Dysprosium (Dy), chemical element, a rare-earth metal of the lanthanide series of the periodic table. Dysprosium has no biological role. It is considered to be moderately toxic. Dysprosium is good at absorbing neutrons and so it is used in dysprosium-oxide-nickel cement in control rods in nuclear reactors. Dysprosium is used in data storage applications such as compact discs and hard discs. It is also used in medium source rare-earth lamps (MSRs) in the film industry. Dysprosium iodide is used these lamps to produce an intense white light. Dy-164 has two medical applications. It is used in the production of Dy-165, which is used in arthritis therapy. Dysprosium isotope Dy-164 is also used for the production of Dy-166, which decays to Ho-166 and this used in cancer therapy.