Abstract

Proliferative nodules are uncommon benign lesions that arise in the background of giant congenital melanocytic nevi in the neonatal period and rarely in nevi of adults. Despite their benign clinical course, due to concerning clinical and histopathologic features, these proliferations are often mistaken for melanoma. Recognition of this phenomenon, and a high index of suspicion, are necessary to allow for exclusion of melanoma through ancillary techniques. However, proliferative nodules arising outside of the neonatal period and within other histologic types of nevi pose diagnostic challenges due to lack of recognition. Herein, we describe two cases of proliferative nodules arising in melanocytic proliferations with the appearance of cellular blue nevi, which has not been previously reported, where the initial impression was that of blue nevus-like melanoma. The first case involved an 11-year-old male with a buttock lesion composed of a dermal and subcutaneous multinodular proliferation of highly cellular atypical spindle to epithelioid melanocytes lacking maturation and with occasional mitoses arising within an otherwise cellular blue nevus. Chromosomal microarray demonstrated whole chromosomal gains (+7, 11, 13, and 20). The second case involved a 34-year-old female with an orbital lesion composed of a hypercellular subcutaneous nodule of atypical epithelioid to spindle melanocytes with scattered mitoses surrounded by a rim of otherwise cellular blue nevus. Chromosomal microarray demonstrated whole chromosomal gains (+6, 7, 9, 13, 17, 19, 20, and 22) and copy neutral loss of heterozygosity of whole chromosome 10. Multiple gains or losses of whole chromosomes, as seen in these cases, are typical of proliferative nodules, whereas melanoma demonstrates complex chromosomal aberrations. These cases highlight the importance of recognizing proliferative nodules, and that they may occur outside of the typical clinicopathologic context, to avoid a misdiagnosis of melanoma.

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