[Melanotic neurofibroma]

Melanotic neurofibromas are rare tumours. The clinical and histological diagnosis is often difficult to make.

A 41 year-old woman with type-1 neurofibromatosis presented with an old, large (16 cm by 6 cm) pigmented tumour on her left arm. It was initially considered to be a congenital naevus. Partial surgical resection was performed. Histological examination showed a loose proliferation of spindle-cells within the dermis and subcutaneous layers, with multiple foci of melanin-laden cells but no mitotic figures or atypical cells. There was no melanocytic theca. The tumour had immunoreactivity for the S-100 protein, neuron-specific-enolase, neurofilaments, synaptophysin, A-103 and HMB-45. The association of a benign pigmented tumour producing melanin and the presence of Schwann cells and nervous cells, led to the diagnosis of diffuse melanotic neurofibroma.

Melanotic neurofibromas can occur on their own or be associated with neurofibromatosis. They must be distinguished from classical neurofibromas when pigmentation occurs in the latter. Melanotic neurofibromas usually appear in the second or third decade of life and rarely in childhood. It is worth noting that hairs may overlie a melanotic neurofibroma, mimicking a giant naevus or a neurocristic cutaneous hamartoma. These are the two main differential diagnoses among children. Among adults, the main difficulty is to distinguish melanotic neurofibroma from pigmented dermatofibrosarcoma, because of the clinical and histological similarities between these two.