Clinical Question: What is the risk that a congenital melanocytic nevus will undergo malignant transformation?

Setting: Outpatient (any)

Study Design: Meta-analysis (other)

Synopsis: Congenital melanocytic nevi are pigmented lesions of varying size that are present at or shortly after birth. They occur in about 0.2 to 2.1 percent of infants and range in size from less than one square centimeter to hundreds of square centimeters. Previous case series and cohort studies have provided varying estimates of the risk of malignant transformation. This systematic review identified 14 relevant articles after a comprehensive literature search. Studies with fewer than 20 patients or those with fewer than three years of follow-up were excluded. Eight studies were retrospective and six were prospective; the mean follow-up ranged from 3.4 to nearly 24.0 years. Six studies had fewer than 100 patients; and two studies were large, with 1,008 and 3,922 patients.

The proportion of patients with melanoma ranged from 0.0 to 10.7 percent (six of 56 patients in one small retrospective study). The age at diagnosis of melanoma ranged from birth to 57 years, with a mean age of 15 years. Overall, 49 melanomas were reported in 46 of 6,571 patients (0.7 percent). Most melanomas (75 percent) arose in so-called “garment nevi,” which are greater than 15.7 in (40 cm) in diameter. Because of the higher rate in small studies, suggesting selection bias, this figure of 0.7 percent probably represents an upper bound. The largest, and perhaps best, population-based study found that 0.2 percent of all newborns were registered as having congenital melanocytic nevi, of whom only two (0.05 percent) underwent malignant transformation after a median follow-up of 10 years.

Bottom Line: Less than 1 percent of congenital melanocytic nevi reported in the medical literature underwent malignant transformation, although the heterogeneous nature of the studies permits only a broad estimate of risk. The true rate of malignant transformation may be lower because of selection bias inherent in smaller studies. (Level of evidence: 2a)