There is no single isotope used to treat cancer. Oncologists now draw from more than a dozen radioactive isotopes, each chosen for a specific tumor type, location, and treatment goal. The most widely used include iodine-131 for thyroid cancer, lutetium-177 for prostate and neuroendocrine tumors, radium-223 for bone metastases, and yttrium-90 for liver cancer. What ties them together is a shared principle: attach a radioactive atom to a molecule that finds cancer cells, and let the radiation do its work at close range, sparing most of the body in the process.
How Radioactive Isotopes Kill Cancer Cells
Every therapeutic isotope works by emitting radiation that damages DNA inside cancer cells. The radioactive atom is paired with a targeting molecule, sometimes called a radiopharmaceutical, that binds to receptors or antigens found on the surface of specific tumor cells. Once the compound locks onto its target, the isotope’s decay delivers a concentrated dose of radiation to the tumor while keeping exposure to healthy tissue relatively low.1PubMed Central. Effects of Targeted Radionuclide Therapy on Cancer Cells Beyond the Ablative Radiation Dose
The type of radiation an isotope emits determines where it works best. Beta emitters like lutetium-177 and iodine-131 release electrons that travel a few millimeters through tissue, making them effective against larger or more spread-out tumors. Alpha emitters like actinium-225 and radium-223 release heavier particles that travel only a fraction of a millimeter but deposit far more energy along that short path, making them exceptionally destructive to individual cancer cells.2PubMed Central. Alpha and Beta Emitters in Translational Nuclear Medicine: Clinical Advances, Challenges, and Future Direction For the same radiation dose, alpha particles are much more damaging to cells than beta particles or gamma rays.3Frontiers in Medicine. Revisiting the Radiobiology of Targeted Alpha Therapy
Beyond directly shredding DNA in the cells they hit, these isotopes also trigger what researchers call bystander effects: neighboring cancer cells that were not directly irradiated still show reduced survival, apparently affected by signals released from damaged cells. Studies have found that yttrium-90 and lutetium-177 produce stronger bystander toxicity than iodine-125 in certain cell lines, which adds a secondary layer of tumor killing that extends beyond the radiation’s immediate physical range.4PubMed Central. Assessing the radiation-induced bystander effect in radionuclide therapy using targeted radiopharmaceuticals
Iodine-131 and Thyroid Cancer
Iodine-131 is the longest-established isotope in cancer treatment and remains the standard of care for differentiated thyroid cancer after surgery. Its success rests on a biological quirk: thyroid cells naturally absorb iodine from the bloodstream via a dedicated transporter called the sodium-iodide symporter. When a patient swallows a capsule of radioactive iodine-131, any remaining thyroid tissue and most thyroid cancer cells soak it up preferentially, concentrating the radiation exactly where it is needed.5Current Radiopharmaceuticals. The role of 131I in the treatment and management of thyroid cancer
Doctors typically administer iodine-131 after thyroidectomy to destroy any microscopic remnants of thyroid tissue and to treat metastases that still take up iodine. Treatment doses are considerably higher than the tiny traces used in diagnostic scans. Patients usually need to follow a low-iodine diet beforehand and observe radiation safety precautions for a few days afterward, since iodine-131 also emits gamma rays that can expose people nearby. Despite its age, this therapy remains one of the most effective isotope treatments, with cure rates for early-stage thyroid cancer well above 90 percent.
Lutetium-177 for Prostate and Neuroendocrine Tumors
Lutetium-177 has arguably been the biggest story in isotope-based cancer therapy over the past decade. It is a beta emitter with a convenient half-life of about 6.7 days, long enough to deliver sustained radiation to a tumor but short enough that the body clears it within weeks. Two major applications have reshaped how oncologists think about advanced cancers that were previously very difficult to treat.
