Yttrium-90 (⁹⁰Y) is a beta-emitting radioisotope used for brachytherapy in liver cancer treatment due to its 12 mm tissue penetration range (Dong et al., 2024; Christie et al., 2011; Talebi and Rajabi, 2022). Early ⁹⁰Y resin microspheres faced discontinuation due to significant ⁹⁰Y leaching and adverse effects. Subsequent efforts focused on mitigating leaching by incorporating stable ⁹⁰Y into Yttrium oxide (Y₂O₃) combined with alumina and silica, followed by high-temperature melting and neutron bombardment to create ⁹⁰Y glass microspheres. This process reduced toxicity associated with ⁹⁰Y leakage.

The Affiliated Zhongshan Hospital of Fudan University developed ⁹⁰Y glass microspheres (30–50 μm) with a density of 3.27 g/mL, which require further clinical validation (Yan et al., 1993). High density can hinder injection, and limited microsphere count may inadequately irradiate large tumors. Resin microspheres, with their lower density, offer easier injection. To address ⁹⁰Y leaching from resin microspheres, an alkali precipitation step was added post-ion exchange (Lau et al., 1994). This reduced the leakage rate while maintaining safety, enabling its usage for tumors with large blood supply. However, some leaching persisted, necessitating glucose or contrast agents for dispersion (Westcott et al., 2016). Metal ions in tumor tissue contributed to leakage rates of 1/1000 to 4/1000. Amthauer et al. detected radioactivity in urine from patients treated with ⁹⁰Y resin microspheres, potentially from reversible ion exchange (Grosser et al., 2018). Commercial options include SIR-Spheres^® (polymer) and TheraSphere^® (glass), containing 1.2 million spheres per vial at 3 GBq activity (2500 GBq/microsphere). Glass microspheres are denser than blood and resin microspheres three times as much as blood (Baino et al., 2021). Zhen et al.'s pooled analysis of 16 studies demonstrates that TARE with ⁹⁰Y microspheres achieves median overall survival of 14.3 months (95% CI: 11.9-17.1) in unresectable ICC (Intrahepatic Cholangiocarcinoma), with disease control rates of 77.2% (RECIST criteria: 11.5% partial response, 61.5% stable disease). Both glass (14.0 months OS) and resin (14.3 months OS) microspheres show comparable efficacy, with predominantly mild adverse events requiring no intervention (Zhen et al., 2019).

Phosphorus-32 (³²P) is a pure beta-emitting radionuclide characterized by a half-life of 14.3 days and a maximum tissue penetration radius of 8 mm (Order et al., 1996; Rahman et al., 2020). These properties render ³²P particularly suitable for long-distance transport and prolonged internal radiation therapy. Wallner et al. were the first to report the application of ³²P colloidal chromic phosphate bound to albumin particles for brachytherapy of unresectable pancreatic tumors. However, the small particle size of ³²P colloidal chromic phosphate and the high interstitial pressure of tumors may cause radioactive toxicity side effects when injected, no subsequent clinical reports using this method were seen. Masakazu Kawashita et al. from Kyoto University mixed Y₂O₃, Al₂O₃, and SiO₂ and melted them at 1600°C to produce rectangular glass pieces material with a size of 1 cm in length and width, then bombarded red phosphorus with an electron beam to produce phosphorus ions (P⁺) and implanted them into the glass material to produce P⁺ rich glass material, which was finally bombarded by a high-flux neutron reactor to obtain ³²P/⁹⁰Y/⁸⁸Y/⁹¹Y/¹⁵⁴Eu glass microspheres (Kawashita et al., 1999). Therefore, Masakazu Kawashita from Kyoto University also successfully prepared YPO₄ and Y₂O₃ microspheres with diameters ranging from 20 to 30 μm using a high-frequency induction heating plasma method (Kawashita, 2002). How to stably bind ³²P onto microsphere carriers is an urgent problem that needs solving before applying ³²P microspheres in clinical liver cancer brachytherapy treatments. Recent clinical studies demonstrate promising outcomes for ³²P in pancreatic cancer treatment. A propensity-score weighted analysis of 104 LAPC patients showed that combining chemotherapy with EUS-guided ³²P implantation significantly improved outcomes versus standard therapy (chemotherapy ± chemoradiotherapy). The combination therapy group exhibited 189 days longer restricted mean survival time (527.2 vs. 338.0 days), 168.6 days longer local progression-free survival, and 23.9% higher downstaging probability within 30 months. These results highlight ³²P’s potential when combined with systemic therapy for localized tumor control (Lim et al., 2025).

