The Histopathological Diagnosis and Reporting of Melanoma: A New Look at an Old Challenge

The goal of a standardized and reproducible histopathological diagnosis and reporting of melanoma is far from being fully achieved. Clark's and McGovern's historical classification into lentigo maligna melanoma, superficial spreading melanoma, nodular melanoma, and acral (and mucosal) lentiginous melanoma can be still kept as an acceptable startpoint. The WHO 2006 classification recognizes additional subtypes of melanoma but is still largely incomplete. The differential diagnosis of melanoma with benign melanocytic proliferations is one of the most difficult fields in Dermatopathology because it stems from the evaluation of a constellation of diagnostic criteria whose implementation, meaning, and relative weight considerably vary depending on the overall morphological context; thus the histopathological diagnosis of a melanocytic tumor is a mere assessment of probability, and is subject to considerable interobserver disagreement. Ancillary (Immunohistochemistry and molecular biology) techniques have been increasingly performed in order to assist the histopathological diagnosis; as a rule, however, no single information achieved with these techniques is expected to give clear-cut information for the differential diagnosis between nevus and melanoma. The histopathological reporting should include at least the following compulsory parameters: ulceration (absent vs present); mitotic rate (integer number/square millimeter); regression (if present); lymphovascular invasion (if present) perineurial invasion (if present); Breslow's thickness; microsatellitosis (if present); status of the surgical margins (with microscopically measured distances between tumor and lateral or deep margins). Even is molecular techniques are expected to completely change our landscape of histopathology of melanoma within the next few years; these ‘classical' histopathological parameters are still the cornerstone for the management of melanoma patients.