The Role and Impact of Holmium-166 Microspheres in Brachytherapy

Holmium-166 Microspheres, commercially known as 166Ho-QuiremSpheres®, represent an innovative advancement in brachytherapy, particularly for treating unresectable liver tumours. Developed by the University Medical Center of Utrecht, these microspheres are designed for direct implantation into hepatic tumours through the hepatic artery, emulating the mechanism of the previously utilised 90Y-microspheres. The core distinction lies in their material composition; 166Ho-microspheres are formulated from microparticles loaded with Holmium-166 (166Ho), encapsulated within poly-L-lactic acid (PLLA) particles, and activated through high-neutron flux. This article outlines the development, mechanism, and application of 166Ho-microspheres in liver cancer treatment, highlighting their CE marking achievement in April 2015.

Introduction to Holmium-166 Microspheres

Holmium-166 Microspheres are a contemporary brachytherapy solution tailored to patients with unresectable liver tumours. These microspheres offer a targeted therapeutic approach by employing beta electrons (β–) radiation from Holmium-166. Their development at the University Medical Center of Utrecht showcases a blend of radiopharmaceutical innovation and medical device precision, aiming to enhance the efficacy of liver cancer treatment.

Development and Certification

The inception of Holmium-166 Microspheres was driven by the need for more effective and targeted treatment options for liver cancer. The particles are synthesised by integrating Holmium-165 into PLLA particles, subsequently activated to 166Ho through exposure to a high-neutron flux reactor. This meticulous process ensures the stable and efficient production of 166Ho, which is pivotal for the microspheres’ therapeutic efficacy. In April 2015, 166Ho-QuiremSpheres® attained the CE mark, underscoring its safety and performance as per European Union standards.

Mechanism of Action of Holmium-166 Microspheres

The therapeutic premise of 166Ho-microspheres lies in their targeted delivery and the subsequent beta electron radiation emission. Once implanted into the hepatic tumour via the hepatic artery, these microspheres emit β– radiation, confining the therapeutic effect within the tumour while minimising exposure to surrounding healthy tissue. This localised radiation therapy impedes tumour growth and potentially induces tumour regression, presenting a promising treatment avenue for patients with inoperable liver cancer.

Comparison with 90Y-Microspheres

Holmium-166 Microspheres are often compared to 90Y-microspheres due to their analogous application in hepatic tumours. Both utilise beta radiation for tumoural therapy; however, 166Ho-microspheres offer certain advantages, including the possibility for post-implantation imaging due to the gamma emission properties of 166Ho. This facilitates precise dosimetry and treatment monitoring, providing an edge over 90Y-microspheres, which lack direct imaging capabilities post-implantation.

Clinical Application

The primary application of Holmium-166 Microspheres is in the treatment of unresectable liver tumours. By delivering high-dose radiation directly to the tumour site, this therapy aims to maximise tumoural dose while safeguarding healthy liver tissue, a crucial consideration given the liver’s vital functions. The use of 166Ho-microspheres has demonstrated promising outcomes in clinical settings, offering an effective treatment modality for patients who are not candidates for surgical resection or have limited treatment options.

Advantages and Considerations

The advantages of 166Ho-microspheres extend beyond their targeted therapeutic action. The ability to perform precise imaging post-implantation enables personalised treatment planning and efficacy assessment. Additionally, the safety profile and minimally invasive nature of the procedure make it a viable option for patients with limited alternatives. However, the treatment’s success is contingent upon careful patient selection, tumour accessibility, and meticulous procedural execution.

Future Directions

The development and clinical integration of Holmium-166 Microspheres mark a significant advancement in liver cancer therapy. Ongoing research and clinical trials continue to explore this treatment’s full potential, including its applicability to other tumour types and combination therapies. As understanding and technology evolve, 166Ho-microspheres may offer broader therapeutic horizons for cancer patients.


Holmium-166 Microspheres, or 166Ho-QuiremSpheres®, represent a pivotal innovation in the field of brachytherapy, offering a targeted and effective treatment option for patients with unresectable liver tumours. By harnessing the therapeutic potential of beta radiation within a precisely delivered microsphere, this treatment modality stands as a testament to the advancements in radiopharmaceutical therapy. With its CE marking and proven clinical efficacy, 166Ho-microspheres promise to play an integral role in the future of cancer treatment, blending radiological precision with therapeutic impact.

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