Nevic mitoses: a review of 1041 cases

The presence of dermal mitotic figures is helpful in identifying malignant melanocytic lesions but occasionally occur in benign nevi. We aim to determine a "benchmark" mitotic frequency for a wide variety of nevi. We prospectively collected 1041 cases of benign melanocytic nevi and reviewed them for the presence of mitotic figures. Specimens were collected from female (62%) and male (38%) participants, ages ranged from 1 to 90 years. Nevus types included compound melanocytic nevi (CMN), intradermal melanocytic nevi, junctional melanocytic nevi, lentiginous CMN (LCMN), lentiginous junctional melanocytic nevi, blue nevi, deep penetrating nevi, and pigmented spindle cell nevi. Nevi with congenital, mildly dysplastic, and Spitzoid features were included. A total of 82 of 1041 (7.9%) nevi contained one or more mitotic figures. Most (76.1%) mitoses were found in the papillary dermis. Single mitotic figures were more common (80.4%) than 2 (15.9%) or 3 (3.7%) within a single specimen. Of all cases containing mitotic figures, the most common nevus type was CMN. The nevus type most frequently demonstrating mitotic figures was CMN. The most common body site among cases with mitotic figures was trunk (53.7%), whereas the body site with the largest proportion of nevi demonstrating mitotic figures was special site (10.9%). The percentage of nevi containing mitotic figures was nearly the same among female (7.9%) and male (7.8%) participants. Results of this large review confirm that mitotic figures, even multiple ones, do not preclude benignity in a variety of melanocytic nevi.