Actin-rich desmoplastic malignant melanoma: report of three cases.

We report three cases of desmoplastic malignant melanoma (DMM) rich in smooth 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 excised, respectively, 3 and 1 years previously. In Case 3, DMM was associated with lentigo maligna at the time of presentation. Morphologically, DMMs were composed of spindle neoplastic cells organized in haphazardly orientated long fascicles separated by collagen bundles. Perineural invasion was present and mitotic activity was prominent in all cases. The neoplastic spindle cells were intensely positive with S100 protein and smooth muscle actin antisera and negative with HMB45 and Melan-A (Mart-1) antibodies. Double staining for smooth muscle actin and S100 protein revealed no definite coexpression 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 elements differentiate toward mesenchymal elements, paralleling the phenotypic changes seen in sarcomatoid carcinomas. Therefore, multidirectional differentiation may explain the mesenchymal (sarcomatoid) differentiation of neoplastic melanocytes and may be responsible for the different biologic behavior of DMMs, which is closer to mesenchymal tumors than to conventional melanomas.