Abstract

An 81-year-old woman presented for 0.6 cm flesh-colored “rubbery” feeling papule located on the left upper posterior lateral thigh. A punch biopsy was performed revealing a dermal myxoid lesion containing spindled cells with tapered nuclei associated with large numbers of mast cells. The spindled cells were negative for S-100 protein, SOX10, and Factor XIIIa, but positive for epithelial membrane antigen (EMA) and CD34. Interestingly, the mast cells showed cytoplasmic positivity for the nuclear stain microphthalmia transcription factor (MiTF), in addition to the expected cytoplasmic Giemsa positivity. The histopathologic features of this case showing an uncircumscribed, spindle cell proliferation in a myxoid stroma with scattered Giemsa positive mast cells, favor myxoid neurofibroma. However, the S100 and SOX10 by immunohistochemistry are negative, excluding a Schwann cell proliferation, such as a neurofibroma or Schwannoma. A negative Factor XIIIa excludes a myxoid dermatofibroma, and the positive EMA helped confirm a myxoid extraneural perineurioma. Although it is well known that mast cells are seen in high frequencies in neurofibromas, they are often commonly also seen in other nerve sheath tumors and soft tissue sarcomas, including perineurioma. This case demonstrates an uncommon but likely underreported entity in dermatopathology, a perineurioma. The difficulty in establishing this diagnosis lay with its mimicker, a myxoid neurofibroma, due to the similar architecture and presence of mast cells.

Financial Disclosure:
No current or relevant financial relationships exist.