A zonal comparison of MIB1-Ki67 immunoreactivity in benign and malignant melanocytic lesions

Differentiation between malignant melanomas and benign nevi can sometimes be difficult by conventional histopathology, and additional diagnostic markers may be helpful. This study investigated the immunoreactivity of the cell proliferation marker MIB1-Ki67 in 23 compound nevi, 17 dysplastic nevi, 8 Spitz nevi (SN), and 24 malignant melanomas (MMs) and evaluated its ability in separating benign nevi from MMs. In each lesion, the average number (percentage) of MIB1-positive nuclei (%MIB1-Mean) and the maximal number (percentage) of MIB1-positive nuclei (%MIB1-Max) were determined from each of the superficial, middle, and deep dermal zones of the lesion as well as from the entire lesion. The %MIB1-Max was determined from subjectively selected area(s) of high count. Malignant melanomas had a significantly greater %MIB1-Mean and %MIB1-Max than all benign nevi in all individual zones and in the entire lesion (p < 0.05). Discriminant analysis showed that the %MIB1-Mean and %MIB1-Max counted from the whole lesions had better discriminating abilities than from the individual zones. By using the %MIB1-Mean from all zones, all lesions except 1 SN and 3 MMs could be correctly classified as benign or malignant. When using the %MIB1-Max from all zones, all but 2 SN could be correctly separated as benign or malignant. Thus, MIB1-Ki67 immunoreactivity closely correlates with the benignancy or malignancy of melanocytic lesions and may assist in the differentiation of benign nevi from MMs.