Abstract

A 37-year-old female presented with a recurrent nodule on the dorsal thumb after an excision four years previously. Histopathology showed a haphazard proliferation of bland spindle cells within the dermis and superficial subcutis with occasional storiform areas, short fascicles, and a few admixed mast cells. Immunohistochemistry revealed a mixed population or cells consisting of fibrohistiocytic cells with positivity for factor 13a and melanocytes with positivity for Melan-A and HMB45 in addition to Sox10 and S100. Cytologic atypia, mitoses, and pigment were not present. The previous excision from this patient was reviewed and showed nearly identical findings. Due to the presence of melanocytes in an unusual spindle cell proliferation, the previous excision underwent molecular testing to exclude a subtle desmoplastic melanoma. Interestingly, this showed a mosaic loss of chromosome 5q with no other abnormalities. The previous excision had multifocal positive margins likely accounting for the recurrence. Review of the literature revealed a single case report by Nair et al which described a similar pattern and coined the term “Storiform melano-fibrous histiocytoma”. Our case appears to fit this term perfectly. This unusual lesion may represent benign fibrous histiocytoma/dermatofibroma colonized by benign spindled melanocytes.