Examination of hematoxylin-eosin–stained sections revealed a poorly circumscribed dermal nodule composed of spindle cells exhibiting a dermatofibromalike pattern with an infiltrative margin. The cells were arranged in sheets and short haphazard fascicles entrapping slightly fibrotic collagen bundles. The tumor extended to the deep dermis without involving the subcutaneous tissue. There was no marked inflammatory infiltrate. The overlying epidermis appeared slightly hyperplastic with central areas containing focal spongiosis, parakeratosis, and coagulated plasma. On the lateral aspect of the lesion, mainly at the tips of the rete ridges, there was a minimal increase in the number of junctional melanocytes without nesting. Melanocytes within the dermis were predominantly elongated, with oval to fusiform nuclei and indistinct, lightly eosinophilic cytoplasm. The nuclei consisted of uniformly fine chromatin, occasionally exhibiting small nucleoli and, rarely, intranuclear inclusions. Mitoses were not found in any section. There was no evidence of pigment in most of the cells, but small, scattered foci of cells containing minimal brownish yellow granular pigment were seen. Hematoxylin-eosin staining indicated that the pigment could be either hemosiderin or melanin. On Prussian blue staining, however, no hemosiderin was identified, but Fontana-Masson silver impregnation highlighted the pigment, indicating that the pigment was melanin (Figure 4). Immunohistochemical analysis revealed only weak staining for HMB-45 and S100 protein and slightly more intense staining for vimentin.
A Painless Nodule on the Dorsum of the Foot. Arch Dermatol. 2002;138(8):1091-1096. doi:10.1001/archderm.138.8.1091