Lipomatous congenital melanocytic nevus presenting as a neck mass in a young adult

Congenital melanocytic nevus (CMN) is a melanocytic proliferation that has its onset at birth or shortly thereafter and shows characteristic histopathologic features including symmetric proliferation of benign melanocytes, extension of nevus cells into the deep reticular dermis and subcutis, maturation of melanocytes with descent, tracking of melanocytes around and within adnexal structures, vessels, or nerves and splaying of collagen bundles by nevus cells arranged in single rows or cords. We report the case of a 34 year old previously healthy woman who presented with a progressively enlarging soft tissue mass in the right neck and back adjacent to a medium sized CMN. Magnetic resonance imaging showed multiple lipomatous masses within the soft tissues of the posterior superficial neck. Subsequent excision of the soft tissue mass showed a well circumscribed lipomatous lesion with diffuse infiltration by benign appearing melanocytes within the fat lobules. Excision of the mass was not accompanied by overlying skin and, thus, posed a diagnostic challenge. Sudden increase in the size of a CMN is worrisome for the development of a melanoma, however, this lesion lacked significant cytologic atypia and mitotic figures, and had a low proliferative index by Ki-67 immunohistochemistry. This case serves to illustrate the initial diagnostic dilemma as well as the plasticity of the neural crest cells.