Benign melanocytic nevus cells in axillary lymph nodes. A prospective incidence and immunohistochemical study with literature review.

Benign nevus cell aggregates (NCAs) in lymph nodes usually present as intracapsular or trabecular collections of small, uniform melanocytes that resemble those seen in intradermal melanocytic nevi. The surgical pathologist must be aware of nodal NCAs because they can mimic micrometastasis of carcinoma. Although not uncommon, the frequency with which NCAs occur is controversial. Two previous studies attempted to determine the case incidence of NCAs in axillary lymphadenectomies; widely different results were reported, ranging from .33% to 6.2%. In this study, the authors examined prospectively 300 axillary lymph node dissections containing 5186 lymph nodes, using S-100 protein immunohistochemistry as a supplemental evaluation measure, to determine the incidence of NCAs. Twenty-eight NCA-positive lymph nodes from 22 cases were found, for a 7.3% case incidence and a .54% nodal incidence; these figures were higher than those previously reported. The possible pathogenesis of this phenomenon is discussed, with a review of the literature.