# Holmium-166

Source: https://nuclide.cc/isotopes/ho-166/  
Nuclide record `ho-166` (Isotope). Data CC BY-NC 4.0, attribute "Data from Nuclide (nuclide.cc)"; commercial use needs a licence.

## TL;DR

A radioactive metal built into tiny beads that are injected into the artery feeding a liver tumour, where they lodge and irradiate it from close range. Unlike similar beads made with yttrium-90, these can also be seen clearly on a scan.

## Summary

Holmium-166 poly-L-lactic acid (PLLA) microspheres (QuiremSpheres, QuiremScout) deliver radioembolisation to liver tumours much as yttrium-90 microspheres do, but 166Ho's shorter half-life (26.8 hours) concentrates over 90% of the dose within about 4 days, versus roughly 11 days for 90Y. Its beta spectrum has two main components near 1.77 MeV (48.7%) and 1.85 MeV (50.0%), with a mean tissue range around 2.2-2.5 mm and maximum around 8.7-11 mm, broadly comparable to 90Y.

The key practical advantage is imageability: 166Ho emits an 81 keV gamma photon (about 6.6-6.7% abundance) suitable for SPECT/CT, and holmium's paramagnetic properties also make the microspheres visible on MRI, so the same product used for a diagnostic 'scout' dose and the therapeutic dose can both be imaged directly, rather than relying on a technetium-99m-labelled surrogate as with 90Y (which requires a separate macroaggregated-albumin lung-shunt scan).

166Ho microspheres are manufactured by first embedding stable holmium-165 in the PLLA polymer matrix, then neutron-activating the finished microspheres in a research reactor (165Ho(n,γ)166Ho); production has run at the BR2 reactor (Belgium) and the Maria reactor (Poland). As a single-manufacturer product (Quirem Medical, part of Terumo) tied to specific reactor irradiation slots, availability is narrower than for 90Y microspheres but has not been reported as a major bottleneck.

## Fields

- Kind: Isotope
- Status: approved
- Last checked: 2026-09-22
- Also known as: 166Ho; Ho-166
- Tags: therapy; beta emitter; gamma emitter; radioembolisation; MRI-visible

## Sources

- Holmium-166 Radioembolization: Current Status and Future Perspectives (Cancers, 2022): https://pmc.ncbi.nlm.nih.gov/articles/PMC9626412/
- NICE: QuiremSpheres for unresectable hepatocellular carcinoma: https://www.nice.org.uk/guidance/ta985/chapter/2-Information-about-QuiremSpheres
- Safety and efficacy of holmium-166 radioembolization in HCC - the HEPAR Primary study (J Nucl Med, 2022): https://jnm.snmjournals.org/content/jnumed/early/2022/05/19/jnumed.122.263823.full.pdf

## Connected records

- technologies: [Radioembolisation (TARE / SIRT, yttrium-90)](https://nuclide.cc/technologies/radioembolisation-tare/)
- isotopes: [Yttrium-90](https://nuclide.cc/isotopes/y-90/)
- indications: [Hepatocellular carcinoma](https://nuclide.cc/indications/hcc/), [Intermediate hepatocellular carcinoma (BCLC B)](https://nuclide.cc/indications/hcc-intermediate/)
- drugs: [Holmium-166 microspheres](https://nuclide.cc/drugs/ho-166-microspheres/)
- companies: [Quirem Medical](https://nuclide.cc/companies/quirem-medical/)

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