Abstract

Background: Cutaneous Syncytial Myoepithelioma (CSM) is a rare variant of cutaneous myoepithelioma with EWSR1-PBX3 fusion, which, by immunohistochemistry, is typically positive for EMA, S100, and SMA while negative for cytokeratins, desmin, ALK-1, and other melanocytic markers. We present a case of CSM with positive ALK-1 immunostaining but negative ALK gene rearrangement. Case description: A 38-year-old female with no significant dermatologic history presented with a 1.0 cm pink plaque on the mid-back, which had been present for over a year and treated with over-the-counter topical wart medications. A shave biopsy was performed with a clinical suspicion of traumatized nevus or verrucoid keratosis. Microscopic examination demonstrated verrucous epidermal change and a dermal neoplasm composed of epithelioid cells arranged in diffuse syncytial sheets. The neoplastic cells had relatively uniform oval-to-round nuclei with occasional larger nuclei. Mitotic figures were rare, with relatively low Ki-67 proliferative activity. The tumor cells expressed EMA and S100 protein but were negative for SOX-10, MART-1, PRAME, and pancytokeratin. These findings were consistent with a diagnosis of CSM. However, ALK-1 immunostaining showed patchy expression, with some cells showing a perinuclear dot-like pattern. Fluorescent in situ hybridization (FISH) was positive for ESWR1 gene rearrangement but negative for ALK gene rearrangement. The FISH findings supported a diagnosis of CSM and argued against the possibility of epithelioid fibrous histiocytoma, which was an alternate consideration given the positive ALK-1 immunostaining. Conclusion: CSM showing ALK-1 positivity on immunostaining, to our knowledge, has never been reported. ALK-1 expression in CSM could cause confusion with epithelioid fibrous histiocytoma. Molecular testing for EWSR1 and ALK gene fusions can help distinguish between these entities.

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