Behaviour of 99mTc-Evans Blue in the lymphatic system

Abstract

Lymphatic mapping for sentinel node biopsy in breast cancer and melanoma usually involves peritumoral injection of a 99mTc-colloid to locate the sentinel lymph node (SLN) several hours prior to surgery, as well as peri-operative injection of a blue dye. The lymphatic uptake and distribution of 99mTc-Evans Blue (EB) was investigated in a study assessing its potential as a lymphoscintigraphic agent. The popliteal node and afferent lymphatic channel were displayed surgically in six anaesthetised rabbits, and following subdermal administration of doses containing 99mTc-EB plus EB, simultaneous digital and scintigraphic images were obtained at regular time intervals to 30 minutes. Results from image analyses showed a high correlation (r=0.96) between 'blueness' and radioactivity as a function of time, indicating that 99mTc-EB migrated in lymph to concentrate in the SLN, at the same rate as EB. Instant thin layer chromatographic analysis of blue efferent lymph confirmed 99mTc-EB was mainly bound to endogenous proteins, and gel permeation chromatography determined the mechanism of uptake depends on a specific molecular structure of the dye. The 99mTc-complex of 1,7-diamino-8-naphthol-2,4-disulphonic acid was identified in (non- blue) urine, indicating 99mTc-EB is metabolised to this product by hepatic reduction prior to excretion. Excised popliteal nodes exposed to aqueous and organic solvents did not dissociate any blue colouration or radioactivity, suggesting a strong retention mechanism was present.