A subspecialty of medicine based on the use of radioactive substances in medical diagnosis, treatment, and research. Cyclotron-produced radioactive materials were introduced in the early 1930s, but the invention of the nuclear reactor during World War II made carbon-14, hydrogen-3, iodine-131, and later technetium-99m available in large quantities. Today most biomedical research and the care of many patients depend on the use of radioactive materials.