Prostate Cancer
In advanced prostate cancer, most tumor cells overexpress a protein called PSMA (prostate-specific membrane antigen). Lutetium-177 is attached to a small molecule that locks onto PSMA, delivering radiation directly to prostate cancer cells throughout the body. The landmark VISION trial showed that adding lutetium-177-PSMA-617 to standard care extended median overall survival from about 11 months to about 15 months in men with metastatic castration-resistant prostate cancer, while also more than doubling the time before the disease progressed on imaging.6PubMed Central. Lutetium-177-PSMA-617 for Metastatic Castration-Resistant Prostate Cancer Those results led to regulatory approval and made lutetium-177-PSMA-617 a standard option for men whose cancer has stopped responding to hormonal therapies and chemotherapy.7PubMed. Lutetium-177-prostate-specific membrane antigen therapy for prostate cancer: current status and future prospects
More recent trials have pushed lutetium-177-PSMA into earlier treatment lines. The PSMAfore trial compared it against switching to a different hormonal therapy in men who had not yet received chemotherapy. Lutetium-177 prolonged the time before imaging showed progression and had a favorable safety profile, suggesting it could become an option earlier in the disease course rather than being reserved as a last resort.8PubMed Central. 177Lu-PSMA-617 versus a change of androgen receptor pathway inhibitor therapy for taxane-naive patients with progressive metastatic castration-resistant prostate cancer (PSMAfore)
Neuroendocrine Tumors
Neuroendocrine tumors are uncommon cancers that arise from hormone-producing cells, most often in the gastrointestinal tract. Many of these tumors display somatostatin receptors on their surfaces, and lutetium-177 can be coupled to a somatostatin analog (a molecule that mimics the natural hormone) to home in on them.9PubMed. Prognostic Significance of Somatostatin Receptor Heterogeneity in Progressive Neuroendocrine Tumor Treated with Lu-177 DOTATOC or Lu-177 DOTATATE The NETTER-1 phase 3 trial established lutetium-177-DOTATATE as a treatment for advanced midgut neuroendocrine tumors that were progressing despite standard therapy, and it has since become a cornerstone of care for these patients.10PubMed Central. Phase 3 Trial of 177Lu-Dotatate for Midgut Neuroendocrine Tumors
Radium-223 for Bone Metastases
Radium-223 occupies a unique niche. It is a calcium-mimicking alpha emitter: because radium sits in the same column of the periodic table as calcium, the body naturally directs it to areas of active bone formation, precisely the spots where bone metastases are remodeling the skeleton.11PubMed Central. Radium 223 dichloride for prostate cancer treatment Once there, its alpha particles deliver intense, short-range radiation to the tumor deposits. This approach is approved for men with castration-resistant prostate cancer that has spread to the bones but not significantly to other organs. It was the first alpha emitter to demonstrate an overall survival benefit in a large randomized trial, a milestone that energized the entire field of alpha-particle therapy.
Yttrium-90 for Liver Cancer and Lymphoma
Yttrium-90 appears in two very different treatment settings. In liver cancer, it is embedded in tiny glass or resin microspheres that are threaded through a catheter directly into the hepatic artery feeding a tumor. The microspheres lodge in the tumor’s blood supply and irradiate it from the inside, a procedure called radioembolization. A large single-center study of nearly 300 patients with hepatocellular carcinoma treated with yttrium-90 microspheres found tumor response rates of roughly 42 to 57 percent, depending on how response was measured, and median survival of about 17 months in patients with relatively preserved liver function.12Gastroenterology. Radioembolization for Hepatocellular Carcinoma Using Yttrium-90 Microspheres: A Comprehensive Report of Long-term Outcomes Researchers continue to study how radioembolization affects liver function over the long term, since the liver must remain functional enough to sustain the patient even as the tumor is treated.13PubMed Central. Long-term outcome analysis of Y90 radioembolization in hepatocellular carcinoma
Separately, yttrium-90 is used in radioimmunotherapy for certain blood cancers. Attached to an antibody that targets the CD20 protein on B-cell lymphoma cells, yttrium-90-ibritumomab tiuxetan delivers radiation to lymphoma cells throughout the body.14PubMed Central. Yttrium ibritumomab tiuxetan in the treatment of non-Hodgkin’s lymphoma: current status and future prospects Clinical trials have shown high efficacy as a first-line standalone treatment in follicular lymphoma and marginal zone lymphoma.15Clinical Lymphoma, Myeloma and Leukemia. Real World Long-term Follow-up Experience with Yttrium-90 ibritumomab tiuxetan in Previously Untreated Patients with Low-Grade Follicular Lymphoma and Marginal Zone Lymphoma
Isotopes Used in Brachytherapy
Not all isotope-based cancer treatment involves injecting or swallowing a radiopharmaceutical. Brachytherapy places sealed radioactive sources directly into or next to a tumor, and it relies on its own set of isotopes. Iodine-125 seeds are commonly implanted in early-stage prostate cancer and have also been used as an alternative to iridium-192 for breast cancer boost treatments after lumpectomy, where they deliver a more favorable dose distribution and expose medical staff to less radiation.16PubMed. The use of iodine-125 seeds as a substitute for iridium-192 seeds in temporary interstitial breast implants Iridium-192, meanwhile, is the workhorse of high-dose-rate brachytherapy and has been used in prostate, cervical, and other cancers for decades. In localized prostate cancer, iridium-192 high-dose-rate brachytherapy has demonstrated effectiveness in patients who are not candidates for surgery, with follow-up data extending beyond ten years.17PubMed. Iridium 192 high-dose-rate brachytherapy–a useful alternative therapy for localized prostate cancer?
