Actin-rich desmoplastic malignant melanoma - Report of three cases

We report three cases of desmoplastic malignant melanoma (DMM) rich in smoo th muscle actin. They occurred in two men (Cases 1 and 3) and in one woman (Case 2). Cases 1 and 2 were recurrent lesions from common melanomas excise d, respectively, 3 and I years previously. In Case 3, DMM was associated wi th lentigo maligna at the time of presentation. Morphologically, DMMs were composed of spindle neoplastic cells organized in haphazardly orientated lo ng fascicles separated by collagen bundles. Perineural invasion was present and mitotic activity was prominent in all cases. The neoplastic spindle ce lls were intensely positive with S100 protein and smooth muscle actin antis era and negative with HMB45 and Melan-A (Mart-l) antibodies. Double stainin g for smooth muscle actin and S100 protein revealed no definite coexpressio n of the two antigens. Follow-up was available for patients 1 and 2 who had local recurrences and are still alive, It is possible that actin rich elem ents differentiate toward mesenchymal elements, paralleling the phenotypic changes seen in sarcomatoid carcinomas. Therefore, multidirectional differe ntiation may explain the mesenchymal (sarcomatoid) differentiation of neopl astic melanocytes and may be responsible for the different biologic behavio r of DMMs, which is closer to mesenchymal tumors than to conventional melan omas.