Cobalt-60 is a radioactive isotope of cobalt. Szilard used Cobalt-60 to treat his bladder cancer because it is a highly energetic form of radiation that can kill cancer cells. The radiation from Cobalt-60 is absorbed by the cancer cells, damaging their DNA and causing them to die. This type of treatment is called radiotherapy.
Unveiling Leo Szilard: A Quiz on the Remarkable Life and Contributions of a Visionary Physicist