The tissue distribution of Evans blue dye in a sheep model of sentinel node biopsy

Tc-Evans blue is a 'single dose' agent for lymphatic mapping combining radioactivity and blue dye for sentinel node identification. The mechanism and distribution of blue dye retention in the lymph node is not clearly understood.

To demonstrate the cellular distribution of Tc-Evans blue in sheep sentinel lymph nodes by measuring the radioactivity of different tissue components and correlating this with pathological examination.

Tc-Evans blue was used to identify sheep lymph nodes. Part of each node was sent for pathological examination including imprint cytology, and frozen and permanent section examination. Sections were examined without stains, with only red stains and conventional haematoxylin & eosin staining. The remaining nodal tissue was homogenized and components separated by enzymatic digestion and density gradient centrifugation. Fractions representing each tissue component were counted in a gamma counter and the distribution of Tc-Evans blue calculated.

A dispersed population of blue staining cells was found. Their distribution, number and size indicated that they were histiocytes such as macrophages or antigen presenting cells. Radioactivity was distributed throughout the lymph node. Over 70% remained in the plasma, 19% in the leukocyte layer, and 10% was associated with erythrocytes and undigested tissue.

The accumulation of radioactivity and blue colour in the lymph nodes indicates the mechanism of retention is a result of the binding interaction between Tc-Evans blue-protein and lymph node histiocytes including macrophages and antigen presenting cells.