The known isotopes of lutetium range in mass number from 149 to 190.
The primary decay mode before the most abundant stable isotope, 175Lu, is electron capture (with some alpha and positron emission), and the primary mode after is beta emission.
All isotopes of lutetium are either radioactive or, in the case of 175Lu, observationally stable, meaning that 175Lu is predicted to be radioactive but no actual decay has been observed.
Lutetium (177Lu) chloride, sold under the brand name Lumark among others, is used for radiolabeling other medicines, either as an anti-cancer therapy or for scintigraphy (medical radio-imaging).
Its most common side effects are anaemia (low red blood cell counts), thrombocytopenia (low blood platelet counts), leucopenia (low white blood cell counts), lymphopenia (low levels of lymphocytes, a particular type of white blood cell), nausea (feeling sick), vomiting and mild and temporary hair loss.