This presentation, case number 12 from a medical lecture (likely dermatopathology), details the diagnosis and differentiation of an unusual, deep intramuscular tumor found in a 20-year-old male. The lesion is an epithelioid malignant peripheral nerve sheath tumor (MPNST), which is challenging because it presents deep within the muscle, completely enveloping a large central nerve bundle, and exhibits striking cellular atypia. Crucially, the epithelioid MPNST strongly mimics metastatic melanoma due to its epithelioid morphology, strong and diffuse positivity for S100 and SOX10, and negativity for Melan-A and HMB-45. The speaker emphasizes that this tumor differs significantly from the conventional spindle-cell MPNST (which is rarely S100 positive and often associated with NF1). Diagnostic distinction relies heavily on the clinical context (deep location adjacent to a major nerve in a young patient with no melanoma history) and confirmatory immunohistochemistry, notably the near-universal loss of nuclear INI1/SMARCB1 expression (seen in 75% of epithelioid MPNSTs). The presentation concludes by warning dermatopathologists that they may encounter this entity if it presents subcutaneously, or when consulting on poorly differentiated tumors staining positive for S100.
Differentiating Epithelioid MPNST from Melanoma: A Case Study
00:00:08 Introduction and Apology: The speaker begins case number 12, apologizing for running over time during the presentation and noting this was intended to be delivered in person in Australia.
00:00:42 Deep Intramuscular Nodule: The case involves a deep intramuscular nodule in the calf of a 20-year-old male; this tumor type is rare in the skin but has been observed subcutaneously.
00:01:00 Importance of Nerve Identification: The tumor's deep location is confirmed by the presence of large, central nerve bundles, which a dermatologist-only trained pathologist might miss, emphasizing the need for careful orientation.
00:02:27 Perineural Invasion: The tumor is shown completely surrounding and invading a nerve bundle within the perineurium, with the tumor seemingly arising from and expanding outward from the nerve structures.
00:03:03 Cytological Features and Mimicry: The tumor cells are atypical, forming nests, cords, and sheets, strongly resembling other high-grade tumors, including epithelial angiosarcoma, epithelioid sarcoma, and most importantly, metastatic melanoma.
00:04:03 Clinical Context for Suspicion: In a young patient with no melanoma history, a solitary deep nodule wrapped around a large nerve makes metastatic melanoma statistically unlikely.
00:04:21 Immunohistochemistry (IHC) Results: The tumor is strongly and diffusely positive for S100 and SOX10, but negative for Melan-A and HMB-45, creating a strong IHC mimic of melanoma.
00:04:56 Diagnosis: Epithelioid MPNST: Given the clinical setting (deep nerve association) and IHC findings, the diagnosis is Epithelioid Malignant Peripheral Nerve Sheath Tumor (MPNST).
00:05:05 Distinction from Conventional MPNST: The epithelioid variant is clinically, molecularly, and immunophenotypically distinct from the conventional spindle-cell MPNST (which usually arises in NF1 patients and is paradoxically S100 negative).
00:06:34 Key Discriminator (INI1): A helpful test for differentiation is assessing INI1 (SMARCB1) expression; loss of nuclear INI1 is seen in about 75% of epithelioid MPNSTs, while it is not a feature of melanoma.
00:07:31 Relevance to Dermatopathology: Dermatopathologists must be aware of this entity as it can present subcutaneously or be referred when it mimics melanoma based on S100/SOX10 positivity.
00:08:15 Warning about Spindle Cell Tumors: In contrast, a spindle cell tumor in the skin that is strongly S100/SOX10 positive (e.g., on the scalp of an elderly person) should be considered spindle cell melanoma until proven otherwise, as conventional MPNST rarely shows this strong staining pattern.
00:08:59 Conclusion: The speaker concludes the presentation, apologizing again for exceeding the allotted time.