Metastatic desmoplastic malignant melanoma associated with low-grade myofibroblastic sarcoma

The authors report a 63-year-old man with a 2-year history of a primary desmoplastic malignant melanoma on the right heel with lymph node metastasis in the right groin, presented with a large painless mass in the right inguinal area. Microscopically, there was a dense spindle cell proliferation in the reticular dermis, subcutis, and subjacent soft tissue. The neoplastic cells were moderately pleomorphic and showed mitotic figures including atypical ones. Immunohistochemical studies showed a biphasic cellular composition, with one component being S-100 protein positive and alpha-smooth muscle actin negative and another component with a reverse phenotype, namely, alpha-smooth muscle actin positive and S-100 protein negative. Pleomorphic cells and atypical mitotic figures were evident in both components. Electron microscopy identified cells with peripheral bundles of thin cytoplasmic myofilaments with focal dense bodies, fibronexus junctions, abundant rough endoplasmic reticulum, micropinocytic vesicles, and strikingly folded nuclei. These cells had features of myofibroblasts. We conclude that the myofibroblastic proliferation in this case clearly exceeded just a stromal reaction and fulfilled the criteria of a low-grade sarcoma.