Iodine-131 (¹³¹I) is a diagnostic and therapeutic radionuclide (half-life: 8.04 days) that emits both β-rays (99%) and γ-rays (1%). SPECT imaging can be performed, and its maximum tissue range radius is 2 mm (Larson et al., 2015; Li Y. et al., 2025). In 1992, Li et al. labeled gelatin microspheres with ¹³¹I and combined them with chemotherapeutic drugs to treat nine patients with unresectable liver cancer (Chen et al., 1992). However, the microspheres caused severe ectopic embolism, leading to one fatality due to systemic ¹³¹I leakage. Chen et al. synthesized biodegradable ¹³¹I/¹²⁵I dual-labeled gelatin microspheres for rabbit liver embolization and metabolism studies using the chloramine T labeling method, and found that a small amount of ¹³¹I/¹²⁵I would be released into the blood along with the degradation of gelatin microspheres as well as taken up by the thyroid gland (Ma et al., 2012). In 2016, Li et al. prepared ¹³¹I-gelatin-chitosan microspheres by reverse emulsion crosslinking method and chloramine T labeling method to overcome the problem of glass microspheres being non-degradable and unable to repeat brachytherapy treatment (Cai et al., 2016). Four years later, Li et al. synthesized a novel biodegradable chitosan-collagen composite microsphere labeled with ideal sedimentation efficiency, good in vitro and in vivo stability (Pang et al., 2020). Zhang et al. prepared a kind of silk protein microsphere labeled with ¹³¹I (¹³¹I-SFMs) for brachytherapy treatment of rat liver tumors by emulsification crosslinking method combined with chloramine T labeling method (Wu M. R. et al., 2022). Song et al. developed a type of poly (lactic acid) microsphere coated with copper sulfide nanoparticles and paclitaxel labeled with ¹³¹I (¹³¹I-HCuSNPs-MS-PTX), which can be used for SPECT and photoacoustic dual-modal imaging, as well as effective chemotherapy, radioembolization therapy, and photothermal therapy (Liu et al., 2018). Clinical studies of ¹³¹I SPECT/CT in differentiated thyroid cancer (DTC) demonstrate its prognostic value for lymph node metastases (LNM). A retrospective analysis of 942 DTC patients revealed that those without LNM achieved complete response (CR) faster (median 9.4 months vs. 44 months with LNM, HR 2.2) and had better progression-free survival (PFS, HR 0.46). Patients with enlarged ¹³¹I-negative LNM showed the longest time to CR (24 months), while those with small LNM had PFS comparable to LNM-negative cases. Treatment strategies should be individualized, with reoperation preferred for enlarged LNM (13.5 months to CR) and second radioiodine therapy for small LNM (better PFS, HR 4.0) (Heinrich et al., 2025).

Recent advancements in ¹³¹I microspheres focus on multifunctional designs. Sun et al. developed dual-functional ¹³¹I-PDA@PVA microspheres combining radioembolization and photothermal therapy (Sun et al., 2025). These microspheres exhibited high stability (76.5% retention in serum), excellent elasticity, and synergistic tumor inhibition (10% cell viability) via β/γ radiation and NIR-triggered hyperthermia (ΔT > 20°C). SPECT/CT enabled real-time tracking, demonstrating clinical potential for HCC theranostics.

