value was later revised to 55 kBq/kg after a NIST-related update to the activity standardization.

Why can Xofigo not be combined with abiraterone and prednisone?

The randomized ERA-223 trial tested radium-223 concurrently with abiraterone acetate plus prednisone or prednisolone and found no improvement in symptomatic skeletal event-free survival but a markedly higher rate of fractures (roughly 29% versus 11%) in the radium-223 arm. On the strength of that signal, the FDA label and a 2018 EMA restriction advise against this combination, and Xofigo is not indicated for use with abiraterone plus prednisone/prednisolone. Bone-health agents and appropriate patient selection are now emphasized.

Is radium-223 a contamination hazard, and what precautions are needed?

External exposure from a treated patient is low because radium-223 is predominantly an alpha emitter with only modest photon output, so structural shielding is rarely the issue. The dominant radiation-safety concern is contamination control, because radium-223 is excreted mainly in feces, with a smaller urinary component. Staff should use gloves and standard precautions when handling the dose and body fluids, and patients receive instructions on toilet hygiene, handwashing, and managing spills of blood, urine, feces, or vomit for a defined period after each injection.

Can patients be released after a radium-223 injection?

Yes. Because administered activities are low (a few megabecquerels) and photon emissions are limited, radium-223 patients are typically released immediately under 10 CFR 35.75 and the dose-to-others criterion, with written instructions. The physicist or authorized user documents that the projected dose to any other individual is within the release limit and provides body-fluid and hygiene precautions. This differs from higher-activity photon-emitting therapies that may require retention or more detailed release calculations.

What is the medical physicist and RSO role in a Xofigo program?

The medical physicist and radiation safety officer support authorized-user oversight, dose-calibrator setup and quality control for radium-223, written-directive and patient-release procedures, contamination surveys and spill response, staff training on alpha-emitter handling, waste management, and license documentation under 10 CFR Part 35 or the equivalent Agreement State program. They ensure the measured administered activity matches the written directive and that body-fluid precautions and survey records are defensible on inspection.

Key Takeaways

• Radium-223 is a bone-seeking alpha therapy that extends survival. ALSYMPCA showed median overall survival of 14.9 versus 11.3 months (HR 0.70) and delayed time to first symptomatic skeletal event (15.6 versus 9.8 months). ^{1, 2}
• The mechanism is calcium mimicry plus high-LET, short-range alphas. Radium-223 incorporates into hydroxyapatite at osteoblastic metastases and irradiates them over less than about 100 µm, sparing distant marrow. ^{1, 4}
• Dosing is weight-based: 55 kBq/kg, every 4 weeks, six injections. An 80 kg patient receives about 4.4 MBq per injection and about 26.4 MBq total. ^{4, 5}
• Do not combine with abiraterone plus prednisone. ERA-223 showed no benefit and roughly 29% versus 11% fractures, prompting a label warning and a 2018 EMA restriction. ^{3, 6}
• It is a contamination problem, not a shielding problem. Excretion is predominantly fecal; body-fluid precautions and surveys matter more than lead. ⁴
• Patients are usually released immediately under 10 CFR 35.75 with written instructions, and the program runs under 10 CFR Parts 20 and 35 or the Agreement State equivalent. ^{8, 9, 10}

Conclusion

Radium-223 dichloride is a clear example of physics enabling clinical benefit: an alkaline-earth radionuclide that follows calcium chemistry to bone, then delivers a dense, short-range alpha dose exactly where osteoblastic prostate metastases are remodeling. That mechanism produced a real survival benefit in ALSYMPCA and a meaningful delay in symptomatic skeletal events, and it explains why the therapy's radiation-safety burden centers on contamination control and body-fluid precautions rather than structural shielding.

The two most important operational lessons are behavioral, not computational. First, respect the ERA-223 result: the abiraterone-plus-prednisone combination is restricted because it caused harm, and patient selection and bone-health management matter. Second, treat radium-223 as the low-external-dose, contamination-sensitive alpha emitter it is — validate the dose-calibrator setting, match the measured activity to the written directive, instruct patients thoroughly on fecal-route hygiene, and document the release. A medical physicist and RSO who build those disciplines into the workflow give the clinical team a program that is both safe and defensible.

