Erbium

¹⁶⁹Er (with a half-life of 9.4 days) is used in radiosynovectomy, which is a regularly practiced radiotherapy, on rheumatoid arthritis patients whose condition is resistant to standard methods of treatment (Fig. IUPAC.68.1). Rheumatoid arthritis is a chronic, inflammatory, autoimmune disease of the joint capsule (synovial sac), which is lined with a thin membrane called the synovium, of an individual’s moveable joints (synovial joints). In radiosynovectomy, the radiopharmaceutical called ¹⁶⁹Er- citrate colloid, which contains colloidal particles that are labeled with β-emitting ¹⁶⁹Er, is directly injected into the synovial cavity (the cavity between the bones in a moveable joint inside of the synovium) of the affected joint. These radioactive-colloid particles are then phagocytized (engulfed) by macrophage-like synoviocytes as well as other phagocytizing inflammatory cells in the patient’s synovium. Necrosis (tissue death) and the inhabitation of cell proliferation (increase in number of cells) result from the radiation of the synovium and therefore, temporarily halts synovitis (which is the condition of when the synovium thickens with inflammation) and improves synovial joint function [481], [482], [483], [484].