Rhenium-186 (¹⁸⁶Re) and Rhenium-188 (¹⁸⁸Re) (half-lives: 3.8 days and 0.71 days, respectively) are β- and γ-emitting radionuclides suitable for SPECT imaging with tissue range radii are 4.5 mm and 11 mm (Häfeli et al., 2007; Pourhabib et al., 2019). In 1998, Ehrhardt et al. prepared glass microspheres enriched with ¹⁸⁶Re and ¹⁸⁸Re by mixed ReO₂ powder with glass frits and calcined them at 1050°C, then bombarded them with a neutron reactor (Conzone et al., 1998). However, the in vitro stability was poor. Hafeli et al. prepared a biodegradable ¹⁸⁶Re/¹⁸⁸Re polylactic acid microsphere by neutron reactor bombardment method (Häfeli et al., 1999; Häfeli et al., 2001). Subsequently, Hafeli et al. prepared ¹⁸⁶Re/¹⁸⁸Re fibrin glue with high adhesion strength in moist tissue by bombarding Re-enriched fibrinogen with a low-flux neutron reactor for intratumoral irradiation therapy (Häfeli et al., 2007). Mostafa et al. prepared a polylactic acid microsphere loaded with ¹⁸⁸Re sulfide colloid nanoparticles, with a microsphere diameter range of 13–48 μm and a¹⁸⁸Re labeling efficiency of up to 99% (Jamre et al., 2018). Lee et al. prepared a type of ¹⁸⁸Re microsphere by mixing ¹⁸⁸Re (OH₂)₃ (CO)³⁺, which was formed by the interaction of amino borane reduction of ¹⁸⁸ReO₄ and carbon monoxide, with human serum albumin microspheres, with a diameter of 13–48 μm, for the treatment of in situ hepatocellular carcinoma in rats (Ni et al., 2015), Saatchi et al. mixed polylactic acid containing dimethylpyridine amine with polycaprolactone as the dispersed phase and used microfluidic technology to prepare a type of mixed biodegradable microsphere for chelating labeling of ¹⁸⁸Re (De La Vega et al., 2019). A Phase 1 trial (NCT01906385) evaluated ¹⁸⁶Re-nanoliposomes (¹⁸⁶RNL) delivered via convection-enhanced delivery in 21 recurrent glioblastoma patients. The maximum tolerated dose was not reached (up to 22.3 mCi), with most adverse events unrelated to treatment. Median overall survival was 11 months (17 months for patients receiving >100 Gy tumor absorbed dose vs. 6 months for <100 Gy), exceeding standard care outcomes. Disease control was achieved in 61.9% of patients (57.1% stable disease, 4.8% partial response), demonstrating ¹⁸⁶Re’s potential for targeted brain tumor therapy (Brenner et al., 2025).

Holmium-166 (¹⁶⁶Ho) possesses a half-life of 1.1 days, a maximum tissue range of 8.7 mm, and emits both β-rays and γ-rays (81 keV, 62%), making it a diagnostic-therapeutic nuclide for SPECT and magnetic resonance dual-modal imaging (Kühnel et al., 2022; Seevinck et al., 2012; Vente et al., 2014). Nijsen et al. prepared polylactic acid microspheres by solvent evaporation method, then added non-radioactive acetylacetone complexed ¹⁶⁵Ho compound (¹⁶⁵Ho-acetylacetone) to bind with polylactic acid microspheres (Nijsen et al., 1999; Nijsen et al., 2001). They filtered out microspheres with a particle size range of 20-50 μm through a filter screen and finally obtained ¹⁶⁶Ho-acetylacetone-polylactic acid microspheres by bombarding them with a neutron reactor for 6 h. Under Good Manufacturing Practice (GMP) guidelines, Nijsen et al. optimized the evaporation temperature, sieving, and raw material selection in the solvent evaporation process to achieve gram-scale microsphere production. (Zielhuis et al., 2006). To improve the specific activity and ¹⁶⁶Ho stability of ¹⁶⁶Ho microspheres in vivo, Nijsen et al. formed two kinds of inorganic-like ¹⁶⁶Ho microspheres (¹⁶⁶HoPO₄ microspheres and ¹⁶⁶Ho(OH)₃ microspheres) by ion exchange and neutron reactor irradiation of solid acetylacetone holmium microspheres with NaH2PO4 or NaOH (Arranja et al., 2020). A prospective phase 2 study (NCT03379844) in 31 HCC patients demonstrated the clinical utility of ¹⁶⁶Ho-microspheres radioembolization. Hepatobiliary scintigraphy revealed significant reductions in treated liver volume (17%, p = 0.0027) and function (16%, p = 0.0017), with cirrhotic patients showing limited functional compensation (10% decrease vs. 0% in non-cirrhotics). The technique effectively maintained liver function in Child Pugh ≤ B7 patients (median MELD 9), with hepatic clearance rates correlating with biochemical markers (bilirubin, albumin, ALT; p < 0.05), supporting its safety profile in selected HCC populations (Reinders et al., 2025).