How DRPS Can Help

Diagnostic Radiation Physics Services helps nuclear medicine and oncology programs implement radium-223 therapy as a documented, inspection-ready process. That support can include dose-calibrator setup and quality control for radium-223, written-directive and patient-release procedure development, contamination-survey and spill-response planning, staff training on alpha-emitter handling, radioactive-waste and decay-in-storage guidance, and license documentation aligned with NRC or Agreement State requirements — delivered through PET/CT and nuclear medicine physics, radiation safety officer, radioactive material license support, and medical physicist consulting services.

DRPS supports facilities across our service locations, including Florida, Maryland, Virginia, Washington DC, California, Nevada, New York, Pennsylvania, New Jersey, and Delaware. To discuss a new or existing radium-223 program, contact our team.

A strong therapy program makes the safe process the easy process — accurate dose measurement, clear patient instructions, and clean records that hold up on inspection.

Related Resources

• Lu-177 theranostics dosimetry
• Common PET & RPT isotopes
• Dose calibrator quality control
• Nuclear medicine decontamination best practices
• RPT shielding for Lu-177, Ra-223, and Ac-225
• Patient release after radiopharmaceutical therapy
• PET/CT and nuclear medicine physics
• Radiation Safety Officer consulting
• Radioactive material license support

References

1. Parker C, Nilsson S, Heinrich D, et al. Alpha emitter radium-223 and survival in metastatic prostate cancer (ALSYMPCA). N Engl J Med. 2013;369(3):213-223. doi:10.1056/NEJMoa1213755. PubMed
2. Sartor O, Coleman R, Nilsson S, et al. Effect of radium-223 dichloride on symptomatic skeletal events in patients with castration-resistant prostate cancer and bone metastases: results from a phase 3, double-blind, randomised trial. Lancet Oncol. 2014;15(7):738-746. doi:10.1016/S1470-2045(14)70183-4. PubMed
3. Smith M, Parker C, Saad F, et al. Addition of radium-223 to abiraterone acetate and prednisone or prednisolone in patients with castration-resistant prostate cancer and bone metastases (ERA 223): a randomised, double-blind, placebo-controlled, phase 3 trial. Lancet Oncol. 2019;20(3):408-419. doi:10.1016/S1470-2045(18)30860-X. PubMed
4. Poeppel TD, Handkiewicz-Junak D, Andreeff M, et al. EANM guideline for radionuclide therapy with radium-223 of metastatic castration-resistant prostate cancer. Eur J Nucl Med Mol Imaging. 2018;45(5):824-845. doi:10.1007/s00259-017-3900-4. PubMed
5. U.S. Food and Drug Administration. Xofigo (radium Ra 223 dichloride) injection — prescribing information (NDA 203971). accessdata.fda.gov
6. European Medicines Agency. EMA restricts use of prostate cancer medicine Xofigo. 2018. ema.europa.eu
7. Wilson JM, Parker C. The safety and efficacy of radium-223 dichloride for the treatment of advanced prostate cancer. Expert Rev Anticancer Ther. 2016;16(9):911-918. doi:10.1080/14737140.2016.1222273. PubMed
8. U.S. Nuclear Regulatory Commission. 10 CFR Part 20: Standards for Protection Against Radiation. ecfr.gov
9. U.S. Nuclear Regulatory Commission. 10 CFR Part 35: Medical Use of Byproduct Material. ecfr.gov
10. U.S. Nuclear Regulatory Commission. Regulatory Guide 8.39: Release of Patients Administered Radioactive Materials. nrc.gov
11. U.S. Nuclear Regulatory Commission. FSME-13-002: Program-Specific Guidance About Radium-223 Dichloride. 2013. nrc.gov
12. International Commission on Radiological Protection. ICRP Publication 107: Nuclear Decay Data for Dosimetric Calculations. Annals of the ICRP. 2008;38(3). icrp.org