Holmium microspheres have recently become available in the European market as the third type of microspheres for radioembolization of unresectable liver malignancies. Holmium microspheres come with a dedicated administration system, and since these microspheres contain holmium-166 (166Ho) instead of yttrium-90, unique dosing and imaging possibilities have become available as well. In addition, a scout dose of 166Ho microspheres (Conformité Européenne mark is now granted and not pending anymore) can be used instead of 99mTc-macroaggragated albumin during the preparatory angiography procedure. So far, two prospective phase I and phase II clinical studies have been performed on 166Ho radioembolization in a population of liver metastases from mixed origins. These studies showed that a mean whole-liver dose of 60 Gy is safe and induces tumor response. Ongoing trials investigate the effect of 166Ho radioembolization in patients with neuroendocrine tumor metastases, hepatocellular carcinoma, and colorectal cancer metastases. Data derived from these studies will be used to refine the dosing schedule of 60 Gy to the whole liver and determine the optimal level of activity for each patient. This paper discusses several basics and provides an overview of relevant dosing aspects, technical aspects of performing holmium radioembolization, as well as a summary of completed and ongoing clinical studies and the upcoming developments regarding these microspheres.
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