Dermatofibroma with Monster Cells: A Very Atypical Presentation

Summary: Dermatofibroma with monster cells, or pseudosarcomatous dermatofibroma, is a rare dermatofibroma variant first described in 1987. Due to the lack of immunohistochemical and molecular studies, many cases would be reclassified as sarcomas nowadays. We present a case of a 57-year-old female with a mass on her right upper inner thigh since 2020, initially diagnosed as dermatofibrosarcoma protuberans (DFSP). Excision of the thigh lesion demonstrated a dermal spindle cell neoplasm with a storiform and fascicular pattern and extension into the subcutaneous fat. The cells were spindled and elongated with vesicular nuclei and moderate amounts of eosinophilic cytoplasm. There was another component of scattered foamy histiocytes and multinucleated giant cells with hyperchromatic nuclei. Few mitotic figures were observed. The epidermis was acanthotic with hyperkeratosis and orthokeratosis. The tumor cells were positive for CD34 around the lesion's periphery, weakly positive for factor XIIIa and CD68, and negative for p53. Fluorescence in situ hybridization (FISH) testing was negative for PDGFB, and next-generation sequencing (NGS) testing was positive for the LAMTOR1::PRKCD fusion gene. The lesion was diagnosed as dermatofibroma with monster cells, although the biopsy was initially interpreted as DFSP. This case underscores the importance of molecular studies for the definitive diagnosis of atypical lesions.