Effect of high specific-activity sulfur colloid preparations on sentinel node count rates

Purpose: Preliminary results by other investigators suggest that increasing the specific activity of Tc-99m nanocolloid preparations increases the measured counts in sentinel nodes compared with lower specific-activity (SA) preparations using the same initial injected dose. The authors set out to determine whether a similar result could be perceived with Tc-99m sulfur colloid (SC) preparations. Methods: Twenty-three consecutive patients (low SA group) with successful visualization of sentinel nodes by lymphoscintigraphy before our standard protocol was changed to a higher SA preparation were compared with 28 patients (high SA group) just after the switch. Injection techniques were similar in both groups: peritumoral injections at two to four points of a mixture of half-filtered (0.22 mum filter) and unfiltered Tc-99m sulfur colloid in 6 ml followed immediately by intradermal injections of filtered sulfur colloid above the tumor. Activity levels for both types of injections ranged from 3.7 to 11.1 mBq (100 to 300 muCi). Preparation of the higher SA mixture of sulfur colloid was achieved by using only one eighth of the sulfur colloid vial contents when the same activity (125 mCi) of Tc-99 was added. Regions of interest were drawn around the images of sentinel nodes and the initial injection site in the anterior and lateral projections. Ratios of sentinel node to initial injection site count were calculated for both groups. Results: The mean ratio of sentinel node to injection site count in the high SA group was 2.9 times greater than that in the low SA group. The median ratio value was 2.7 times greater in the high SA group. Conclusion: These preliminary results suggest higher counts in the sentinel node are possible with a higher SA preparation.