The lifetime risk of developing melanoma in Caucasian patients with large congenital melanocytic nevi (LCMN) is estimated to be between 4.5% and 10%. Cohort studies of LCMN and the risk of melanoma development in an Asian population are not available.

We sought to determine the risk of melanoma development in a retrospective cohort of patients presenting with LCMN to a dermatology tertiary referral center in Singapore from January 1989 to December 2004.

Patients with congenital melanocytic nevi (CMN) that covered at least 5% of the body surface area were included in the study. Data were obtained from electronic records and photographic documentation. A search for malignancy was done using the National Cancer Registry database.

In all, 39 patients (23 male and 16 female) met the study criteria of LCMN; 15 of 39 patients also met the criteria of having a giant CMN (ie, a CMN that is predicted to be at least 20-cm diameter in adulthood). There were 29 Chinese, 6 Malay, 1 Indian, and 3 Caucasian patients. Their ages ranged from 23 months to 60 years (mean 18.8 years). They presented at a mean age of 26 months and were followed up for an average of 16.9 years. The size of the LCMN ranged from 5% to 40% of body surface area, with a mean of 12.2%. The most common sites were the back (54%), lower limb (28%), and abdomen (26%). Satellite lesions were present in 22 patients. Magnetic resonance imaging of the head or thoracolumbar spine was performed in 7 patients with LCMN on the scalp/face or back, respectively; all produced normal findings. Only one patient was treated: he had carbon-dioxide laser ablation and Q-switched neodymium:yttrium-aluminum-garnet laser treatment of a small part of his LCMN. Skin biopsies were done in 5 patients who had developed nodules; histology showed no evidence of malignancy. No patients had developed any form of malignancy.

The addition of 3 adult patients born before the start of the cancer registry may have led to survivor bias. The small sample size did not allow a precise estimate of the risk of melanoma development in our study population.

The risk of melanoma development in LCMN within a predominantly Southeast Asian cohort appears to be very low. Prophylactic complete excision of LCMN is ideal, but seldom achievable. Hence, patient education, regular melanoma surveillance, and biopsy of suspicious lesions are very important.