Certain radioactive isotopes have an affinity or liking for certain body cells. Thus iodide given to a patient will bunch up and help machines "visualize" the thyroid so that a gland expert doctor can determine if there is something wrong. A drink called "Go Litely" can help MDs "see" what might be wrong with an intestinal tract. Merely diagnostic or fact-finding.
Someday, perhaps, specially developed isotopes will seek out and destroy tumor or cancer cells.