Immunohistochemistry in a Spindle Cell Lesion: A Case of Cellular Dermatofibroma

Abstract

Cellular dermatofibroma (CDF) is an uncommon hyper-cellular variant of benign fibrous histiocytoma that may clinically and histo-pathologically mimic malignant spindle cell neoplasms, particularly dermatofibrosarcoma protuberans (DFSP) and spindle cell squamous cell carcinoma (SpSCC). Accurate distinction is essential because these entities differ substantially in biological behaviour, therapeutic management, and prognosis. This study reports a diagnostically challenging case of CDF in a 56-year-old woman presenting with a gradually enlarging keratotic nodule over the extensor aspect of the left forearm. Clinically, the lesion resembled keratoacanthoma, while preliminary histopathological evaluation suggested a spindle cell neoplasm. Histopathological examination revealed a hypercellular dermal spindle cell proliferation arranged in fascicular and focal storiform patterns without significant atypia, necrosis, or atypical mitoses. No honeycomb infiltration of subcutaneous fat was identified. Immunohistochemistry demonstrated negativity for CD34, AE1/AE3, STAT6, SOX10, desmin, smooth muscle actin, caldesmon, calponin, and ERG, with focal S100 positivity. Excision followed by histopathological examinations such as correlation of histomorphological and immunophenotypic findings revealed a well-circumscribed, densely cellular dermal spindle cell lesion, and immunohistochemistry confirmed a diagnosis of cellular dermatofibroma (CD34, Pan-cytokeratin negative). This case highlights the diagnostic utility of immunohistochemistry in the evaluation of cutaneous spindle cell lesions and emphasises the importance of distinguishing CDF from malignant mimickers to prevent unnecessary aggressive surgical management.