Pain Relief from Bone Metastases
Some isotopes are used not to cure cancer but to relieve the severe bone pain that metastatic disease causes. Strontium-89 and samarium-153 are both approved in the United States and Europe for palliating pain from bone metastases.18PubMed. Radioisotopes for the palliation of metastatic bone cancer: a systematic review Like radium-223, strontium-89 naturally accumulates in bone. Samarium-153 is linked to a bone-seeking molecule. Both work by irradiating the tumor-bone interface where pain signals originate. Response rates for pain relief range widely, from about 40 to 95 percent across studies, but the relief is real and meaningful for patients whose pain is difficult to control with conventional analgesics. Combining these isotopes with external beam radiation appears to boost pain relief further: one study found complete pain relief in 50 to 60 percent of patients receiving combination therapy compared with 33 to 40 percent with the isotope alone.19Reports of Practical Oncology & Radiotherapy. Efficacy of samarium 153 and strontium 89 treatment for bone metastases in prostate cancer patients
The Theranostic Approach
One of the most compelling developments in isotope therapy is the concept of theranostics, a blend of “therapy” and “diagnostics.” The idea is simple and elegant: use one isotope of a compound to scan the patient and find the cancer, then swap in a different isotope of the same compound to treat it. The gallium-68/lutetium-177 pairing is the most established example. Gallium-68, a positron emitter, lights up tumors on a PET scan. Lutetium-177, a beta emitter attached to the same targeting molecule, then treats the tumors the scan revealed.
This approach is already standard practice in PSMA-targeted prostate cancer therapy, where a gallium-68-PSMA scan confirms the tumor expresses the target before the patient receives lutetium-177-PSMA treatment. Researchers are now extending this pairing to new targets beyond PSMA, including fibroblast activation protein (FAP), which is expressed in the supportive tissue surrounding many solid tumors. Early studies of gallium-68/lutetium-177-labeled FAP-targeting compounds have shown promising tumor uptake and retention in preclinical models, and first-in-human imaging has demonstrated good distribution in a lung cancer patient.20PubMed. (68)Ga/(177)Lu-Labeled Theranostic Pair for Targeting Fibroblast Activation Protein with Improved Tumor Uptake and Retention Other research teams are refining the PSMA pairing itself by adding albumin-binding groups to keep the compound in the bloodstream longer, improving tumor accumulation.21PubMed. Improving Theranostic Gallium-68/Lutetium-177-Labeled PSMA Inhibitors with an Albumin Binder for Prostate Cancer
Combining Isotope Therapy with Immunotherapy
Radiation has long been known to provoke an immune response against tumors: as cancer cells die from radiation damage, they release proteins and signals that can alert the immune system. Researchers are now deliberately exploiting this by pairing isotope therapy with immune checkpoint inhibitors, the drugs that remove the brakes the immune system places on itself. Preclinical and early clinical evidence suggests that the combination may improve outcomes beyond what either treatment achieves alone.22PubMed Central. Combining Targeted Radionuclide Therapy and Immune Checkpoint Inhibition for Cancer Treatment
The choice of isotope matters here, too. In a recent study comparing yttrium-90 and actinium-225 combined with checkpoint inhibitors, the two isotopes triggered different immune responses. Yttrium-90 reduced a type of immune cell that suppresses anti-tumor responses, improving the ratio of killer T cells to those suppressive cells. Actinium-225 strongly activated interferon signaling, a different immune pathway. The greatest survival benefit in the study came from actinium-225 combined with checkpoint inhibitors at a specific dose, underscoring that the radiation type and dose both shape the immune outcome.23PubMed Central. Radionuclide-Dependent Stimulation of Antitumor Immunity in GD2-Targeted Radiopharmaceutical Therapy Combined with Immune Checkpoint Inhibitors
Where These Isotopes Come From
