An synthetic compound containing the organic phosphonate hydroxyethylidene diphosphonate (HEDP) labeled with the radioisotope rhenium Re 186. Re-186 hydroxyethylidene diphosphonate binds to hydroxyapatitie in bone, delivering a cytotoxic dose of beta radiation to primary and metastatic bone tumors. Re-186 is a beta emitter with a short half-life, a radioisotope profile that provides localized antitumor radiocytotoxicity while sparing extramedullary bone marrow tissues.