A drug used to treat prostate cancer that has spread to the bone and has not gotten better with other treatment. It is also being studied in the treatment of other types of cancer. Radium 223 dichloride contains a radioactive substance called radium 223. Radium 223 collects in bone and gives off radiation that may kill cancer cells. Radium 223 dichloride is a type of radiopharmaceutical. Also called Xofigo.