[Proliferative nodule in a congenital melanocytic nevus].

There are several benign lesions that may appear on conventional melanocytic nevi and clinically or histologically simulate malignant transformation while they actually behave in a benign manner.1 These include proliferative nodules, which show a particular tendency to develop on giant congenital melanocytic nevi (although they are also seen on smaller lesions) and, to a lesser degree, on acquired nevi.2-7 Clinically, they tend to present as a recent change in the color of the central part of the nevus, although they can also occasionally appear on lesions with no recent changes.3,5,6 Proliferative nodules show a benign biological behavior in spite of the architectural abnormalities they present and care must be taken to avoid confusion with melanoma. Identification is especially important in prepubertal children, in whom melanoma is very rare.7 We present the case of an 8-year-old boy referred to us with a congenital nodular lesion on the back of the left-hand. The nodule measured 0.8 cm in diameter, had well-defined borders, a smooth and shiny surface, and a brownish-pink color (Figure 1). The lesion appeared before the boy was 1 year-old and had grown in proportion with the child, showing no recent changes. The patient reported occasional pain with trauma. The nodule had a rubbery consistency and Darier sign was negative. Provisional diagnosis of a granular cell tumor, mastocytoma, adnexal tumor, or melanocytic lesion prompted complete removal of the tumor. histology revealed a dermal melanocytic proliferation in which the superficial stratum had the characteristics of a common congenital nevus, but with an extensive underlying area of greater cell density, less clearly defined margins, and with expansive nodular growth at the base (Figure 2A-C). The cellular composition of the deep section was heterogeneous, with small uniform cells alternating with areas of large cells with very few mitoses (Figure 2D,E). Immunohistochemical analysis showed generalized immunostaining for melan-A and reactivity for human Melanoma Black-45 (hMB-45) limited to the large cells (Figure 2F). Immunostaining with Ki67 revealed a proliferation index of 1%. Based on these findings, a diagnosis of a proliferative nodule on a melanocytic nevus was made. Proliferative nodules consist of the proliferation of a clone of melanocytes that make up the nevus, but that acquire a morphology that differs from the other cells predominant in the lesion. Although these proliferations occasionally present architectural alterations that can be confused with melanoma, they have a benign behavior.1-6 nodules can be identified under a microscope at a low magnification and there is a clear contrast between the cells that make up the nodule and the adjacent melanocytes predominant in the nevus. The nodules are generally located in the papillary or mid dermis, although they occasionally extend into the deep dermis.5