We report a case of a 16-year-old boy who presented a soft-tissue mass in the anterior compartment of the right thigh discovered by positron emission tomography/computed tomography within the work-up of unexplained prolonged inflammatory syndrome. The mass has no calcification. Subsequently, magnetic resonance imaging of the femoral triangle was carried out. Axial short tau inversion recovery images showed a 3.5-cm ill-defined mass in the femoral triangle with focal areas of hypointensity, which suggests that there might be fibrosis or hemosiderin within the tumor. Axial T1-weighted images showed a slight hyperintense mass involving the iliopsoas muscle. Contrast-enhanced fat-suppressed T1-weighted imaging showed a heterogeneous solid enhancement. Adjacent thick fascia enhancement of the vastus intermedius and the vastus lateralis muscles extending from the mass as a tail-like margin suggested the infiltrative spread of the tumor along the fascial plane. The mass and the lymphadenopathy were excised. Immunohistochemically, tumor cells were staining for muscle actin and desmin. Many plasma cells were IgG4+ (175per high-power field) with a ratio IgG4+/IgG+ of 50%. The diagnosis of IgG4-related disease (IgG4-RD) was made. Although a diffuse array of musculoskeletal symptoms has been observed in IgG4-related disease, reports of biopsy-proven musculoskeletal involvement of the limb are rare. We showed the radiological features of IgG4-RD presenting as a soft-tissue mass of the thigh. Musculoskeletal involvement, clinical significance, and treatment of IgG4-RD are also discussed.

中文翻译：

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IgG4相关疾病：在青少年大腿上很少见为软组织肿块。

我们报道了一个16岁男孩的病例，该男孩在右大腿前部出现了软组织肿块，该肿块是由正电子发射断层扫描/计算机断层扫描发现的，原因是无法解释的长期炎症综合征。肿块没有钙化。随后，进行股骨三角的磁共振成像。轴向短tau反转恢复图像显示股骨三角区有一个3.5厘米的边界不清晰的肿块，并伴有低强度灶区，表明肿瘤内可能存在纤维化或含铁血黄素。轴向T1加权图像显示有轻度高强度肿块，累及involving肌。对比增强的脂肪抑制的T1加权成像显示异质性固体增强。中间股韧带和邻近股外侧肌的厚筋膜增强从尾部开始延伸，呈尾状边缘，表明肿瘤沿筋膜平面浸润扩散。切除肿物和淋巴结肿大。免疫组织化学法检测肿瘤细胞的肌动蛋白和结蛋白染色。许多浆细胞是IgG4 +（每高倍视野为175），IgG4 + / IgG +之比为50％。进行了IgG4相关疾病（IgG4-RD）的诊断。尽管在IgG4相关疾病中观察到了一系列弥漫性的肌肉骨骼症状，但鲜有活检证明四肢肌肉骨骼受累的报道。我们显示了IgG4-RD的放射学特征，表现为大腿的软组织肿块。还讨论了骨骼肌肉受累，临床意义以及IgG4-RD的治疗。