14. Ancient melanocytic naevus (AN): A simulator of malignant melanoma

Abstract

Background and aims: Ancient melanocytic naevus (AN) is an unusual but distinct melanocytic neoplasm within the spectrum of simulators of malignant melanoma. The name ‘ancient’ is used because it shares numerous features with ancient schwannoma. Ancient naevi are found most commonly on the face of older persons. The neoplasm is usually a dome-shaped, skin-coloured papule. Histopathologically, ancient naevi are exo-endophytic, mostly intradermal proliferations of two populations of melanocytes: one with large pleomorphic nuclei and the other with small monomorphic ones. A few mitotic figures may be present. We report a case of 73-year-old male who presented with an exo-endophytic pigmented lesion 5 mm in diameter on his right lateral thigh. It had been present for several years. An excision biopsy was done. Methods: Here we present a case of an ancient melanocytic naevus. Results: The histological examination of routine H&E stains showed a predominately dermal compound melanocytic lesion in the upper dermis composed of two populations of intermingled melanocytes. One with large pleomorphic round, oval, polygonal and spindled melanocytes with large hyperchromatic nuclei, some with prominent nucleoli and scant cytoplasm. The other with conventional, small, round to oval mature melanocytes arranged in a ‘congenital’ pattern. A few multinucleate pleomorphic melanocytes were also present. Ki-67 immunostain showed a proliferation index of less than 1%. Morphological features and immunohistochemistry were consistent with ancient melanocytic naevus. Conclusions: Ancient naevi may be misdiagnosed as melanoma arising in an intradermal naevus. Knowledge of the histopathologic characteristics of this benign melanocytic neoplasm should enable pathologists to avoid over diagnosis of melanoma arising in an intradermal naevus.