Melanoma risk in congenital melanocytic naevi: a systematic review.

BACKGROUND: The risk of malignant melanoma in congenital melanocytic naevi (CMN) is a matter of controversial and ongoing debate.
OBJECTIVES: The purpose of this systematic review is to provide a careful and detailed summary of the published data, including several recently published studies.
METHODS: Articles on CMN (n=1424) were retrieved from Medline, 1966-October 2005. Case reports and studies lacking relevant clinical information were excluded. Only systematic collections of cases were taken into consideration. Series with fewer than 20 patients or studies with a mean follow-up of <3 years were regarded as epidemiologically less significant.
RESULTS: Fourteen articles were finally chosen for further analysis. The studies varied significantly with respect to study design (source of cases; retrospective vs. prospective analysis), age of patients, follow-up time, and naevus characteristics. The frequency of melanomas ranged between 0.05% and 10.7% and was significantly higher in smaller studies (P<0.0001). In a total of 6571 patients with CMN who were followed for a mean of 3.4-23.7 years, 46 patients (0.7%) developed 49 melanomas. The mean age at diagnosis of melanoma was 15.5 years (median 7). By comparison with age-adjusted data from the Surveillance, Epidemiology and End Results database, we calculated that patients with CMN carry an approximately 465-fold increased relative risk of developing melanoma during childhood and adolescence. Primary melanomas arose inside the naevi in 33 of 49 cases (67%). In seven cases (14%), metastatic melanoma with unknown primary was encountered; in four cases (8%) the melanoma developed at an extracutaneous site. The risk of developing melanoma and the rate of fatal courses were by far highest in CMN>or=40 cm in diameter.
CONCLUSIONS: The overall risk of melanoma of 0.7% in all 14 studies was lower than expected. The higher incidence of melanomas in smaller studies indicates selection bias. The melanoma risk strongly depends on the size of CMN and is highest in those naevi traditionally designated as garment naevi. The median age of 7 years at diagnosis of melanoma points to a risk maximum in childhood and adolescence. Future studies on CMN should report: (i) diameter, percentage of body surface, and localization of the CMN; (ii) percentage of naevus area removed by excision or subject to dermabrasion or other superficial treatments; (iii) mean and median age at entry into the study; (iv) mean and median follow-up time; (v) details on each melanoma case; (vi) standardized morbidity ratio of melanoma; and (vii) percentage of neurocutaneous melanosis.