7 - Immunohistology of Melanocytic Neoplasms

Abstract

Melanoma continues to represent one of the greatest diagnostic challenges in surgical pathology and, unfortunately, is an important source of litigation. Both as a primary lesion in the skin and in metastatic sites, this neoplasm is capable of assuming many different macroscopic and histologic appearances that mimic other diseases, both benign and malignant. The four main histologic phenotypes of primary melanoma include (1) superficial spreading (intermittent sun exposure), (2) lentigo maligna (marked sun exposure), (3) acral lentiginous mucosal, and (4) nodular melanoma. Other morphologic variants include nevoid, spitzoid (with all its variants), balloon (clear) cell, pleomorphic sarcomatoid, spindle cell/desmoplastic/neuroid, small cell (neuroendocrine-like), signet-ring cell, myxoid, metaplastic, and rhabdoid. All these variants may lack obvious melanin pigment (i.e., are amelanotic), which may render the diagnosis difficult. Accordingly, immunohistochemistry and other molecular analyses (FISH, CGH, gene arrays, mass spectrometry) have become exceedingly important in the accurate recognition of melanoma. Furthermore, immunohistochemistry now plays a crucial role in selecting patients for targeted therapies and to evaluate the response. This discussion is directed principally toward questions concerning diagnosis, prognosis, and treatment of melanocytic tumors in general.