Large nested melanoma: a clinicopathological, morphometric and cytogenetic study of 12 cases.

A group of melanomas characterised by predominant growth as large nests within the epidermis has been described. These cases present a diagnostic challenge, as many traditional architectural criteria for the recognition of melanoma are absent. We report the clinical, histological, immunohistochemical, morphometric and cytogenetic features of a series of 12 cases of large nested melanoma. In this series, large nested melanoma accounted for 0.2% of cases of melanoma. The majority occurred on the trunk of middle aged patients with absent or minimal solar elastosis and 42% were associated with a component of benign intradermal melanocytic naevus, speaking to classification of these melanomas as falling within the spectrum of lesions developing in skin with low cumulative sun damage. In 67% of cases invasive melanoma was present. Criteria such as asymmetry, variation in nest size and intraepidermal nests with an underlying rim of junctional keratinocytes appear to be highly specific, and are strongly predictive of typical cytogenetic abnormalities of melanoma, which were identified in 92% of cases. Conversely, in addition to features which are definitionally absent or limited, features such as solar elastosis and cytological atypia do not appear to be particularly helpful in recognition of this variant.