Benign nevus cell aggregates in the thymus: A case report

Aggregates of benign nevus cells occurring in lymph nodes are a well-descri bed incidental finding. Nevus cell aggregates (NCAs) can mimic foci of meta static carcinoma or other disease processes, so the surgical pathologist sh ould be familiar with this lesion. The purpose of this report is to describ e the potential diagnostic difficulties created by benign NCAs within the t hymus of a 32-year-old man with dysplastic nevus syndrome and malignant mel anoma involving mediastinal lymph nodes and the right lung. Morphologically , the NCAs in this case elicited the differential diagnoses of metastatic m elanoma and thymoma. Immunohistochemical studies helped to establish the co rrect diagnosis by demonstrating reactivity for S-100 protein and negative staining for keratin and HMB-45. Unlike malignant melanomas, NCAs show no p 53 protein immunoreactivity, and low proliferative activity was detected by Ki-67 antigen immunostaining. Although melanocytic cells were rarely repor ted in thymic neoplasms, we are not aware of any previous reports of NCAs o ccurring in the normal thymus.