Tiny Radioactive Seeds Offer New Hope for Thyroid Cancer That Resists Iodine Therapy

For most patients with differentiated thyroid cancer, the outlook after surgery and radioactive iodine treatment is excellent. Yet a stubborn minority of tumors eventually stop absorbing iodine altogether, slipping beyond the reach of the very therapy that defines thyroid cancer care. When that happens, patients enter a category known as radioiodine-refractory differentiated thyroid cancer, or RAIR-DTC, a condition with a median survival of only three to five years and a ten-year survival rate of roughly ten percent. A new review published in Holistic Integrative Oncology by Dingyi Zhang, Zhijun Chen and colleagues at Jiangxi Cancer Hospital in Nanchang, China, examines a quietly powerful alternative for these patients: iodine-125 seed brachytherapy, a technique that plants miniature radioactive sources directly inside tumors that can no longer be removed or treated with conventional radioiodine.

The clinical problem these seeds address is substantial. Differentiated thyroid carcinomas, which include papillary and follicular subtypes, account for about ninety percent of all thyroid cancers, and while most patients do well after standardized treatment, approximately twenty-three percent still develop distant metastases. The standard approach for such patients is total thyroidectomy followed by iodine-131 radiation therapy, which exploits the thyroid cell’s natural ability to concentrate iodine. But roughly one-third of patients treated with iodine-131 experience degenerative changes in tumor cell morphology and function during the course of the disease, losing iodine-avidity and progressing to the refractory state. Once tumors shed this ability, systemic targeted therapy and immunotherapy become the mainstays, while local lesions may be managed with surgery, external beam radiotherapy, or ultrasound-guided ablation.

Local treatment takes on particular urgency when recurrent or metastatic lesions press against critical structures such as the airway, the esophagus, or the major cervical vessels, or when they cause breathing difficulty, symptomatic bone metastases, or brain metastases. Surgery, the conventional answer, becomes progressively harder with each operation because scar tissue and fibrosis complicate dissection, and in some cases resection is simply not feasible. It is in this therapeutic gap that iodine-125 seed implantation has carved out its role, offering a high localized radiation dose with rapid fall-off in surrounding tissue and no need for fractionated hospital visits. The seeds deliver sustained, targeted radiation over several months, achieving persistent tumor cell eradication while sparing the patient the cumulative burden of repeated interventions.

The physics behind these tiny devices explains much of their appeal. Iodine-125 is an artificial radioactive isotope that decays into an excited state of tellurium-125 through electron capture, emitting a 35.5 kiloelectronvolt gamma ray and a series of characteristic X-rays in the 27.4 to 31.4 kiloelectronvolt range. These low-energy photons penetrate only a short distance through tissue, with a half-value layer of about two centimeters, and their intensity falls off steeply according to the inverse square law, producing an extremely sharp dose gradient around each seed. With a half-life of 59.4 days, an implanted seed delivers an initial dose rate of approximately 0.07 to 0.12 Gy per hour, releasing about ninety percent of the prescribed dose within three months and completing its treatment cycle over roughly a year.

That continuous low-dose-rate exposure produces radiobiological effects that differ fundamentally from the high-dose bursts of external beam radiotherapy. Tumor cells repair radiation-induced DNA damage between external beam fractions, a major contributor to radiation resistance, but the uninterrupted photon emission from iodine-125 seeds induces DNA double-strand breaks continuously, disrupting the tumor cell proliferation cycle primarily at the G2/M checkpoint. The low-dose-rate mode also simulates a hyperfractionation effect, enhancing the killing of hypoxic cells, and as tumor blood vessels readjust and cells die, some oxygen-starved regions undergo reoxygenation, becoming more radiosensitive. There may be immune benefits as well, with antigen release from dying cells potentially activating dendritic cell-mediated adaptive responses. Meanwhile, because most normal tissues proliferate slowly and repair sublethal damage more effectively than tumor cells, the technique inflicts less long-term injury on surrounding healthy structures than fractionated external beam therapy at equivalent tumor doses.

The clinical track record, though still built largely on small studies, is encouraging. Iodine-125 seeds were first deployed against prostate cancer by Whitmore in 1972, and Blasko later achieved excellent results in early-stage disease, providing the foundation for extending the technique to thyroid cancer. Kanitz reported the first use in RAIR-DTC, treating three patients with non-iodine-avid, unresectable local recurrences and achieving local control in all three, with the longest follow-up extending to 41 months. Since then, multiple studies have explored the technique across the spectrum of refractory disease. Chen and colleagues studied six patients with bone metastases and found that pain scores, maximum tumor diameter, and serum thyroglobulin levels all declined at two and four months after implantation, while another report described a follicular thyroid cancer patient whose sternal metastasis shrank significantly five months after seed therapy despite failing surgery, repeated iodine-131 treatments, and external beam radiotherapy.

