Sources Of Error In Interpreting Sentinel Lymph Nodes

Errors may be associated with S-100 protein staining. The major problem here is the interpretation of dendritic leukocytes or sinus macrophages. Dendritic leukocyte interpretation is especially difficult if the dendritic leukocytes are nonden-dritic, as is often the case in the immune-suppressed inactive sentinel nodes (Fig. 3b). With good quality immunohistochemical preparations, sinus macrophages do not stain for S-100 protein; however, if there is background staining, these may present interpretative difficulties. Capsular nevocytes (Figs. 4a-4c) occur in more than 20% of patients undergoing sentinel lymph node dissection and are made more visible by the use of immunohistochemistry. These cells are, in fact, confined to the capsule and trabeculae of the lymph node. They tend to be smaller and more cohesive than melanoma cells and while strongly S-100 protein positive (Fig. 4b) will either express no HMB-45 or HMB-45 at a relatively weak level (Fig. 4c). Nevocyte clusters are often arranged around capsular vessels. Another pitfall is the presence of neural tissue within the lymph node (Fig. 5). If the nerve has associated Schwann cells, these may stain relatively strongly, and if the nerve is cut transversely, an appearance suggestive of a cluster of S-100-protein-positive melanoma cells may result.

Errors may also be associated with HMB-45 staining. These are fewer than those encountered with S-100 protein, but some care is necessary. A minority of melanomas (10-15%) are made up of cells that do not express HMB-45. In hya-linized and calcified connective tissue within lymph nodes, especially lymph nodes from the groin and internal iliac area, extracellular HMB-45 positivity may be seen and care is necessary to avoid overcalling this appearance.

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