OBJECTIVETo study the clinicopathologic features, differential diagnosis and prognosis of proliferative nodules(PNs) in congenital melanocytic nevi(CMN).

METHODSHistopathologic evaluation and immunohistochemical study by EnVision method were carried out in 8 cases of PNs in CMN. The clinical information and follow-up data were analyzed.

RESULTSThe age of patients ranged from 1 to 54 years (mean 27.6 years). Tumors were located on face (3 cases), on back (2 cases), upper extremities (2 cases) and lower extremities(1 case). Microscopically, PNs with expansile growth were observed in 8 cases of CMN. Melanocytes in PNs show variable pleomorphism with a mitotic activity of 0 to 4 per 10 high power fields. Blending of cells with adjacent CMN was noted in 6 cases. According to the morphology of melanocytes in PNs, it was classified into different types including large oval melanocytes (4 cases), small melanocytes (2 cases) and Spitz-nevus-like forms (2 cases). Immunohistochemically, melanocytes in PNs were consistent with those in adjacent CMN. They were diffusely positive for S-100 protein, but were either negative or focally positive for HMB45. Less than 5% of melanocytes were positive for Ki-67 in 8 cases of PN. Follow-up was available in all cases, ranging from 9 to 82 months. Seven patients with excision of single PN in CMN were alive with no evidence of disease, while 1 patient with multiple PNs in CMN was stable with disease after 62 months follow-up.

CONCLUSIONSPN is a rare melanocytic lesion arising in CMN. Recognition of its specific histologic features can help to avoid being misdiagnosed as melanoma. Long term follow-up should be recommended in patients with PNs, especially in those with atypical histologic features. Further investigation is needed to elucidate its clinical behavior.