Nevus cell aggregates massively occupying parenchyma of an external iliac lymph node: A case report and review of the literature.

We report a case of nevus cell aggregates (NCAs) in an external iliac lymph node from a patient with a compound congenital nevus in the corresponding drainage skin. Melanocytes in parenchyma were in band, nest-like or nodular fashion, and partly continuous with those in capsule and trabeculae. The largest nodule in parenchyma measured 6.5 mm. Melanocytes mostly exhibited benign appearance identical to cutaneous nevus. A few regions abundant in cells displayed atypical features, including increased nucleo-cytoplasmic ratio, small nucleoli, and occasional mitotic figures. Immunohistochemistry showed that melanocytes stained positive for p16, but negative for HMB-45 and nestin. Ki-67 labeling was less than 1% and reticulin mainly surrounded individual melanocytes. Besides, Vysis melanoma fluorescence in situ hybridization (FISH) plus another 2 probes targeting 9p21(CDKN2A) and 8q24(MYC) showed normal results. The patient is alive without malignant tumor after 52-month follow up. Our case provides a new evidence for the existence of intraparenchymal NCAs in deep lymph node and indicates that melanocytes with some atypical features can occur in nodal nevi. Nevus cells in parenchyma connected to those in capsule and trabeculae are a significant clue to distinguish nodal nevi from metastatic melanomas. Additionally, immunohistochemistry and FISH assay are useful in differential diagnosis.