Histopathological characteristics of small diameter melanocytic naevi

Abstract

Background/aims: The clinical definition of an atypical naevus ("dysplastic naevus" or "naevus with architectural disorder and cytological atypia of melanocytes") stresses size larger than 5 mm in diameter as a major diagnostic criterion. Because malignant melanomas and their precursors may arise in smaller lesions, a histological study of melanocytic lesions smaller than 4 mm in diameter was conducted to evaluate their histological appearance.

Methods: Two hundred and sixty one naevi smaller than 4 mm in diameter were collected and characterised by histological examination into benign naevi without architectural disorder and naevi with architectural disorder and mild, moderate, and severe atypical melanocytes according to criteria used on larger lesions.

Results: Small melanocytic naevi covered the same complex histological spectrum from benign naevi to severely atypical naevi when compared with larger lesions. A high proportion of small naevi (72%) exhibited features diagnostic for naevi with architectural disorder and cytological atypia.

Conclusion: There is a discrepancy between histological and clinically defined atypical naevi. The same generally accepted criteria for the histological diagnosis of atypical naevi should be used for small melanocytic naevi in addition to large ones. Thus, small naevi exhibiting atypical features on histological examination should be categorised as atypical naevi, regardless of their small diameter.

Figure 1

Different categories of small melanocytic naevi: (A, B) ordinary benign naevi without architectural disorder; naevi with architectural disorder and (C, D) mild, (E, F) moderate, or (G, H) severe cytological atypia of melanocytes (haematoxylin and eosin stain; original magnification, ×10 for A, C, E, and G; ×40 for B, D, F, and H).