Track: Case Reports

Abstract:

Epithelioid malignant peripheral nerve sheath tumors (EMPNSTs) represent an exceedingly rare subset of soft tissue sarcomas. Even more uncommon is their occurrence in the orbital or cutaneous regions, with only two previously reported cases in the literature, to our knowledge. We present a diagnostically challenging case of an EMPNST located in the left lower eyelid. Histopathological examination revealed malignant epithelioid cells with abundant cytoplasm. The tumor demonstrated a striking nested architecture, extending into the deeper dermis and surrounded by myxoid stroma on a per-cell basis, mimicking the morphology of myoepithelial carcinoma. Immunohistochemical staining showed strong and diffuse positivity for SOX10 and S100 and notable negativity for HMB-45, Melan-A, Tyrosinase, and MITF. Interestingly, a small collision congenital nevus component was also present, which resulted in a misdiagnosis in a prior biopsy. However, the excisional sample revealed morphology that was not evident in the original biopsy and was more consistent with EMPNST than desmoplastic or conventional melanoma, or conventional MPNST. Following a multidisciplinary tumor board discussion, the patient underwent wide local excision with delayed reconstruction using a Hughes flap. At one-year follow-up, there has been no evidence of local recurrence or metastasis. This exceptionally challenging case highlights the diagnostic complexity of EMPNST, particularly in rare anatomical locations or unusual histologic features. EMPNST can closely mimic melanoma due to its diffuse and strong S100 protein expression, unlike the conventional MPNST variant, which typically shows only patchy staining. Accurate diagnosis relies heavily on recognizing key histologic features in conjunction with a comprehensive immunohistochemical panel.

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Jeremy Tan