THE ACTIVE MOIETY OF RADIUM RA 223 DICHLORIDE IS THE ALPHA PARTICLE-EMITTING ISOTOPE RADIUM-223 (AS RADIUM RA 223 DICHLORIDE), WHICH MIMICS CALCIUM AND FORMS COMPLEXES WITH THE BONE MINERAL HYDROXYAPATITE AT AREAS OF INCREASED BONE TURNOVER, SUCH AS BONE METASTASES (SEE TABLE 2). THE HIGH LINEAR ENERGY TRANSFER OF ALPHA EMITTERS (80 KEV/MICROMETER) LEADS TO A HIGH FREQUENCY OF DOUBLE-STRAND DNA BREAKS IN ADJACENT CELLS, RESULTING IN AN ANTI-TUMOR EFFECT ON BONE METASTASES. THE ALPHA PARTICLE RANGE FROM RADIUM-223 DICHLORIDE IS LESS THAN 100 MICROMETERS (LESS THAN 10 CELL DIAMETERS) WHICH LIMITS DAMAGE TO THE SURROUNDING NORMAL TISSUE.