Melanoma Micromets

Melanoma micrometastases (micromets) consist of small groups of melanoma cancer cells, originating from the original tumor site, that have spread through the lymphovascular system to another part of the body. The presence of metastasis in sentinel lymph nodes (SLN) may be the first indicator that cutaneous melanoma has spread to other sites in the body and helps to guide physicians in the diagnosis, prognosis, and therapy of their patients. In patients with clinical stage I/II melanoma, SLN status is the strongest predictor of survival.
Histologic evaluation of lymph nodes using a microscope is the most accurate way to assess for lymph node metastasis, since a significant number of patients have clinically negative lymph nodes. Histologic evaluation for melanoma involves sectioning of the tissue block at three different levels and staining each level with H&E and one of the levels with HMB45 and Melan A immunohistochemical (IHC) stains. HMB45 is a protein that facilitates maturation of stage I pre-melanosomes to stage II and is expressed in the majority of malignant melanomas as well as in tumors with melanocytic differentiation. Melan A is a lineage specific marker that recognizes a cytoplasmic protein involved in formation of stage II melanosomes and aids in diagnosis of metastatic melanoma. Malignant melanoma may be negative for at least one lineage-specific marker, so using a combination of markers, such as HMB45 and Melan A, is helpful for melanoma micromet detection. Correlation of the IHC and H&E slides is recommended and comparison with the cytomorphology of the primary tumor may be helpful in difficult cases.