Biological and chemical evaluation of various radiocolloids used for clinical radiosynovectomy

Radiopharmaceuticals used for radiosynovectomy are the following: 90Y-citrate colloid, 90Y-silicate colloid, 169Er-citrate colloid, 166Ho-phytate and 188Re-tin colloid. Radiochemical purity and stability in all cases were higher than 99.0%. The particle sizes of the radiocolloids were different; the mean values were as follows: 90Y-citrate colloid: 3.1 μm, 90Y-silicate colloid: 0.9 μm, 167Er-citrate colloid: 1.1 μm, 166Ho-phytate: 0.66 μm, 188Re-tin colloid: 0.6 μm. During biodistribution studies in rabbits the activity values in various organs (liver, blood, spleen, kidneys, lymph nodes, skeleton) outside of the injected knee ranged up to 10% of injected dose (ID) even after 14 days since injection. Activity values retained in the knee were in the range 95-87% of ID. Activity recoveries during animal studies were also high during the initial time period (90-95%) and a week after injection only dropped slightly below 90%. Owing to the favourable intra-articular retention and very low leakage values in all cases, the calculated absorbed dose showed high values in the target synovial surface (∼40 Gy), low effective dose (5.3 mGy) and also low whole body absorbed dose (1.9 mGy) values. According to preclinical data obtained during this study, the authors concluded that all the colloid radiopharmaceuticals tested showed very low leakage and that organs only accumulate insignificant amounts of radioactivity. This leads to very favourable dosimetric calculations. (author)