It's difficult to use that which has never been found in nature. An estimated one gram of astatine exists at any given time in Earth's crust. However, once formed from the decay of heavier elements, astatine doesn't remain around for long. Its half-life is a paltry 8.1 hours! Nobody has ever even seen elemental astatine, because a mass large enough to be viewed with the naked eye would instantly be vaporized from the heat resulting from its own radioactivity.
Astatine-211, an isotope, has found limited application in medicine. It's occasionally used in radiation therapy for cancer or to label cells.
(Image: Greg Robson)