The effect of sodium thiosulfate levels on filtered Tc-99m sulfur colloid yields

Objectives In our nuclear pharmacy, filtered Tc-99m sulfur colloid (SC) is formulated from high concentration Tc-99m SC using low thiosulfate levels. The objective of the study was to evaluate the effect of various thiosulfate levels on filtered Tc-99m SC yields.

Methods Previously, high specific concentration Tc-99m SC was formulated using a small fraction, 0.25 mg (0.125 ml) thiosufate from a sulfur colloid kit (Pharmalucence, Inc.). Our lab investigated the effect of increasing thiosulfate levels including 0.2 mg (0.1 ml), 0.4 mg (0.2 ml), and 0.6 mg (0.3 ml) on the yield of filtered Tc-99m SC. For each experimental design, between 101-103 mCi of Tc-99m pertechnetate was added to thiosulfate along with the acid solution. The solution was boiled for 5 minutes, cooled, and neutralized with the respective buffer. The radiochemical purity of each thiosulfate preparation was evaluated using miniaturized chromatography consisting of Whatman 31ET paper with acetone. Duplicate aliquots of each thiosulfate formulation, ranging from 30-40 mCi, was removed and transferred to a 0.1 um membrane filter (Millex Durapore PVDF membrane filter) which were then washed with 1.0 to 1.4 ml of saline into specific vials (2.0 ml total volume). Activity of each membrane filter and vial was measured using a Biodex Atomlab 500 dose calibrator and mean activity for each experimental design calculated.

Results The radiochemical purity of high specific concentration Tc-99m SC was >98% for all formulations evaluated. The mean yields of filtered Tc-99m SC (0.1 um) utilizing 0.2 mg, 0.4 mg and 0.6 mg thiosulfate were 54%, 57%, and 50%, respectively, indicating that the 0.4 mg thiosulfate formulation was optimal.

Conclusions The effect of altering thiosulfate levels on filtered Tc-99m SC was investigated. As a result, our nuclear pharmacy has adopted the modified formulation consisting of 0.4 mg thiosulfate for filtered Tc-99m SC.