Nevoid Melanoma : Differential Diagnosis
The infiltrating tumor cells of a nevoid melanoma are arranged in sheets or nests and consist of small epithelioid melanocytes resembling type A nevus cells. They have pale or eosinophilic cytoplasm, round to oval vesicular nuclei, and small punctate nucleoli. Mild to moderate nuclear pleomorphism is seen and mitotic activity is increased, especially in the deeper portions of the tumor. Uncommonly seen features include pigmentation, tumor-infiltrating lymphocytes, and perineural invasion. Distinction of nevoid melanoma from common melanocytic nevi can be made by careful attention to cytologic features and the noting the presence of dermal mitotic activity. One must, of course, keep a high index of suspicion in the first place when handling such cases. Immunohistochemical stains such as MIB-1, cyclin D1, and HMB-45 can be very helpful as well. Benign nevi show only scattered positive cells for these markers in the superficial layers of dermis. Melanomas show numerous positive cells scattered throughout the full thickness of the lesion.