Producing medical isotopes is a surprisingly fragile enterprise. Most therapeutic isotopes are made in nuclear reactors or particle accelerators (cyclotrons), and some short-lived isotopes are generated at the hospital itself using compact generators that extract a useful daughter isotope from a longer-lived parent.24PubMed Central. Production Review of Accelerator-Based Medical Isotopes The supply chain has been periodically disrupted when major reactors go offline for maintenance or repairs. A notable crisis occurred when two major research reactors shut down simultaneously, exposing how dependent the world’s supply was on a handful of aging facilities.25PubMed Central. Shortages no more: Fixing the isotope supply chain
This vulnerability has driven investment in accelerator-based production as an alternative. Cyclotrons and linear accelerators can produce many of the same isotopes without relying on a nuclear reactor, and they can sometimes be located closer to the hospitals that use them, reducing the logistical challenge of shipping rapidly decaying material across long distances. Actinium-225, one of the most promising alpha emitters for next-generation therapies, is particularly difficult to produce in large quantities, and scaling up its supply is widely seen as one of the biggest practical obstacles to broader use of alpha-particle therapy.26PubMed Central. Actinium-225 in Targeted Alpha Therapy
Practical Concerns for Patients
If you are told you need isotope therapy, the experience varies considerably depending on which isotope and delivery method your oncologist recommends. Iodine-131 for thyroid cancer typically requires a brief period of isolation (one to a few days) because the gamma rays it emits can expose others. Lutetium-177-PSMA treatment is usually given as an intravenous infusion in an outpatient nuclear medicine department, with patients returning every six to eight weeks for a series of treatments. Yttrium-90 radioembolization involves an interventional radiology procedure where a catheter is guided into the liver’s blood supply. Brachytherapy with iodine-125 seeds for prostate cancer involves a single procedure to implant the seeds, which then deliver radiation over weeks to months as they decay in place.
Side effects depend on both the isotope and what it targets. Lutetium-177-PSMA commonly causes dry mouth (because salivary glands also express some PSMA), fatigue, and temporary drops in blood cell counts. Iodine-131 can cause neck swelling, nausea, and changes to taste and salivary function. Radium-223 frequently causes nausea, diarrhea, and bone marrow suppression. All isotope therapies carry some risk of affecting the bone marrow, since blood-forming cells are sensitive to radiation, but the targeted nature of these treatments means the marrow effects are usually milder and more manageable than with whole-body radiation. Hospitals that administer these therapies must maintain specific infrastructure for handling radioactive materials and managing waste, since even trace long-lived impurities in isotope preparations can create disposal challenges.27PubMed Central. Dealing with dry waste disposal issues associated with (177m)Lu impurities: a long-term challenge for nuclear medicine departments
What Is Coming Next
The field is expanding fast in several directions at once. Actinium-225, already mentioned as a potent alpha emitter, is in clinical trials for prostate cancer and other malignancies, with researchers working to solve the production bottleneck. Astatine-211, another alpha emitter with a short half-life of about seven hours, has attracted attention for prostate cancer theranostics because its rapid decay could allow for intense but brief treatment sessions.28PubMed. The advent of Astatine-211 in targeted radionuclide therapy in prostate cancer: will it come to true fruition? Other isotopes entering the field include terbium-161, copper-67, and iodine-131 in new PSMA-targeting formulations, each with slightly different physical properties that may suit different tumor sizes and locations.
Access remains uneven worldwide. Even as clinical evidence grows, the broad implementation of isotope-based cancer therapy is slowed by logistical constraints, financial limitations, supply chain gaps, and regulatory hurdles that vary dramatically from country to country.29PubMed Central. The use of radiopharmaceuticals in targeted cancer therapy: a narrative review In well-resourced settings, isotope therapy has already moved from a niche last-resort option to a frontline treatment for certain cancers. In much of the world, it remains difficult to obtain. Closing that gap is as much an infrastructure and policy problem as it is a scientific one.