Atypical umbilical naevi: histopathological analysis of 20 cases.

AIMS: Melanocytic naevi on the umbilicus have been described as a form of flexural naevi, with the most common feature being a 'nested and dyshesive pattern'. We have encountered a distinct group of umbilical naevi with more significant atypia and prominent fibrosis, not reported previously. This study aimed to characterize these naevi more clearly.

METHODS AND RESULTS: Eighty-one umbilical naevi from 2000 to 2013 were reviewed retrospectively, 20 cases of which showed lamellar fibrosis and atypia and were designated as atypical umbilical naevi (AUN). Lamellar fibrosis in AUN was extensive and frequently entrapped dermal melanocytes, resulting in low-grade cytological atypia (74% of cases) and impaired maturation (47%) of the dermal component. Other common features included bridging (95%), shoulder architecture (94%), lentiginous growth (85%) and high-grade junctional cytological atypia (85%). 'Nested and dyshesive pattern' was observed in only 20% of AUN. Ki-67 immunostaining performed on 12 AUN revealed a consistently low proliferation index of <1%. All AUN in this series lacked junctional confluence, florid pagetoid spread and dermal mitoses.

CONCLUSION: A subset of special site naevi in the umbilical region demonstrate characteristic lamellar fibrosis, architectural disorder and cytological atypia. Familiarity with these site-related atypical features would avoid overdiagnosis of melanoma.