Lymph node metastases, among the most common sites of refractory recurrence, have also responded well. Zhang studied 44 patients with lymph node metastases who underwent seed implantation, with CT scans at two, four, and six months showing complete response in two patients, partial response in nine, stable disease in 29, and progression in only two, for an overall efficacy rate of 95.24 percent and a significant drop in average lymph node diameter and serum thyroglobulin. Yu followed 15 patients with 24 metastatic lymph nodes for a median of 48 months and observed no local regional recurrence after brachytherapy, with only three patients developing new nodes outside the treated areas and no significant adverse events. In one striking case reported by Zhai, a patient who had endured four surgeries, eight iodine-131 administrations, and immunotherapy over fifteen years still carried a 30-millimeter necrotic neck node; a multidisciplinary team aspirated the liquefied component, applied radiofrequency ablation, and implanted 33 seeds, and the mass disappeared within about a month.

The technique is also proving useful in combination strategies and in the most aggressive thyroid malignancy. Niu described an anaplastic thyroid cancer patient treated with two seed implantations plus the targeted drug apatinib, with follow-up showing significant tumor shrinkage, a notable result given that surgery alone does not meaningfully improve survival in advanced stages of that disease. Chen compared anlotinib alone, seed therapy alone, and the two combined in refractory patients, finding that local progression-free survival was significantly longer in the combination group at 42.2 months versus roughly 18.6 months in each single-modality group, although overall survival did not differ significantly. Separately, Chen reported 18 patients with 36 metastatic cervical lymph nodes treated under ultrasound guidance: compression and esophageal symptoms improved, 69 percent of nodes shrank during 50 months of follow-up, and a third of tumor volumes nearly vanished. Ren documented objective response rates peaking at 80 percent at six months and one-, two-, and three-year survival rates of 83.3, 72.2, and 61.1 percent respectively.

Beyond treatment, the seeds have found a second life as surgical guides. Radioactive seed localization involves implanting a low-activity seed into a lesion before surgery, then tracking it intraoperatively with a gamma detection probe, an approach long established in breast cancer. Garner first reported its use for three recurrent thyroid cancer lesions in the thyroidectomy bed, with all patients tolerating the procedure well, and subsequent reports from Pérez, Cambil, and Vilar demonstrated that seeds can guide resection of recurrent medullary and papillary lesions in anatomically complex neck positions with high accuracy. Surgeons gain ample preoperative preparation time, can verify complete resection immediately afterward, and can shorten incisions and operative times without increasing complications, offering a more economical alternative to other localization methods.

The review’s authors are careful to note the caveats. Most of the evidence comes from small, single-center studies with homogeneous patient selection, standardized protocols for seed localization are lacking, and the comparative value of different imaging guidance techniques remains unresolved. Larger, multi-center trials are needed before the technique’s place in guidelines can be fully secured. Still, the trajectory is clear: after decades of development, iodine-125 seed brachytherapy has evolved from a palliative afterthought into a mature local treatment that balances tumor control with quality of life. For patients whose tumors have abandoned iodine and whose anatomy has exhausted the surgeons, these rice-grain-sized sources of sustained radiation may represent the sharpest tool left in the arsenal, a kind of internal radiation scalpel delivering months of therapy from a single outpatient procedure.

Subject of Research: Iodine-125 seed brachytherapy for radioiodine-refractory differentiated thyroid cancer

Article Title: Current advances of iodine-125 seed brachytherapy in radioiodine-refractory differentiated thyroid cancer

Article References: Zhang, D., Su, Y., Tang, X., Wang, W., Wu, Z., & Chen, Z. (2026). Current advances of iodine-125 seed brachytherapy in radioiodine-refractory differentiated thyroid cancer. Holistic Integrative Oncology, 5(1), Article 79. https://doi.org/10.1007/s44178-026-00297-2

Image Credits: AI Generated

DOI: 10.1007/s44178-026-00297-2

Keywords: iodine-125 seed, brachytherapy, thyroid cancer, radioiodine-refractory, RAIR-DTC, bone metastasis, lymph node metastasis, low-dose-rate irradiation, radioactive seed localization, anlotinib, thyroglobulin, interventional oncology