Giant pigmented tumour of the scalp--a diffuse neurofibroma or a congenital naevus showing neurofibromatous changes? Immunohistochemical and electron microscopic studies.

A case of giant pigmented tumour of the scalp which developed in a 47-year-old woman is reported. Macroscopically, the tumour showed a peculiar two-layered structure, consisting of an upper non-pigmented and a lower pigmented portion. Histologically, it was composed of elongated neurofibromatous tumour cells with abundant collagen fibres in the non-pigmented portion and round naevus-like cells with abundant melanin pigment in the pigmented portion. S-100 protein and neurone-specific enolase were demonstrated in most of the tumour cells, but neurofilament and myelin basic protein were not detected. Electron microscopy revealed melanosomes in the tumour cells of the pigmented portion. These findings might support a melanocytic origin for the tumour, but the lack of superficial pigmentation and the associated hair loss were against this. The tumour may represent an example of duality of neural crest differentiation.