Influence of temperature on the radiochemical purity of 99mTc-colloidal rhenium sulfide for use in sentinel node localization

BACKGROUND Some preparations of 99mTc-colloidal rhenium sulfide (Nanocis) contain excess free pertechnetate (99mTcO4) impurity (>5%). OBJECTIVES To improve the radiochemical purity of Nanocis preparations and the quality of preoperative lymphoscintigraphy in sentinel node localization of breast cancer patients, we investigated the effects of temperature on the presence of free 99mTcO4 and nanocolloid size modification. METHODS A Nanocis kit was reconstituted with sodium pertechnetate (650-850 MBq) in a final volume of 3.5 ml and heated for 30 min at 100, 115, or 130 degrees C. The radiochemical purity was determined by paper chromatography, in triplicate. The particle size was evaluated by membrane filtration through a 200-nm and 100-nm filter. The preoperative lymphoscintigraphy images were acquired about 2 h after tracer administration. RESULTS Significantly higher radiochemical purity values were observed with 28 Nanocis preparations heated at 130 degrees C (median: 99.8%, min-max: 97.0-99.9%) compared with values from 37 Nanocis preparations heated at 100 degrees C (median: 96.3%, min-max: 85.2-99.5%) or 26 Nanocis preparations heated at 115 degrees C (median: 95.1%, min-max: 85.7-99.8%). The interbatch variations of the radiochemical purity were reduced at 130 degrees C. A high temperature level (130 degrees C) did not modify the particle size. In lymphoscintigraphy, free 99mTcO4 uptake by the thyroid or stomach, which was sometimes observed with a Nanocis preparation heated at 100 or 115 degrees C, was never visualized with a Nanocis preparation heated at 130 degrees C. CONCLUSION These results indicate that increasing temperature from 100 to 130 degrees C can be used in routine clinical practice to improve the radiochemical purity of the Nanocis preparation.