Case 12B Male, 54 years. Diffuse swelling and induration, skin of forehead. S-100Show diagnosis & comments
The tumour cells of desmoplastic melanoma are strongly S-100 positive, but negative for Melan-A (MART-1), HMB-45 (gp100), usually MITF and tyrosinase. Therefore, an S-100 immunostain is extremely important diagnostically. In any case where a skin biopsy reveals an increased number of enlarged elongated cells within the dermis, and especially when derived from sun-damaged skin and when perivascular round cell infiltrates are present, an S-100 stain is mandatory.
In scar tissue, there may be small numbers of elongated, slender, S-100 positive cells, which should be borne in mind in order to avoid overdiagnosis of this rare melanoma variant.
Robson A, Allen P, Hollowood K. S100 expression in cutaneous scars: a potential diagnostic pitfall in the diagnosis of desmoplastic melanoma. Histopathology. 2001 Feb;38(2):135-40.