Current role of electron microscopy in the diagnosis of pigmented tumors

Melanoma represents the most diagnostically challenging of the group of pigmented lesions to be discussed in this review, but a number of other pigmented lesions may present diagnostic difficulties either because they are uncommon or occur in unexpected clinico-pathologic situations. With the routine use of immunohistochemistry as the initial ancillary diagnostic technique in surgical pathology, electron microscopy has been relegated to a secondary but still essential diagnostic role in a significant number of these cases. There are no stains that are 100% specific or sensitive in depicting these lesions. The practice of using immunohistochemistry first (and in many laboratories exclusively) to address diagnostic dilemmas, although appearing to be a practical and cost-effective approach, is not always the case. Once 3 or more antibodies are used to solve the diagnostic challenge, ultrastructural evaluation becomes less expensive. To solve a difficult diagnostic problem often requires testing for 4 to 5 antigens. Turn around times for both immunohistochemistry and electron microscopy are similar. Ultra-rapid (3 hours) processing of specimens for electron microscopy is available for those cases requiring immediate diagnosis. One genuine problem with using immunohistochemistry as the initial diagnostic technique is that the results may show staining patterns that can be regarded as consistent with the diagnosis of melanoma even when the diagnosis is incorrect, and the need for performing ultrastructural evaluation is never appreciated. Immunohistochemistry and electron microscopy should be considered truly complementary and it is unfortunate that utilization of both diagnostic modalities does not occur more frequently. Pathologists should be aware of the entire immuno-morphological spectrum associated with melanocytic neoplasms to be able to address diagnostic difficulties that may, arise as a consequence of the heterogeneity of these tumors.