Mitotically active proliferative nodule arising in a giant congenital melanocytic nevus: a diagnostic pitfall

Proliferative (cellular) nodules (PN) which mimic malignant melanoma clinically and histologically are described in congenital melanocytic nevi (CMN) and may pose significant diagnostic challenges. We report the case of a 10-day-old male with a giant congenital nevus involving the neck, upper chest, back, and left shoulder containing several nodular lesions, some crusted. Biopsy of a nodule revealed densely packed nevus cells with hyperchromatic round to oval and occasionally irregularly shaped nuclei. There was no necrosis or pushing border, and the nodule blended with the adjacent nevus; however, the lesion demonstrated a significant number of mitoses (27 per mm2) and a 60% labeling index with Ki-67. Further analysis by fluorescence in situ hybridization (FISH) with a 4-color probe set targeting 6p25, 6q23, 11q13, and centromere 6 revealed increased chromosomal copy numbers of all 4 probes, which was interpreted as evidence of polyploidy. In addition, analysis of DNA copy number changes using a single nucleotide polymorphism microarray (Affymetrix, Santa Clara, CA) showed no chromosomal aberrations. The diagnosis of PN in a giant congenital nevus was eventually rendered. At 13-month follow-up, the nodules showed no evidence of growth. Our case illustrates that PNs in the neonatal period might demonstrate extreme mitotic activity. This feature is worrisome when encountered in melanocytic lesions; however, it should not trigger by itself a diagnosis of melanoma in the absence of other histologic criteria of malignancy. In addition, we document polyploidy by FISH in PN, which can potentially be misinterpreted as a FISH-positive result.