Moubarak Simon, McAfee John, Fritchie Karen, Ko Jennifer S, Billings Steven D, Ronen Shira
Department of Pathology, Cleveland Clinic, Cleveland, Ohio, USA.
J Cutan Pathol. 2025 Oct;52(10):605-610. doi: 10.1111/cup.14844. Epub 2025 Jul 10.
We present a case of a 39-year-old woman initially diagnosed with Merkel cell carcinoma (MCC), a highly aggressive neuroendocrine carcinoma, due to the presence of cytokeratin and neuroendocrine marker expression. The tumor was dermal based, showing small round blue cells with fine chromatin, scant cytoplasm, and scattered mitotic figures arranged in sheets, small cohesive nests, and cords within sclerotic to edematous stroma. Provided immunohistochemical stains showed strong pancytokeratin expression coupled with perinuclear dot-like staining for cytokeratin 20 in a distinct regional distribution, predominantly in areas where the tumor cells formed cohesive nests and cords within sclerotic stroma. Stains for neuroendocrine markers, including synaptophysin, INSM1, and CD56, were positive, albeit focal or regional in the more cohesive areas. Given the patient's age and unusual regional staining patterns, additional testing was performed, which revealed diffuse membranous CD99 staining and EWSR1::ERG fusion. These findings led us to revise the diagnosis to cutaneous Ewing sarcoma (ES). The distinction between MCC and cutaneous ES is crucial due to their different survival rates and treatment approaches. This case underscored the importance of considering alternative diagnoses when encountering cutaneous small round blue cell tumors in the presence of unusual histologic and immunohistochemical findings, particularly in younger patients.
我们报告一例39岁女性病例，该患者最初因存在细胞角蛋白和神经内分泌标志物表达而被诊断为默克尔细胞癌（MCC），这是一种高度侵袭性的神经内分泌癌。肿瘤位于真皮层，表现为小圆形蓝色细胞，染色质细腻，胞质稀少，有散在的有丝分裂象，呈片状、小的紧密巢状及条索状排列于硬化至水肿的间质中。免疫组化染色显示，全细胞角蛋白呈强阳性表达，细胞角蛋白20呈核周点状染色，呈独特的区域分布，主要在肿瘤细胞于硬化间质中形成紧密巢状及条索状的区域。神经内分泌标志物染色，包括突触素、胰岛素瘤相关蛋白1（INSM1）和CD56，均为阳性，尽管在更紧密的区域呈局灶性或区域性阳性。鉴于患者年龄及不寻常的区域染色模式，进行了进一步检测，结果显示弥漫性膜性CD99染色及EWSR1::ERG融合。这些发现使我们将诊断修订为皮肤尤因肉瘤（ES）。MCC与皮肤ES的鉴别至关重要，因为它们的生存率和治疗方法不同。该病例强调了在遇到具有不寻常组织学和免疫组化表现的皮肤小圆形蓝色细胞肿瘤时，尤其是在年轻患者中，考虑其他诊断的重要性。