Samarium-153 (¹⁵³Sm) is a radioactive nuclide that can release both β-rays with a maximum energy of 810 keV and γ-rays (103 keV), making it suitable for clinical SPECT imaging and radiotherapy (Wang et al., 2020; Bayouth et al., 1994; Wong et al., 2023). Bai et al. prepared a novel ¹⁵³Sm-labeled biodegradable polylactide microsphere and studied its stability in rabbits. Yeong et al. labeled ¹⁵²Sm on negatively charged acrylic microspheres by electrostatic adsorption then treated them with sodium carbonate solution for alkaline precipitation, and finally irradiated them with a neutron reactor for 6 h to obtain ¹⁵³Sm microspheres (Wong et al., 2019). Subsequently, Yeong et al. used a solid-phase-oil-water solvent evaporation method to prepare ¹⁵²Sm₂O₃ microspheres, which were then irradiated with a neutron reactor to obtain ¹⁵³Sm microspheres with an average particle size of 33 μm, and no longer-lived radioactive nuclides or elemental impurities were found (Tan et al., 2022). Ying et al. also developed a biodegradable ¹⁵²Sm-acetylacetone-polylactide microsphere for intra-arterial radioembolization therapy of liver tumors (Wong et al., 2020). Recent advances in ¹⁵³Sm microspheres demonstrate their potential for combined chemo-radioembolization. Researchers successfully developed sulphonated polystyrene microspheres (31.95 ± 0.26 μm) co-loaded with [¹⁵³Sm]Sm₂O₃ (2.82 ± 0.6 GBq/g) and doxorubicin (55.6% ± 1.1% encapsulation efficiency). These theranostic microspheres showed excellent radionuclide retention (>99% over 300 h) in physiological conditions with no detectable impurities after neutron activation. The formulation meets key requirements for intraarterial liver cancer treatment, offering simultaneous chemotherapy and radiotherapy delivery while maintaining optimal physicochemical properties for clinical translation (Wong et al., 2025).

Ytterbium-175 (¹⁷⁵Yb) is a radioactive isotope for studying in vivo biodistribution. In addition, Lutetium-177 (¹⁷⁷Lu) is the metal nuclide for clinical diagnosis and treatment of diseases (O'Neill et al., 2020; Jamre et al., 2019). Shamsaei et al. designed and developed a novel biodegradable ¹⁷⁵Yb-labeled polylactide microsphere for intratumoral irradiation radiotherapy embolization (Jamre et al., 2019). Gao et al. prepared a¹⁷⁷Lu silica microsphere that can be used for SPECT imaging by directly physically mixing ¹⁷⁷LuCl₃ solution with mesoporous silica microspheres and then performing alkali precipitation, and directly used it for intratumoral injection therapy of tumor-bearing mice and achieved good anti-tumor efficacy (Wu X. et al., 2022).

Recent advancements in ¹⁷⁷Lu-based therapies include the development of injectable 3D hollow porous granular hydrogels (¹⁷⁷Lu-3D-HPGH) for precise brachytherapy (Xu et al., 2023). Synthesized via microfluidics and UV cross-linking, these hydrogels, developed by Xu et al., demonstrate high radiolabeling efficiency (97.85%), uniform tumor distribution, and robust anti-tumor efficacy in preclinical models, offering a promising theranostic platform for HCC treatment. Zhao et al. engineered ¹⁷⁷Lu-PCMs using radiation-induced graft polymerization (Zhao et al., 2024). These phosphocholine-modified microspheres demonstrated ultra-stable Lu coordination (DFT-confirmed chelation), mechanical robustness (117.2 μm size), and precise tumor targeting in rabbit VX2 models. SPECT/CT-guided intra-arterial brachytherapy achieved complete tumor regression without ectopic leakage, highlighting translational potential for image-guided HCC treatment.

Innovative radio-immunotherapy approaches have emerged. Yang et al. created ¹⁷⁷Lu-labeled alginate microspheres co-loaded with IDO1 inhibitor Indoximod (Yang et al., 2023). The 2 μm microspheres achieved >90% labeling efficiency, suppressed kynurenine pathways, and enhanced CD8⁺ T-cell infiltration. Combined with αPD-L1, they inhibited distal tumors in H22 models via DC maturation and Treg downregulation, showcasing a promising immunomodulatory platform.

In summary, most traditional radioactive microspheres are prepared by “cold” microspheres using methods such as melt spraying, solvent evaporation, inverse emulsion cross-linking, and alkali precipitation. Finally, they all need to be bombarded by high-flux neutron reactors to activate the “cold” microspheres into radioactive microspheres.