99Tc-Evans blue dye for mapping contiguous lymph node sequences and discriminating the sentinel lymph node in an ovine model

The aim of this study was to investigate the potential of Tc-99m-Evans blue for discriminating the sentinel lymph node in multitiered lymph node sequences by using an ovine model. Tc-99m-Evans blue is an agent that has both radioactive and color signals in a single dose. Previous studies in smaller animal models suggested that this agent could have advantages over the dual-injection technique of radiocolloid/blue dye. Doses of (99m) Tc-Evans blue (similar to 21 MBq) containing Evans blue dye (approximately 4 mg) were administered to the hind limbs or fore limbs of sheep to map the lymphatic drainage patterns, validate its ability to identify the sentinel lymph node, and examine the reproducibility of the technique. The study protocol was repeated with Tc-99m-antimony trisulfide colloid and Patent Blue V dye. After the operative exposure, lymph nodes were identified with the gamma probe and then excised and analyzed for radioactivity (percentage of injected dose) and blue color. After the administration of Tc-99m-Evans blue, all lymph nodes harvested (35 of 35) in either short chains or long basins were hot and blue. The sentinel lymph nodes concentrated more radioactivity than the second-tier nodes to the extent of 2:1 to 215:1. For radiocolloid/Patent Blue V, the ratios were lower, at 2:1 to 3:1. Tc-99m-Evans blue was found to better discriminate the sentinel lymph node than Tc-99m-antimony trisulfide colloid/Patent Blue V in variable multitier lymph node anatomy, and it is an agent that promises to have positive clinical applications.