Holmium-166

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.