Primary liver cancers (i.e. hepatocellular carcinoma or cholangiocarcinoma) are worldwide some of the most frequent cancers, with rapidly fatal liver failure in a large majority of patients. Curative therapy consists of surgery (i.e. resection or liver transplantation), but only 10-20% of patients are candidates for this. In other patients, a variety of palliative treatments can be given, such as chemoembolization, radiofrequency ablation or recently introduced tyrosine kinase inhibitors, e.g. sorafenib. Colorectal cancer is the second most lethal cancer in Europe and liver metastases are prevalent either at diagnosis or in follow-up. These patients are usually treated by a sequence of surgery, chemotherapy and antibody therapy [Okuda et al. (Cancer 56:918-928, 1985); Schafer and Sorrell (Lancet 353:1253-1257, 1999); Leong et al. (Arnold, London, 1999)]. Radioembolization is an innovative therapeutic approach defined as the injection of micron-sized embolic particles loaded with a radioisotope by use of percutaneous intra-arterial techniques. Advantages of the use of these intra-arterial radioactive compounds are the ability to deliver high doses of radiation to small target volumes, the relatively low toxicity profile, the possibility to treat the whole liver including microscopic disease and the feasibility of combination with other therapy modalities. Disadvantages are mainly due to radioprotection constraints mainly for (131)I-labelled agents, logistics and the possibility of inadvertent delivery or shunting [Novell et al. (Br J Surg 78:901-906, 1991)]. The Therapy, Oncology and Dosimetry Committees have worked together in order to revise the European Association of Nuclear Medicine (EANM) guidelines on the use of the radiopharmaceutical (131)I-Lipiodol (Lipiocis®, IBA, Brussels, Belgium) and include the newer medical devices with (90)Y-microspheres. (90)Y is either bound to resin (SIR-Spheres®, Sirtex Medical, Lane Cove, Australia) or embedded in a glass matrix (TheraSphere®, MDS Nordion, Kanata, ON, Canada). Since (90)Y-microspheres are not metabolized, they are not registered as unsealed sources. However, the microspheres are delivered in aqueous solution: radioactive contamination is a concern and microspheres should be handled, like other radiopharmaceuticals, as open sources. The purpose of this guideline is to assist the nuclear medicine physician in treating and managing patients undergoing such treatment.