Benign "metastatic" cellular blue nevus.

This report illustrates the case of a patient with a pigmented lesion on the dorsal skin of his left foot associated with a bulky homolateral inguinal mass. A metastatic melanoma was clinically suspected; therefore, the pigmented foot lesion was excised and grafted, and the groin mass was dissected. The histological examination of the foot lesion revealed a well-preserved epidermal layer, beneath which spindle cells endowed with regular nuclei yet without any atypia or mitotic figures, were present. Melanin-rich histiocytes surrounded the nerve fibers and the vessels that intermingled with such spindle cells. Under light microscopy, the sections of the inguinal lymph nodes revealed clusters of pigmented cells that looked very much like those found in the foot skin lesion. These spindle-shaped cells infiltrated the nodes' capsule and peripheral sinuses and left the inner parenchyma unaltered. The inguinal mass revealed a thick, fibrous capsule surrounding a heavily pigmented tissue rich with blue nevus cells with islands of melanophages. In the case presented here, the differential diagnosis between cellular blue nevus and nodular melanoma was mandatory. In this case report, we provide the differential diagnosis and review the criteria used for it. Further support for the diagnosis was obtained from immunohistochemical findings that were positive for S-100 protein and not for HMB-45. Wide but conservative surgery appears to be the treatment of choice for cellular blue nevus. In fact, the patient described here is still alive 5 years postoperatively. Hence, the clinical evolution of the patient's lesions can be considered benign.