Abstract：
Epithelioid and spindle cell sarcomas with NR1D1::MAML1/2 gene fusions are rare and emerging entities. Only six cases of NR1D1-rearranged mesenchymal tumors have previously been reported in the literature; they are often characterized by an epithelioid morphology, at least focal pseudogland formation, prominent cytoplasmic vacuoles, and focal to diffuse immunohistochemical expression of keratin. We herein report the first case of an NR1D1::MAML1 epithelioid and spindle cell sarcoma with dual immunohistochemical expression of ERG and FOSB, mimicking a pseudomyogenic hemangioendothelioma (PHE) on core biopsy. The sarcoma arose in the left forearm of a 64-year-old man. Initial biopsy showed a mesenchymal neoplasm composed of epithelioid and spindle cells dispersed in myxoid stroma with scattered stromal neutrophils. The morphologic features, combined with the dual immunohistochemical expression of ERG and FOSB, initially mimicked PHE, representing an important potential diagnostic pitfall. The patient subsequently underwent a radical resection, which showed a much more diffuse epithelioid appearance with nested architecture and pseudogland formation. Next-generation sequencing was performed on the resection specimen, which revealed an NR1D1::MAML1 gene fusion, confirming the final diagnosis. Given the fully malignant potential of this tumor, knowledge and recognition of this rare entity are essential to ensure proper management, prevent misdiagnosis, and further characterize the clinical course of this emerging entity. Comprehensive molecular testing can help to identify these rare tumors and exclude the possibility of epithelioid mimics, including PHE.
摘要：
具有NR1D1::MAML1/2基因融合的上皮样和梭形细胞肉瘤是罕见的新兴实体。以前文献中仅报道了6例NR1D1重排的间充质肿瘤；它们通常以上皮样形态为特征。至少是局灶性的假藻形成，突出的细胞质液泡，并聚焦于角蛋白的弥漫性免疫组织化学表达。我们在此报告了首例NR1D1::MAML1上皮样和梭形细胞肉瘤，具有ERG和FOSB的双重免疫组织化学表达，在核心活检中模拟假单菌性血管内皮瘤（PHE）。肉瘤出现在一名64岁男子的左前臂。最初的活检显示间充质肿瘤由上皮样细胞和梭形细胞组成，分散在粘液样基质中，中性粒细胞分散。形态学特征，结合ERG和FOSB的双重免疫组织化学表达，最初模仿PHE，代表一个重要的潜在诊断陷阱。患者随后接受了根治性切除术，表现出更加弥漫性的上皮样外观，具有嵌套结构和假藻形成。对切除标本进行下一代测序，揭示了NR1D1::MAML1基因融合，确认最终诊断。鉴于这种肿瘤的恶性潜能,对这种罕见实体的了解和认可对于确保适当的管理至关重要，防止误诊,并进一步表征这一新兴实体的临床过程。全面的分子检测可以帮助识别这些罕见的肿瘤，并排除上皮样模拟的可能性，包括PHE。
