Nonepidermal origin of malignant melanoma associated with a giant congenital nevocellular nevus.

We report an adult who developed primary malignant melanoma arising in association with residual nevus cells in the pectoralis muscle under an intact skin graft 13 years following the partial excision of a giant congenital nevocellular nevus (GCNN). The case serves to emphasize the malignant potential of GCNN, for which we estimate a lifetime risk of at least 6.3 percent and an approximate 17-fold risk for melanoma when compared with the general population. This case supports previous observations that melanoma may arise in nonepidermal components of a GCNN and suggests the need to consider excision of the GCNN to muscle fascia or deeper if nevus cells extend beyond this anatomic zone. We estimate the risk of death from general anesthesia during the first year of life to be substantially less than the risk of malignant degeneration during the same time period. The decision to excise the GCNN as soon as the problem is recognized should depend on the general health of the infant and the risk of surgical complications.