Abstract: Boron containing polyphosphonates of the formula ##STR1## wherein Z is a boron-containing radical; R.sub.2 is an alkyl group containing from 1 to about 10 carbon atoms; R.sub.3 is a geminal diphosphonate or a vicinal polyphosphonate containing up to 10 phosphonic acid radicals; R.sub.4 is hydrogen, lower alkyl, amino, benzyl, halogen, hydroxyl, --CH.sub.2 COOH, --CH.sub.2 PO.sub.3 H.sub.2, or --CH.sub.2 CH.sub.2 PO.sub.3 H.sub.2 ; and the salts and esters thereof, have high affinity for calcified tissues, in particular calcific tumors. The compounds are useful in boron neutron capture therapy of such tumors.