Cutaneous Melanoma Arising in Congenital Melanocytic Nevus: A Retrospective Observational Study.

Background: Congenital melanocytic nevi (CMN) are benign proliferations of melanocytes usually present at birth. The magnitude of the melanoma risk for CMN is controversial, generating an ongoing debate on the best approach to manage these lesions.

Objective: To perform a retrospective, observational study with the aim to evaluate the prevalence of CMN-associated melanomas in tertiary referral centers, as well as the eventual correlation between clinical, dermoscopic, and histological features of CMN-associated melanomas.

Methods: A single-center retrospective observational study was performed on all clinical and dermoscopic images of histologically confirmed melanomas arising on CMN over a 14-year period (January 2005 to March 2019).

Results: Our database included 2,159 melanomas in the considered period. Of those, 27 (1.3%) were CMN-associated melanomas. The mean age of patients with CMN-associated melanoma was 33 years (range, 11-70 years). The mean diameter of CMN-associated melanoma was 18 mm (range, 6 mm to 20 cm), and 56% were located on the back. Twenty-one (77.8%) of CMN-associated melanomas arose on small CMN (<1.5 cm), 5 (18.5%) on medium-sized CMN (1.5-19.9 cm), and 1 (3.7%) on a large/giant type (≥20 cm). The majority of CMN-associated melanomas (63%) exhibited a globular dermoscopic pattern in their benign part, while a blue-white veil and irregular blotches were the most frequent dermoscopic features in the malignant part. About three quarters of melanomas occupied 10-50% of the nevus surface. Breslow thickness was higher in melanomas involving less than 10% of nevus surface (mean thickness, 1 mm) than in those affecting 10-50 and >50% of the nevus surface (0.8 and 0.7 mm, respectively).

Conclusions: In our series, small CMN was the most frequent type of CMN-associated melanoma. Although the risk of melanoma is increasing by the increasing size of CMN, our finding is definitely related to the much higher prevalence of small CMN in the general population as compared to the prevalence of intermediate-sized and large CMN.

Limitations: Small sample size, single-center experience, retrospective design.