Since the discovery of radium, the medical community has been interested in the use of unsealed radioactive isotopes to target and treat cancer and benign systemic and loco-regional proliferative conditions such as arthritis. Lack of effective systemic agents, such as chemotherapy during the early period of modern oncology, coupled with the intention to use targeted therapy for treating cancer made radioisotopes an option for use in cancer. Radioactive iodine-131 (I-131), because of abundant beta and gamma emissions and normal physiological uptake of elemental iodine in the thyroid gland, was the most logical choice for treating a number of thyroid disorders. Radioactive phosphorous-32 (P-32) made its way into nuclear medicine therapy, mainly because of the ubiquitous-ness of phosphorous in many biological molecules. It comes as no surprise that it was the first agent utilized in treating hematological malignancies. Since that time, several new isotopes were identified and clinical uses were explored. Advances in scientific research, systematic clinical trial design, and consideration of the properties of radiopharmaceuticals have resulted in finding new effective therapy agents.

Although many radioisotopes have potential applications in nuclear medicine therapy, the ultimate choice of an agent is based on type of emission, energy, half-life, ease of production, availability, and cost. Radioisotopes for therapeutic use can be largely divided into two groups—radiometals and radiohalogens. Radiometals have long been attractive options and have gained widespread popularity for many clinical uses while radiohalogens have maintained their popularity because of their relatively simpler radiolabeling characteristics.

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