Sentinel node biopsy and lymphoscintigraphy with a technetium 99m labeled blue dye in a rabbit model

Lymphatic mapping for sentinel node biopsy in breast cancer and melanoma usually involves initial peritumoral injection of a radioisotope, gamma camera detection of the sentinel lymph node several hours prior to the operation, and separate perioperative injection of a blue dye. We have developed a combined approach using technetium 99m labeled blue dye (Evans Blue) for use in lymphoscintigraphy that may be injected as a single dose just prior to the operation.

In an anesthetized rabbit model we dissected a hind limb to display the popliteal node and afferent lymphatic. Technetium 99m Evans Blue ((99m)Tc-EB) (22 MBq; 0.5 mL) was injected subdermally in the dorsum of the paw. Simultaneous digital and gamma camera images were obtained at 14 time intervals to 30 minutes post injection. For each of these time intervals the percentage of radioactivity and percentage blueness of the popliteal node were determined. Urine and afferent lymphatic fluid were analyzed by chromatography. The popliteal node was excised post mortem, placed into solvent solutions and analyzed for blueness and radioactivity.

Time-activity curves for radioactivity and time-blueness curves for Evans Blue uptake showed strong correlation (r = 0.958). Lymph analysis suggested (99m)Tc-EB is mainly bound to endogenous proteins. Urine was radioactive but not colored, (99m)Tc-EB being metabolized and excreted in the urine as 1,7-diamino-8-naphthol-2,4-disulfonic acid. Prolonged exposure of node to solvents did not dissociate any blue coloration or radioactivity.

(99m)Tc-EB and Evans Blue are simultaneously retained and concentrated in the sentinel lymph node. This process is rapid and reproducible. (99m)Tc-EB migrates at the same rate as Evans Blue in lymph, where it is transported as bound to endogenous proteins. These dye molecules are metabolized by reductive cleavage in the liver and then excreted renally as colorless, radioactive metabolites. This novel agent has the potential to facilitate lymphatic mapping and subsequent sentinel node biopsy for a range of solid malignancies including breast cancer and melanoma.