Nodular fasciitis is a benign myofibroblastic neoplasm that characteristically enlarges rapidly and then usually regresses spontaneously. While the vast majority of tumors are benign, there are rare reports of morphologically benign nodular fasciitis giving rise to metastases, not predictable on histologic grounds. Here, we report what we believe is an example of morphologically malignant nodular fasciitis, which occurred in the upper extremity of a 7-year-old male. The tumor was composed of short, intersecting fascicles of myofibroblastic cells in a loose myxoid matrix, with keloidal hyalinization and admixed osteoclastic giant cells, all characteristic of nodular fasciitis. However, it additionally exhibited striking nuclear pleomorphism, a feature not compatible with conventional nodular fasciitis. Fluorescence in situ hybridization demonstrated a USP6 translocation, confirmed by next-generation sequencing to be the novel CALD1-USP6 fusion. No other somatic or germline mutations were detected. This case adds to the expanding morphologic and molecular genetic spectrum of nodular fasciitis.

结节性筋膜炎是一种良性肌纤维母细胞肿瘤，其特征是迅速扩大，然后通常会自行消退。虽然绝大多数肿瘤是良性的，但很少有关于形态学良性结节性筋膜炎引起转移的报告，这在组织学上是不可预测的。在这里，我们报告我们认为是形态学上恶性结节性筋膜炎的一个例子，它发生在一名 7 岁男性的上肢。肿瘤由松散的粘液样基质中短的、交叉的肌纤维母细胞束组成，具有瘢痕疙瘩透明化和混合的破骨细胞巨细胞，所有这些都是结节性筋膜炎的特征。然而，它还表现出惊人的核多形性，这一特征与传统的结节性筋膜炎不相容。USP6易位，经新一代测序证实为新型CALD1 - USP6融合。未检测到其他体细胞或种系突变。该病例进一步扩大了结节性筋膜炎的形态学和分子遗传谱。