A nonepidermal, primary malignant melanoma arising in a giant congenital melanocytic nevus 40 years after partial surgical removal

Abstract

Giant congenital melanocytic nevi (GCMN) are defined by their size, measuring at least 20 cm in diameter in adulthood.1 These lesions are extremely rare and have an estimated incidence of 1 in 20,000 newborns.2 The most common anatomic site of a GCMN is the trunk, followed by the extremities and head/neck areas.3 Many GCMN can be associated with smaller satellite congenital nevi involving a variety of anatomic locations. Patients with GCMN have an increased risk of developing melanoma, and malignant transformation is most likely to occur during the first decade of life.4 The lifetime risk of developing melanoma in a large congenital nevus is approximately 3% to 6%.3 Although melanomas arising in large congenital nevi are usually of cutaneous origin (epidermal or nonepidermal), reports have also shown that transformation can occur in extracutaneous sites, such as in neurocutaneous melanocytosis.5