A radioactive drug used to treat prostate cancer that has spread to the bone and is causing symptoms but has not spread to other organs. It is used in patients whose cancer is castration resistant (has not responded to treatments that lower testosterone levels). It is also being studied in the treatment of other types of cancer. Radium Ra 223-dichloride contains a radioactive substance called radium Ra 223. It collects in bone and gives off radiation that may kill cancer cells. Radium Ra 223-dichloride is a type of radiotherapeutic agent.