{"entity":{"id":"ho-166","kind":"isotope","name":"Holmium-166","aka":["166Ho","Ho-166"],"tldr":"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.\n\nThe 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).\n\n166Ho 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.","status":"approved","asOf":"2026-09-22","links":[{"label":"Holmium-166 Radioembolization: Current Status and Future Perspectives (Cancers, 2022)","url":"https://pmc.ncbi.nlm.nih.gov/articles/PMC9626412/"},{"label":"NICE: QuiremSpheres for unresectable hepatocellular carcinoma","url":"https://www.nice.org.uk/guidance/ta985/chapter/2-Information-about-QuiremSpheres"},{"label":"Safety and efficacy of holmium-166 radioembolization in HCC - the HEPAR Primary study (J Nucl Med, 2022)","url":"https://jnm.snmjournals.org/content/jnumed/early/2022/05/19/jnumed.122.263823.full.pdf"}],"tags":["therapy","beta emitter","gamma emitter","radioembolisation","MRI-visible"],"related":["radioembolisation-tare","y-90","hcc","hcc-intermediate"],"indications":[],"sections":[],"technologies":[],"targets":[],"drugs":[],"companies":[],"institutions":[],"pathways":[],"terms":[],"trials":[],"people":[],"bottlenecks":[],"keyPapers":[],"journals":[],"dependsOn":[],"notes":[],"symbol":"166Ho","element":"Holmium","massNumber":166,"halfLife":"26.8 hours","emissions":["β− 1.77 MeV (48.7%) and 1.85 MeV (50.0%)","γ 81 keV (6.6-6.7%)"],"emissionClass":"beta-minus","use":"both","tissueRange":"Mean about 2.2-2.5 mm, maximum about 8.7-11 mm","production":"Reactor: neutron activation of stable holmium-165 already embedded in the finished PLLA microspheres, 165Ho(n,γ)166Ho.","supply":"adequate","supplyNote":"Single-manufacturer product (Quirem Medical/Terumo) irradiated at specific reactors (e.g. BR2 in Belgium, Maria in Poland); narrower supplier base than 90Y microspheres but not reported as a bottleneck.","chelators":[],"pairedWith":[]},"route":"/isotopes/ho-166/","neighbours":{"technology":[{"id":"radioembolisation-tare","kind":"technology","name":"Radioembolisation (TARE / SIRT, yttrium-90)","route":"/technologies/radioembolisation-tare/"}],"isotope":[{"id":"y-90","kind":"isotope","name":"Yttrium-90","route":"/isotopes/y-90/"}],"indication":[{"id":"hcc","kind":"indication","name":"Hepatocellular carcinoma","route":"/indications/hcc/"},{"id":"hcc-intermediate","kind":"indication","name":"Intermediate hepatocellular carcinoma (BCLC B)","route":"/indications/hcc-intermediate/"}],"drug":[{"id":"ho-166-microspheres","kind":"drug","name":"Holmium-166 microspheres","route":"/drugs/ho-166-microspheres/"}],"company":[{"id":"quirem-medical","kind":"company","name":"Quirem Medical","route":"/companies/quirem-medical/"}]}}