S100 expression in cutaneous scars: a potential diagnostic pitfall in the diagnosis of desmoplastic melanoma

Aims: The histological distinction of desmoplastic melanoma from cutaneous scar tissue, particularly in the context of re-excision specimens or possib le recurrence, may be very difficult. Immunostaining for S100 protein is of ten used to discriminate although there are little data on S100 expression in scar tissue. The aim of this study was to assess whether S100-positive c ells are present in dermal scars and, if so, their extent, distribution and nature. Methods and results: Twenty-two re-excision specimens of previously biopsie d nonmelanocytic skin lesions were reviewed, Formalin-fixed paraffin-embedd ed sections containing dermal scars were stained by a standard ABC immunope roxidase technique for S100 protein, CD1a and neurofilaments. The distribut ion and morphology of positive cells within the dermal scar tissue were doc umented, Cells expressing S100 protein were identified within the scars of 21 of the 22 cases. The number of S100-positive cells varied between cases but in four specimens was substantial. They displayed a variety of morpholo gical appearances but the majority were spindle-shaped, A few showed mild c ytological atypia. It is suggested that the majority represent Schwann cell s with a minority of Langerhans cells and cells of uncertain lineage. Conclusion: S100-positive cells, including spindle cells showing mild atypi a, are found in cutaneous scars. S100 staining of re-excision specimens or putative recurrences of desmoplastic melanoma should be interpreted with ca ution.