Cellular nodules in congenital pattern nevi

Background: The diagnosis of 'cellular dermal nodule in melanocytic nevi' is challenging, as it can simulate invasive and tumorigenic melanoma. Because cellular nodules are presumed benign, it is essential to differentiate them from true malignant melanoma arising within a nevus, which portends a poor prognosis. Although presumed benign, to date, no published long-term follow-up studies have proved the benignity of this lesion. Methods: To better understand the clinical behavior and histological features of these lesions, we present a clinical pathological study of a cohort of 26 cases, for which we obtained clinical follow up by chart review and questionnaires. Results: Clinical follow up was obtained on 16 out of 26 patients (61.5%) and ranged from 12 to 132 months (average 62.3 months). The cellular nodules in congenital pattern nevi, reported here, had an invariably benign clinical course. Conclusions: Features useful in differentiating cellular nodule from melanoma include: (1) lack of high-grade uniform cellular atypia; (2) lack of necrosis within the nodule; (3) rarity of mitoses; (4) evidence of maturation in the form of blending or transitional forms between the cells in the nodule and the adjacent nevus cells; (5) lack of pagetoid spread into the overlying epidermis; and (6) no destructive expansile growth. The difference between cellular nodule and melanoma arising from nevus can be very subtle, and it is important to emphasize that these lesions must be assessed on an individual case-by-case basis.