In vivo stability of ferric hydroxide macroaggregates (FHMA). Is it a suitable carrier for radionuclides used in synovectomy?

Ferric hydroxide macroaggregates (FHMA) have been widely used as a carrier for several radionuclides used in radiation synovectomy. Different rates of extra-articular leakage of radioactivity have been observed with ⁹⁰Y and ¹⁶⁵Dy. In order to understand the mechanism(s) involved in the extra-articular leakage of radioactivity, the in vivo stability of FHMA carrier was studied. Following an injection of [⁵⁹Fe]Fe-FHMA into the knees of normal rabbits, the cumulative leakage of [⁵⁹Fe]Fe-FHMA was 2.9% at 5 days and 12.3% at 14 days. More than 60% of this activity was in the blood. But when FHMA was double labeled with ⁵⁹Fe and ¹⁶⁶Ho, the ⁵⁹Fe leakage significantly increased to 18.5% at 5 days and 27% by 14 days. The instability of FHMA is accelerated when it is complexed with ¹⁶⁶Ho and may be due to the "mass effect" of ¹⁶⁶Ho or due to radiolysis induced by high energy β particles from ¹⁶⁶Ho. These results suggest that FHMA is a suitable carrier only for the short lived radionuclides used in synovectomy.